ISSI IS31LT3948 User manual

IS31LT3948 MR16 DEMO Board Guide
Integrated Silicon Solution, Inc. – www.issi.com 1
R1.0, 18/12/2012
Description
IS31LT3948 is a PFM step-up DC-DC converter
designed for driving the white LED arrays for large size
LCD panel backlighting applications. With internal OVP
circuit, the chip and the system can be safe even if the
load is not connected. The device features external
PWM dimming or DC dimming, which allows the
flexible control of the backlighting luminance.
IS31LT3948 incorporates a unique FB scheme which
automatically adjusts the integrated DC/DC converter
to the optimum output voltage for the system,
maximizing the efficiency.
IS31LT3948 general DEMO board is used in general
lamp.
Features
Wide input voltage range: 5V-100V
Constant Current Output limited only by external
component selection (Note)
No loop compensation required
Internal over-voltage protection
Internal over-temperature protection
Note: The maximum output current is determined by Vout/Vin
ratio as well as the external components. If output current and
Vout/Vin ratio is high, high current components of inductor
and NMOS are needed.
Order information
Part Number Package Type
IS31LT3948-GRLS2-EBMR16 SOP-8
Quick Start
Recommended Equipment
30VDC Power supply
LED panel (1W LED, 7 LEDs in series)
Multi-meter
Recommended Input and Output Ratings
Input: 10-17VDC
Output: 7LEDs in series/333mA
Note: The input voltage must be lower than the output
voltage(total Vf).
Absolute Maximum Ratings
Input voltage ≤ 35VDC
Output voltage≤ 28VDC
Caution: Do not exceed the conditions listed above, otherwise
the board will be damaged or the output current will be
limited.
Procedure
The IS31LT3948 DEMO Board is fully assembled and
tested. Follow the steps listed below to verify board
operation.
Caution: Do not turn on the power supply until all connections
are completed.
1) Connect the negative terminal of the power supply
to the VIN1 pin and the positive terminal to the
VIN2 pin.
2) Connect the negative of the LED panel (LED
arrays) to the LED- terminal.
3) Connect the positive of the LED panel (LED arrays)
to the LED+ terminal.
4) Turn on the power supply and the LED panel (LED
arrays) will be light.
Bill of Materials
No.
Name
Description
Ref Des.
Qty.
Mfr
1
AL Capacitor
470uF±10%,35V
C1
1
2
AL Capacitor
220uF±10%,50V,Low ESR
C3
1
3
SMD Capacitor
10uF±10%,50V
C4
1
4
SMD Resistor
2.4KΩ±1%,1206
R1
1
5
SMD Resistor
24KΩ±1%,0805
R4
1
6
SMD Resistor
270KΩ±1%,0805
R3
1
7
SMD Resistor
10KΩ±1%,0805
R7
1
8
SMD Resistor
0.21Ω±1%,0805
R5
1
9
SMD Resistor
0.9Ω±1%,0805
R8
1
10
Schottky Diode
SS36,3A,60V,SMA
D1-D5
5
11
Diode
1N4007,1A,1000V,SMA
Ds
1
12
SMD Inductor
100uH,Isat≥3A
L1
1

IS31LT3948 MR16 DEMO Board Guide
Integrated Silicon Solution, Inc. – www.issi.com 2
R1.0, 18/12/2012
13
MOS
3A,60V,AP2310,SOT-23,NMOS
T1
1
14
IC
IS31LT3948,SOP8
U1
1
Figure 1 IS31LT3948 MR16 DEMO Board Schematic
Detailed Description
Component Selection
The component selection is very important. They have
a significant effect on the operating state of the demo
board. The output capacitor must be a low ESR
capacitor so as to minimize it’s affect on the line
regulation and load regulation.
Please read the datasheet carefully to get more
information about the component selection.
PCB layout consideration
As for all switching power supplies, especially those
providing high current and using high switching
frequencies, layout is an important design step. If layout
is not carefully done, the regulator could show
instability as well as EMI problems.
•Wide traces should be used for connection of the
high current loop to minimize the EMI and
unnecessary loss.
•The external components ground should be
connected to IS31LT3948 ground as short as
possible. Especially theRcs and Rfb ground to
IS31LT3948 ground connection should be as short
and wide as possible to have an accurate LED
current.
•The capacitor C1, C2, C3 should be placed as close
as possible to IS31LT3948 for good filtering.
Especially the output capacitor C3 connection should
be as short and wide as possible.
•NMOS drain is a fast switching node. The inductor
and Schottky diode should be placed as close as
possible to the drain and the connection should be
kept as short and wide as possible. Avoid other
traces crossing and routing too long in parallel with
this node to minimize the noise coupling into these
traces. The feedback pin (e.g. CS, FB, OVP) should
be as short as possible and routed away from the
inductor, the schottky diode and NMOS. The
feedback pin and feedback network should be
shielded with a ground plane or trace to minimize
noise coupling into this circuit.
The thermal pad on the back of NMOS package must
be soldered to the large ground plane for ideal power
dissipation.

IS31LT3948 MR16 DEMO Board Guide
Integrated Silicon Solution, Inc. – www.issi.com 3
R1.0, 18/12/2012
PCB Layout
Figure 2 Board PCB Layout- Top Layer Figure 3 Board PCB Layout-Bottom Layer
Figure 4 Component Placement Guide -Top Layer Figure 5 Component Placement Guide -Bottom Layer
Revision history
Date Revision Changes
2012.03.06 R1.0 Initial release
2012.04.19 R1.1
1. Recommend output 1-7LEDs change to 7LEDs
2. BOM: Add tolerance to cap.
3. BOM: R1 3KΩchange to 2.4KΩ.
4. BOM: L1 Isat>2.5A change to Isat>3A.
5. BOM: MOS 13N06L change to AP2310.
6. Correct the schematic with ISSI part number.
7. Add order information.
8. Change EVB photo.
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products at any time without notice. ISSI assumes no liability arising out of the application or use of any information, products or services
described herein. Customers are advised to obtain the latest version of this device specification before relying on any published information and
before placing orders for products.