Sipex SP7651 User manual

Sept12-06 SP7651 Evaluation Manual ©2006 Sipex Corporation
SP7651EB SCHEMATIC
Easy Evaluation for the SP7651ER
12V Input, 0 to 3A Output Synchronous
Buck Converter
Built in Low R DS ( ON ) Power FETs
UVLO Detects Both Vcc and V IN
Highly Integrated Design, Minimal
Components
High Efficiency: 88%
Feature Rich: UV IN , Programmable
Softstart, External Vcc Supply and
Output Dead Short Circuit Shutdown
Protection
Solved by
TM
SP7651
Evaluation Board Manual

Sept12-06 SP7651 Evaluation Manual ©2006 Sipex Corporation
Page 2 of 8
USING THE EVALUATION BOARD
1) Powering Up the SP7651EB Circuit
Connect the SP7651 Evaluation Board with an external +12V power supply. Connect
with short leads and large diameter wire directly to the “VIN” and “GND” posts. Connect
a Load between the VOUT and GND2 posts, again using short leads with large
diameter wire to minimize inductance and voltage drops.
2) Measuring Output Load Characteristics
V OUT ripple can best be seen touching the probe tip to the pad for C3 and the scope
GND collar touching the GND side of C3 using short wrapped wire around the collar –
avoid a GND lead on the scope which will increase noise pickup.
3) Using the Evaluation Board with Different Output Voltages
While the SP7651 Evaluation Board has been tested and delivered with the output set
to 3.30V, by simply changing one resistor, R2, the SP7651 can be set to other output
voltages. The relationship in the following formula is based on a voltage divider from the
output to the feedback pin V FB , which is set to an internal reference voltage of 0.80V.
Standard 1% metal film resistors of surface mount size 0603 are recommended.
V OUT = 0.80V (R1 / R2 + 1) => R2 = R1 / [ ( Vout / 0.80V ) – 1 ]
Where R1 = 68.1KΩand for V OUT = 0.80V setting, simply remove R2 from the board.
Furthermore, one could select the value of the R1 & R2 combination to meet the exact
output voltage setting by restricting R1 resistance range such that 50KΩ≤R1 ≤100KΩ
for overall system loop stability.
Note that since the SP7651 Evaluation Board design was optimized for 12V down
conversion to 3.30V, changes of output voltage and/or input voltage will alter
performance from the data given in the Power Supply Data section. In addition, the
SP7651ER provides short circuit protection by sensing V OUT at GND.
POWER SUPPLY DATA
The SP7651ER is designed with a very accurate 1.0% reference over line, load and
temperature. Figure 1 data shows a typical SP7655 Evaluation Board Efficiency plot,
with efficiencies to 87% (including generation of 5V Vcc) and output currents to 3A.
SP7651ER Load Regulation is shown in Figure 2 to have only 0.5% change in output
voltage from 0.5A load to 3A load. Figures 3 and 4 illustrate a 1.5A to 3A and 0A to 3A
Load Step. Start-up Responses in Figures 5, 6 and 7 show a controlled start-up with
different output load behavior when power is applied where the input current rises
smoothly as the Softstart ramp increases. In Figure 8 the SP7651ER is configured for
hiccup mode in response to an output dead short circuit condition and will Soft-start until
the over-load is removed. Figure 9 and 10 show output voltage ripple less than 25mV at
no load to 3A load.
While data on individual power supply boards may vary, the capability of the SP7651ER
of achieving high accuracy over a range of load conditions shown here is quite
impressive and desirable for accurate power supply design.

Sept12-06 SP7651 Evaluation Manual ©2006 Sipex Corporation
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Efficiency vs Load Current
40
50
60
70
80
90
100
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Load current (A)
Efficiency (%)
Output Voltage vs Load Current
3.280
3.290
3.300
3.310
3.320
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Load current (A)
Output Voltage (V)
Figure 1. Efficiency vs Load Figure 2. Load Regulation
Figure 3. Load Step Response: 1.5->3A Figure 4. Load Step Response: 0->3A
Figure 5. Start-Up Response: No Load Figure 6. Start-Up Response: 3A Load
Vout
Vin
SoftStart
Ichoke (0.5A/div)
Iout (2A/div)
Vout
Iout (1A/div)
Vout
Vout
Vin
SoftStart
Ichoke (2A/div)
Vi=12V
Vo=3.3V
Vi=12V
Vo=3.3V
Vi=12V
Vo=3.3V
Vi=12V
Vo=3.3V
Vi=12V
Vo=3.3V
Vi=12V
Vo=3.3V

Sept12-06 SP7651 Evaluation Manual ©2006 Sipex Corporation
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Figure 7. Output Load Short Circuit Figure 8. Output Load Short Circuit (Zoom-in)
Figure 9. Output Ripple: No Load Figure 10. Output Ripple:3A Load
Ichoke (2A/div)
Vout ripple = 10mV
Ichoke (1A/div)
Vout ripple = 10mV
SoftStart
Vo
ut
Ichoke
25A/div
SoftStart
Vout
Ichoke
25A/div
Vi=12V
Vo=3.3V
Vi=12V
Vo=3.3V
Vi=12V
Vo=3.3V
Vi=12V
Vo=3.3V

Sept12-06 SP7651 Evaluation Manual ©2006 Sipex Corporation
Page 5 of 8
TYPE III LOOP COMPENSATION DESIGN
The open loop gain of the SP7651EB can be divided into the gain of the error amplifier
Gamp(s), PWM modulator Gpwm, buck converter output stage Gout(s), and feedback
resistor divider Gfbk. In order to cross over at the selecting frequency fco, the gain of
the error amplifier must compensate for the attenuation caused by the rest of the loop at
this frequency. The goal of loop compensation is to manipulate the open loop frequency
response such that its gain crosses over 0dB at a slope of –20dB/dec. The open loop
crossover frequency should be higher than the ESR zero of the output capacitors but
less than 1/5 to 1/10 of the switching frequency fs to insure proper operation. Since the
SP7651EB is designed with Ceramic Type output capacitors, a Type III compensation
circuit is required to give a phase boost of 180°in order to counteract the effects of the
output LC underdamped resonance double pole frequency.
(SRz2Cz2+1)(SR1Cz
3+1)
SR1Cz2(SRz3Cz3+1)(SRz2C
p1+1)
Vin
Vramp_p
p
(SResrCout+
1)
[S^2LCout+S(Resr+Rdc)Co
ut+1]
(R1+R2
)
R2
OR
Vout
Vref
Vout
(Volts
)
Vref
(Volts
)
Type II
I Voltage
Loop
(Volts
)
Vfbk
Gout(S)
Gain Block
Compensation Gamp(S)
PWM Stage
Output Stage
Gain Block
Gain Block
Gpwm
Gain Block
Gfbk
Voltage Feedback
Resr := Output Capacitor Equivalent Series Resitance
Definition
s:
Rdc := Output
Inductor DC Resistance
Vramp_pp := SP7651 Internal RAMP Amplitude Peak to Peak Voltage
Output Load Resistance >> Resr and Rdc
Cz2 >> Cp1 and R1 >> Rz3
Condition
s:
Figure 11. Voltage Mode Control Loop with Loop Dynamic for Type III Compensation

Sept12-06 SP7651 Evaluation Manual ©2006 Sipex Corporation
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The simple guidelines for positioning the poles and zeros and for calculating the
component values for Type III compensation are as follows:
a. Choose fco = fs / 5
b. Calculate fp_LC
fp_LC = 1 / [2π(L ●C) ^ 1/2 ]
c. Calculate fz_ESR
fz_ESR = 1 / 2π(R ESR ) ●(C OUT )
d. Select R1 component value such that 50kΩ≤R1 ≤100kΩ
e. Calculate R2 base on the desired V OUT
R2 = R1 / [(V OUT / 0.80V) – 1]
f. Select the ratio of Rz2 / R1 gain for the desired gain bandwidth
Rz2 = R1 (V RAMP _ PP / V IN _ MAX ) (fco / fp_LC)
g. Calculate Cz2 by placing the zero at ½ of the output filter pole frequency
Cz2 = 1 / [π(Rz2) ●(fp_LC) ]
h. Calculate Cp1 by placing the first pole at ESR zero frequency
Cp1 = 1 / [2π(Rz2) ●(fz_ESR) ]
i. Calculate Rz3 by setting the second pole at ½ of the switching frequency and the
second zero at the output filter double pole frequency
Rz3 = 2 (R1) (fp_LC) / fs
j. Calculate Cz3 from Rz3 component value above
Cz3 = 1 / π(Rz3) ●(fs)
k. Choose 100pF ≤Cf1 ≤220pF to stabilize the SP7651ER internal Error Amplifier
Note: Loop Compensation component calculations discussed in this section are
further elaborated in the application note #ANP16, “Loop Compensation of
Voltage-Mode Buck Converters”.
These calculations shown here can be quickly iterated with the Type III Loop
Compensation Calculator on the web at:
www.sipex.com/files/Application-Notes/TypeIIICalculator.xls

Sept12-06 SP7651 Evaluation Manual ©2006 Sipex Corporation
Page 7 of 8
PCB LAYOUT DRAWINGS
Figure 12. SP7651EB Layout Top Side & Component Placement
Figure 13. SP7651EB PC Layout Bottom Side & Component Placement
Figure 14. SP7651EB PC Layout Inner Layer 1
Figure 15. SP7651EB PC Layout Inner Layer 2

Sept12-06 SP7651 Evaluation Manual ©2006 Sipex Corporation
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Table 1: SP7651EB List of Materials
REF. DES. QTY
MANUFACTURER
NAME Manufacturer Part Number SIZE DESCRIPTION
U1 1 SIPEX SP7651ER DFN-26 2-FETs Buck Ctrl
U2 1 Sipex SPX5205M5-5.0
SOT-23-5
150mA LDO Voltage Regulator
DBST 1 Diodes Inc. SD101AWS-7
SOD-323
Schottky Diode, 15mA
L1 1 Intertechnical SC7232-2R2M 7 X 7mm Inductor, 2.2uH, 8A, 10.4mohm
C3 1 TDK CORPORATION C3225X5R0J226M 1210 Capacitor, Ceramic, 22uF, 6.3V, X5R, 20%
C4 1 TDK CORPORATION C3225X5R1C226M 1210 Capacitor, Ceramic, 22uF, 16V, X5R, 20%
CVCC 1 TDK CORPORATION C1608X5R1A225K 0603 Capacitor, Ceramic, 2.2uF, 10V, X5R, 10%
C2 1 TDK CORPORATION C1608X7R1H104K 0603 Capacitor, Ceramic, 0.1uF, 50V, X7R, 10%
CBST 1 AVX CORPORATION 06035C682KAT2A 0603 Capacitor, Ceramic, 6.8nF, 50V, X7R, 10%
CSS 1 ROHM MCH185CN153KK 0603 Capacitor, Ceramic, 15nF, 50V, X7R, 10%
CP1 1 AVX CORPORATION 06035A100JAT2A 0603 Capacitor, Ceramic, 10pF, 50V, C0G, 5%
CZ2 1 AVX CORPORATION 06035A391JAT2A 0603 Capacitor, Ceramic, 390pF, 50V, C0G, 5%
CZ3 1 AVX CORPORATION 06035A101JAT2A 0603 Capacitor, Ceramic, 100pF, 50V, C0G, 5%
CF1 1 ROHM MCH185A101JK 0603 Capacitor, Ceramic, 100pF, 50V, C0G, 5%
RZ2 1 VISHAY DALE CRCW0603-1502FRT1 0603 Resistor, 15K, 1/16W, 1%
R2 1 SEI ELECTRONICS RMC-1/16W-21.5K-1%-TR 0603 Resistor, 21.5K, 1/16W, 1%
RZ3 1 VISHAY DALE CRCW0603-2001FRT1 0603 Resistor, 2K, 1/16W, 1%
R1 1 VISHAY DALE CRCW0603-6812FRT1 0603 Resistor, 68.1K, 1/16W, 1%
R3 1 VISHAY DALE CRCW0603-2003FRT1 0603 Resistor, 200K, 1/16W, 1%
R4 1 VISHAY DALE CRCW0603-1003FRT1 0603 Resistor, 100K, 1/16W, 1%
RBST 1 ROHM MCR03EZPEFX20R0 0603 Resistor, 20, 1/16W, 1%
VIN,VOUT,
GND,GND2
4 Vector Electronic K24C/M .042 Dia Input/Output Terminal Posts
ORDERING INFORMATION
Model Temperature Range Package Type
SP7651EB…................................-40°C to +85°C...............…SP7651 Evaluation Board
SP7651ER..............................…. -40°C to +85°C.................................……26-pin DFN