Linear Technology LT8330 User manual

LT8330
1
8330fa
For more information www.linear.com/LT8330
Typical applicaTion
FeaTures DescripTion
Low IQBoost/SEPIC/
Inverting Converter
with 1A, 60V Switch
The LT
®
8330 is a current mode DC/DC converter capable
of generating either positive or negative output voltages
using a single feedback pin.It can be configured as a
boost, SEPIC or inverting converter consuming as low as
6µA of quiescent current.Low ripple Burst Mode opera-
tion maintains high efficiency down to very low output
currents while keeping the output ripple below 15mV in
a typical application.The internally compensated current
mode architecture results in stable operation over a wide
range of input and output voltages.Integrated soft-start
and frequency foldback functions are included to control
inductor current during start-up.The 2MHz operation
combined with small package options,enables low cost,
area efficient solutions.
48V Boost Converter
applicaTions
n 3V to 40V Input Voltage Range
n Ultralow Quiescent Current and Low Ripple Burst
Mode
®
Operation: IQ= 6µA
n 1A, 60V Power Switch
n Positive or Negative Output Voltage Programming
with a Single Feedback Pin
n Fixed 2MHz Switching Frequency
n Accurate 1.6V EN/UVLO Pin Threshold
n Internal Compensation and Soft-Start
n Low Profile (1mm) ThinSOT™ Package
n Low Profile (0.75mm) 8-Lead (3mm × 2mm) DFN
Package
n Industrial and Automotive
n Telecom
n Medical Diagnostic Equipment
n Portable Electronics
L,LT,LTC ,LT M,Linear Technology,the Linear logo and Burst Mode are registered trademarks
and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
Efficiency and Power Loss
EFFICIENCY
POWER LOSS
LOAD CURRENT (mA)
0
40
80
120
160
0
10
20
30
40
50
60
70
80
90
100
0
100
200
300
400
500
600
700
800
900
1000
EFFICIENCY (%)
POWER LOSS (mW)
8330 TA01b
4.7µF
34.8k
4.7µF
1µF
4.7pF
V
IN
SW
FBX
GND
EN/UVLO
LT8330
VIN
12V
6.8µH
VOUT
48V
135mA
V
CC
INT
1M

LT8330
2
8330fa
For more information www.linear.com/LT8330
absoluTe MaxiMuM raTings
SW............................................................................60V
VIN, EN/UVLO............................................................40V
EN/UVLO Pin Above VIN Pin........................................6V
INTVCC (Note 2) ..........................................................4V
FBX ...........................................................................±4V
(Note 1)
orDer inForMaTion
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LT8330ES6#PBF LT8330ES6#TRPBF LTGMQ 6-Lead Plastic TSOT-23 –40°C to 125°C
LT8330IS6#PBF LT8330IS6#TRPBF LTGMQ 6-Lead Plastic TSOT-23 –40°C to 125°C
LT8330HS6#PBF LT8330HS6#TRPBF LTGMQ 6-Lead Plastic TSOT-23 –40°C to 150°C
LT8330EDDB#PBF LT8330EDDB#TRPBF LGRC 8-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C
LT8330IDDB#PBF LT8330IDDB#TRPBF LGRC 8-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C
LT8330HDDB#PBF LT8330HDDB#TRPBF LGRC 8-Lead (3mm × 2mm) Plastic DFN –40°C to 150°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on nonstandard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
TOP VIEW
9
DDB PACKAGE
8-LEAD (3mm ×2mm) PLASTIC DFN
5
6
7
8
4
3
2
1FBX
NC
SW
SW
EN/UVLO
INTVCC
VIN
GND
θJA = 60°C/W
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
1
2
3
6
5
4
TOP VIEW
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
VIN
INTVCC
EN/UVLO
SW
GND
FBX
θJA = 125°C/W, θJC = 102°C/W
pin conFiguraTion
Operating Junction Temperature (Note 3)
LT8330E, LT8330I............................. –40°C to 125°C
LT8330H............................................ –40°C to 150°C
Storage Temperature Range .................. –65°C to 150°C
(http://www.linear.com/product/LT8330#orderinfo)

LT8330
3
8330fa
For more information www.linear.com/LT8330
elecTrical characTerisTics
The ldenotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA= 25°C. VIN = 12V, EN/UVLO = 12V unless otherwise noted.
PARAMETER CONDITIONS MIN TYP MAX UNITS
VIN Operating Voltage Range l3 40 V
VIN Quiescent Current at Shutdown VEN/UVLO = 0.2V
l
0.9
2
2
5
µA
µA
VEN/UVLO = 1.5V
l
2
3.6
5
9.5
µA
µA
VIN Quiescent Current Sleep Mode, Not Switching
l
5.5
8.5
10
15
µA
µA
Active Mode, Not Switching
l
780
840
1100
1200
µA
µA
FBX Regulation
FBX Regulation Voltage FBX > 0V
FBX < 0V
l
l
1.568
–0.820
1.6
–0.80
1.632
–0.780
V
V
FBX Line Regulation FBX > 0V, 3V < VIN < 40V
FBX < 0V, 3V < VIN < 40V
0.005
0.005
0.015
0.015
%/V
%/V
FBX Pin Current FBX = 1.6V, –0.8V l–10 10 nA
Oscillator
Switching Frequency (fOSC) VIN = 24V l1.85 2.0 2.15 MHz
Minimum On-Time VIN = 24V 65 105 ns
Minimum Off-Time VIN = 24V 47 65 ns
Switch
Maximum Switch Current Limit Threshold l1.0 1.2 1.4 A
Switch RDS(ON) ISW = 0.5A 330 mΩ
Switch Leakage Current VSW = 60V 0.1 1 µA
EN/UVLO Logic
EN/UVLO Pin Threshold (Rising) Start Switching l1.620 1.68 1.745 V
EN/UVLO Pin Threshold (Falling) Stop Switching l1.556 1.60 1.644 V
EN/UVLO Pin Current VEN/UVLO = 1.6V l–40 40 nA
Soft-Start
Soft-Start Time VIN = 24V 1 ms
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: INTVCC cannot be externally driven. No additional components or
loading is allowed on this pin.
Note 3: The LT8330E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LT8330I is guaranteed over the full –40°C to 125°C operating junction
temperature range. The LT8330H is guaranteed over the full –40°C to
150°C operating junction temperature range. High junction temperatures
degrade operating lifetimes. Operating lifetime is derated at junction
temperatures greater than 125°C.
Note 4: The IC includes overtemperature protection that is intended to
protect the device during overload conditions. Junction temperature will
exceed 150°C when overtemperature protection is active. Continuous
operation above the specified maximum operating junction temperature
will reduce lifetime.

LT8330
4
8330fa
For more information www.linear.com/LT8330
Typical perForMance characTerisTics
Switching Frequency
vs Temperature Switching Frequency vs VIN
Normalized Switching Frequency
vs FBX Voltage
Switch Current Limit vs Duty
Cycle
Switch Minimum On-Time
vs Temperature
Switch Minimum Off-Time
vs Temperature
FBX Positive Regulation Voltage
vs Temperature
FBX Negative Regulation Voltage
vs Temperature
EN/UVLO Pin Thresholds
vs Temperature
V
IN
= 12V
JUNCTION TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
150
175
1.570
1.580
1.590
1.600
1.610
1.620
1.630
FBX VOLTAGE (V)
8330 G01
V
IN
= 12V
JUNCTION TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
150
175
–0.815
–0.810
–0.805
–0.800
–0.795
–0.790
–0.785
FBX VOLTAGE (V)
8330 G02
V
IN
= 24V
JUNCTION TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
150
175
1.90
1.92
1.94
1.96
1.98
2.00
2.02
2.04
2.06
2.08
2.10
SWITCHING FREQUENCY (MHz)
8330 G04
V
IN
(V)
0
5
10
15
20
25
30
35
40
45
1.85
1.90
1.95
2.00
2.05
2.10
2.15
SWITCHING FREQUENCY (MHz)
8330 G05
V
IN
= 24V
FBX VOLTAGE (V)
–0.8
–0.4
0.0
0.4
0.8
1.2
1.6
0
25
50
75
100
125
NORMALIZED SWITCHING FREQUENCY (%)
8330 G06
V
IN
= 12V
DUTY CYCLE (%)
0
20
40
60
80
100
1.00
1.10
1.20
1.30
1.40
SWITCH CURRENT LIMIT (A)
8330 G07
V
IN
= 24V
JUNCTION TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
150
175
30
40
50
60
70
80
90
100
MINIMUM ON–TIME (ns)
8330 G08
V
IN
= 24V
JUNCTION TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
150
175
30
35
40
45
50
55
60
MINIMUM OFF–TIME (ns)
8330 G09
V
IN
= 12V
EN/UVLO RISING (TURN-ON)
EN/UVLO FALLING (TURN-OFF)
JUNCTION TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
150
175
1.54
1.56
1.58
1.60
1.62
1.64
1.66
1.68
1.70
1.72
1.74
EN/UVLO PIN VOLTAGE (V)
8330 G03

LT8330
5
8330fa
For more information www.linear.com/LT8330
Typical perForMance characTerisTics
Switching Waveforms
(in CCM)
Switching Waveforms
(in DCM/Light Burst Mode)
Switching Waveforms
(in Deep Burst Mode)
VOUT Transient Response: Load
Current Transients from 67.5mA to
135mA to 67.5mA
VIN Pin Current (Sleep Mode, Not
Switching) vs Temperature
VIN Pin Current (Active Mode, Not
Switching) vs Temperature Burst Frequency vs Load Current
VOUT Transient Response: Load
Current Transients from 5mA to
135mA to 5mA
V
IN
= 12V
JUNCTION TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
150
175
0
1.25
2.50
3.75
5.00
6.25
7.50
8.75
10.00
V
IN
PIN CURRENT (µA)
8330 G10
V
IN
= 12V
JUNCTION TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
150
175
600
650
700
750
800
850
900
950
1000
V
IN
PIN CURRENT (µA)
8330 G11
V
IN
= 12V
V
OUT
= 48V
FRONT PAGE APPLICATION
LOAD CURRENT (mA)
0
10
20
30
40
50
0
0.5
1.0
1.5
2.0
2.5
SWITCHING FREQUENCY (MHz)
8330 G12
V
IN
= 12V, V
OUT
= 48V, I
LOAD
= 135mA
1µs/DIV
FRONT PAGE APPLICATION
V
SW
20V/DIV
IL
500mA/DIV
8330 G13
V
IN
= 12V, V
OUT
= 48V, I
LOAD
= 20mA
1µs/DIV
FRONT PAGE APPLICATION
V
SW
20V/DIV
8330 G14
IL
500mA/DIV
V
IN
= 12V, V
OUT
= 48V, I
LOAD
= 2mA
1µs/DIV
FRONT PAGE APPLICATION
V
SW
20V/DIV
8330 G15
IL
500mA/DIV
FRONT PAGE APPLICATION
V
IN
= 12V
V
OUT
= 48V
100µs/DIV
V
OUT
500mV/DIV
IL
100mA/DIV
8330 G16
FRONT PAGE APPLICATION
V
IN
= 12V
V
OUT
= 48V
100µs/DIV
V
OUT
500mV/DIV
IL
100mA/DIV
8330 G17

LT8330
6
8330fa
For more information www.linear.com/LT8330
pin FuncTions
EN/UVLO: Shutdown and Undervoltage Detect Pin.The
LT8330 is shut down when this pin is low and active
when this pin is high.Below an accurate 1.6V threshold
the part enters undervoltage lockout and stops switching.
This allows an undervoltage lockout (UVLO) threshold to
be programmed for system input voltage by resistively
dividing down system input voltage to the EN/UVLO pin.
An 80mV pin hysteresis ensures part switching resumes
when the pin exceeds 1.68V. EN/UVLO pin voltage below
0.2V reduces VIN current below 1µA. If shutdown and
UVLO features are not required,the pin can be tied directly
to system input.
FBX: Voltage Regulation Feedback Pin for Positive or
Negative Outputs.Connect this pin to a resistor divider
between the output and GND.FBX reduces the switching
frequency during start-up and fault conditions when FBX
is close to GND.
GND: Ground Connection for the LT8330. The DFN pack-
age has the best thermal performance due to an exposed
pad (Pin 9) on the bottom of the package.This exposed
pad must be soldered to a ground plane.Pin 5of the DFN
package (and Pin 2of the TSOT package)should also be
connected to a ground plane.The ground plane should be
connectedtolargecopperlayerstospreadheatdissipated
by the LT8330.
INTVCC:Regulated 3V Supply for Internal Loads.The
INTVCC pinmustbebypassedwithaminimum1µFlowESR
ceramic capacitor to ground.No additional components
or loading is allowed on this pin.
NC: No Internal Connection. Tie directly to local ground.
SW: The Output of Internal Power Switch.Minimize the
metal trace area connected to this pin to reduce EMI.
VIN: Input Supply.This pin must be locally bypassed.Be
sure to place the positive terminal of the input capacitor as
close as possible to the VIN pin,and the negative terminal
as close as possible to the GND pin.

LT8330
7
8330fa
For more information www.linear.com/LT8330
block DiagraM
GND
8330 BD
+
–
+
–
ERROR AMP
SELECT
FREQUENCY
FOLDBACK
INTVCC
UVLO
OSCILLATOR
2MHz
SWITCH
LOGIC
BURST
DETECT
A5
A2
A1
ERROR
AMP
ERROR
AMP
SLOPE
VC
SLOPE
SOFT-START
1.6V
FBX
VOUT
R2
R1
–0.8V
UVLO
–
+
–
+A3
+
–
A4
DRIVER
M1
ILIMIT
RSENSE
PWM COMPARATOR
INTVCC
TJ> 170°C
+
–
A6 1.68V(+)
1.6V(–)
EN/UVLO
INTERNAL
REFERENCE
UVLO
VIN
CIN
SW
R4
OPT R3 OPT
VIN
COUT
CVCC
D
L
VOUT
UVLO 3V REGULATOR
M2

LT8330
8
8330fa
For more information www.linear.com/LT8330
operaTion
The LT8330 uses a fixed frequency,current mode control
scheme to provide excellent line and load regulation.Op-
eration can be best understood by referring to the Block
Diagram.An internal 2MHz oscillator turns on the internal
power switch at the beginning of each clock cycle.Current
inthe inductor thenincreasesuntil the currentcomparator
trips and turns off the power switch.The peak inductor
current at which the switch turns off is controlled by the
voltageonthe internal VC node.The error amplifier servos
the VC node by comparing the voltage on the FBX pin with
an internal reference voltage (1.60V or –0.80V, depending
on the chosen topology). When the load current increases
it causes a reduction in the FBX pin voltage relative to
the internal reference.This causes the error amplifier to
increase the VC voltage until the new load current is satis-
fied. In this manner,the error amplifier sets the correct
peak switch current level to keep the output in regulation.
The LT8330 is capable of generating either a positive or
negative output voltage with a single FBX pin.It can be
configured as a boost or SEPIC converter to generate a
positive output voltage,or as an inverting converter to
generate a negative output voltage.When configured as
a boost converter,as shown in the Block Diagram,the
FBX pin is pulled up to the internal bias voltage of 1.60V
by a voltage divider (R1 and R2) connected from VOUT
to GND.Amplifier A2becomes inactive and amplifier A1
performs (inverting) amplification from FBX to VC.When
the LT8330 is in an inverting configuration,the FBX pin
is pulled down to –0.80V by a voltage divider from VOUT
to GND.Amplifier A1becomes inactive and amplifier A2
performs (non-inverting) amplification from FBX to VC.
If the EN/UVLO pin voltage is below 1.6V, the LT8330
entersundervoltagelockout (UVLO),and stopsswitching.
When the EN/UVLO pin voltage is above 1.68V (typical),
the LT8330 resumes switching.If the EN/UVLO pin volt-
age is below 0.2V, the LT8330 only draws 1µA from VIN.
To optimize efficiency at light loads,the LT8330 operates
in Burst Mode operation in light load situations.Between
bursts, all circuitry associated with controlling the output
switch is shut down,reducing the input supply current
to 6µA.
ACHIEVING ULTRALOW QUIESCENT CURRENT
To enhance efficiency at light loads the LT8330 uses
a low ripple Burst Mode architecture.This keeps the
output capacitor charged to the desired output voltage
while minimizing the input quiescent current and output
ripple.In Burst Mode operation the LT8330delivers single
small pulses of current to the output capacitor followed
by sleep periods where the output power is supplied by
the output capacitor.While in sleep mode the LT8330
consumes only 6µA.
As the output load decreases,the frequency of single cur-
rent pulses decreases (see Figure 1) and the percentage
of time the LT8330 is in sleep mode increases,resulting
in much higher light load efficiency than for typical con-
verters. To optimize the quiescent current performance
at light loads,the current in the feedback resistor divider
must be minimized as it appears to the output as load
current. In addition,all possible leakage currents from Figure 1. Burst Frequency vs Load Current
applicaTions inForMaTion
the output should also be minimized as they all add to the
equivalent output load.The largest contributor to leakage
current can be due to the reverse biased leakage of the
Schottky diode (see Diode Selection in the Applications
Information section).
V
IN
= 12V
V
OUT
= 48V
FRONT PAGE APPLICATION
LOAD CURRENT (mA)
0
10
20
30
40
50
0
0.5
1.0
1.5
2.0
2.5
SWITCHING FREQUENCY (MHz)
8330 F01

LT8330
9
8330fa
For more information www.linear.com/LT8330
applicaTions inForMaTion
While in Burst Mode operation the current limit of the
switch is approximately 240mA resulting in the output
voltage ripple shown in Figure 2. Increasing the output
capacitance will decrease the output ripple proportionally.
As the output load ramps upward from zero the switch-
ing frequency will increase but only up to the fixed 2MHz
defined by the internal oscillator as shown in Figure 1. The
output load at which the LT8330 reaches the fixed 2MHz
frequency varies based on input voltage,output voltage,
and inductor choice.
Figure 2. Burst Mode Operation
PROGRAMMING INPUT TURN-ON AND TURN-OFF
THRESHOLDS WITH EN/UVLO PIN
The EN/UVLO pin voltage controls whether the LT8330 is
enabled or is in a shutdown state.A 1.6V reference and a
comparatorA6withbuilt-inhysteresis(typical80mV)allow
the user to accurately program the system input voltage
at which the IC turns on and off (see the Block Diagram).
The typical input falling and rising threshold voltages can
be calculated by the following equations:
VIN(FALLING,UVLO(–)) = 1.60 •(R3+R4)/R4
VIN(RISING, UVLO(+)) = 1.68 •(R3+R4)/R4
VIN current is reduced below 1µA when the EN/UVLO pin
voltage is less than 0.2V. The EN/UVLO pin can be con-
nected directly to the input supply VIN for always-enabled
operation. A logic input can also control the EN/UVLO pin.
When operating in Burst Mode operation for light load
currents, the current through the R3and R4network can
easily be greater than the supply current consumed by the
LT8330. Therefore,R3and R4should be large enough to
minimize their effect on efficiency at light loads.
INTVCC REGULATOR
A low dropout (LDO) linear regulator,supplied from VIN,
produces a 3V supply at the INTVCC pin.A minimum 1µF
low ESR ceramic capacitor must be used to bypass the
INTVCC pin to ground to supply the high transient currents
required by the internal power MOSFET gate driver.
No additional components or loading is allowed on this
pin. The INTVCC rising threshold (to allow soft start and
switching) is typically 2.6V. The INTVCC falling threshold
(to stop switching and reset soft start) is typically 2.5V.
DUTY CYCLE CONSIDERATION
The LT8330 minimum on-time,minimum off-time and
switching frequency (fOSC)define the allowable minimum
and maximum duty cycles of the converter (see Minimum
On-Time, Minimum Off-Time,and Switching Frequency
in the Electrical Characteristics table).
Minimum Allowable Duty Cycle =
Minimum On-Time(MAX) •fOSC(MAX)
Maximum Allowable Duty Cycle =
1 – Minimum Off-Time(MAX) •fOSC(MAX)
Therequired switch duty cycle range for a Boost converter
operating in continuous conduction mode (CCM) can be
calculated as:
DMIN = 1– VIN(MAX)/(VOUT + VD)
DMAX = 1– VIN(MIN)/(VOUT + VD)
where VDis the diode forward voltage drop. If the above
duty cycle calculations for a given application violate the
minimum and/or maximum allowed duty cycles for the
LT8330, operation in discontinuous conduction mode
(DCM) might provide a solution.For the same VIN and
VOUT levels,operation in DCM does not demand as low a
duty cycle as in CCM.DCM also allows higher duty cycle
operation than CCM.The additional advantage of DCM is
the removal of the limitations to inductor value and duty
cycle required to avoid sub-harmonic oscillations and the
right half plane zero (RHPZ). While DCM provides these
benefits,thetrade-offishigherinductorpeakcurrent,lower
available output power and reduced efficiency.
5µs/DIV
V
OUT
5mV/DIV
I
L
200mA/DIV
8330 F02

LT8330
10
8330fa
For more information www.linear.com/LT8330
applicaTions inForMaTion
SETTING THE OUTPUT VOLTAGE
The output voltage is programmed with a resistor divider
from the output to the FBX pin.Choose the resistor values
for a positive output voltage according to:
R1 = R2 •(VOUT/1.60V – 1)
Choose the resistor values for a negative output voltage
according to:
R1 = R2 •(|VOUT|/0.80V – 1)
The locations of R1and R2are shown in the Block Dia-
gram. 1% resistors are recommended to maintain output
voltage accuracy.
Higher-valueFBXdividerresistorsresultinthelowestinput
quiescent current and highest light-load efficiency.FBX
divider resistors R1and R2are usually in the range from
25k to 1M. Most applications use a phase-lead capacitor
from VOUT to FBX in combination with high-value FBX
divider resistors (see Compensation in the Applications
Information section).
SOFT-START
The LT8330 contains several features to limit peak switch
currents and output voltage (VOUT)overshoot during
start-up or recovery from a fault condition.The primary
purpose of these features is to prevent damage to external
components or the load.
High peak switch currents during start-up may occur in
switching regulators.Since VOUT is far from its final value,
the feedback loop is saturated and the regulator tries to
chargetheoutputcapacitorasquicklyaspossible,resulting
in large peak currents.A large surge current may cause
inductor saturation or power switch failure.
The LT8330 addresses this mechanism with an internal
soft-start function.As shown in the Block Diagram,the
soft-start function controls the ramp of the power switch
current by controlling the ramp of VC through M2. This
allowstheoutputcapacitortobechargedgraduallytoward
its final value while limiting the start-up peak currents.
Figure 3 shows the output voltage and supply current for
the first page Typical Application.It can be seen that both
the output voltage and supply current come up gradually.
Figure 3. Soft-Start Waveforms
INTVCC undervoltage (INTVCC < 2.5V) and/or thermal
lockout (TJ> 170°C) will immediately prevent switching,
will reset the internal soft-start function and will pull down
VC. Once all faults are removed,the LT8330 will soft-start
VC and hence inductor peak current.
FREQUENCY FOLDBACK
During start-up or fault conditions in which VOUT is very
low, extremely small duty cycles may be required to
maintain control of inductor peak current. The minimum
on-timelimitationofthepowerswitchmightpreventthese
low duty cycles from being achievable.In this scenario
inductorcurrentrisewillexceedinductorcurrentfallduring
each cycle,causing inductor current to ‘walk up’beyond
the switch current limit.The LT8330 provides protection
from this by folding back switching frequency whenever
FBX pin is close to GND (low VOUT levels). This frequency
foldbackprovidesalargerswitch-offtime,allowinginductor
current to fall enough each cycle (see Normalized Switch-
ing Frequency vs FBX Voltage in the Typical Performance
Characteristics section).
THERMAL LOCKOUT
If the LT8330 die temperature reaches 170°C (typical),
the part will stop switching and go into thermal lockout.
When the die temperature has dropped by 5°C (nominal),
the part will resume switching with a soft-started inductor
peak current.
500µs/DIV
IL
500mA/DIV
V
OUT
20V/DIV
8330 F03