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GM1937
WHITE LED STEP-UP CONVERTER
Description
The GM1937 is a step-up DC/DC converter specifically designed to drive white LEDs with a constant current. The device can drive two, three or four LEDs in series from a Li-Ion cell. Series connection of the LEDs provides identical LED currents resulting in uniform brightness and eliminating the need for ballast resistors. The output capacitor can be as small as 0.22F, saving space versus alternative solutions. A low 95mV feedback voltage minimizes power loss for better
Features
High Efficiency: 84% Typical 36V internal Switch Fast 1.2MHz Switching Frequency Drives Up to three LEDs from a 2.8V Supply Drives Up to six LEDs from a 5V Supply External Matched LED Current Uses Tiny 1mm height inductor Requires Only 0.22 @ F Output Capacitor Low profile TSOT package
Application
Cellular Phones PDA, Handheld Computers Digital Cameras MPS3 Players GPS Receivers
Typical Application Circuits
VIN 3V to 5V 1F VIN 22H
90 EFFICIENCY (%)
SW 0.22F
85 80 75 70 65 60 0 5 10 15 LED CURRENT (mA) 20
VIN = 3.0V VIN = 3.6V
OFF ON
GM1937 FB CE GND R1 6.34
Figure 1 Li-Ion Powered Driver for Three White LEDs
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GM1937V2.00
GM1937
WHITE LED STEP-UP CONVERTER
Marking Information and Pin Configurations (Top View)
SOT25
VCC 5 CE 4
WLYWA
1 SW 2 GND 3 FB
WL: Device Code, Green Product Y: Year W: Week code A: Assembly/testing site
Pin Descriptions
Pin Number
1 2 3
Pin Name
SW GND FB Ground
Pin Function
Switch Pin, (Minimize trace area at this pin to reduce EMI.)
Feedback Pin. Reference voltage is 95 mV. (Calculate resistor value according to the formula RFB=95 mV / ILED .) Shutdown Pin (Connect to 1.5 V or higher to enable device; 0.4 V or less to disable device) Input Supply Pin. (Must be locally bypassed)
4 5
CE VCC
Order Information
Ordering Number GM1937ST25RG Package SOT-25 Shipping 3,000 Units/Tape and Reel
GM1937V2.00
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GM1937
WHITE LED STEP-UP CONVERTER
Absolute Maximum Ratings
PARAMETER Input Voltage SW Voltage FB Voltage CE Voltage Continuous Total Power Dissipation Operating Ambient Temperature Storage Temperature Lead Temperature (Soldering, 10 sec) SOT-25 PD TA Tstg SYMBOL VIN RATINGS 10 36 10 10 300 - 40 to 85 - 65 to 150 + 300 UNITS V V V V mW J J J
Block Diagram
VIN 5 FB 3 COMPARATOR VREF 1.25V 95mV RC CC 0.2 R S Q SW 1
DRIVER Q1
RAMP GENERATOR
CE 4 SHUTDWON
1.2MHz OSCILATOR
2 GND
Figure 2
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GM1937V2.00
GM1937
WHITE LED STEP-UP CONVERTER
Electrical Characteristics (TA = 25J
Parameter Feeback Voltage FB Pin Bias Current Supply Current Switching Frequency Maximum Duty Cycle Switch Current Limit Switch VCESAT Switch Leakage current CE Voltage High CE Voltage Low CE Pin Bias Current ICE Symbol VFB IFB ICC(ON) ICC(OFF) fSW DMAX ISW VSAT ILEAK VCE = 0V
, Vcc = 3V, VCE = 3V, unless otherwise noted) Condition Min 86 10 Typ 95 45 1.9 0.1 0.8 85 1.2 90 320 350 0.00 1 1.5 0.4 65 Max 104 100 2.5 1.0 1.6 Unit mV nA mA A MHz % mA mV 5 A V V A
ISW = 100mA, Duty Cycle = 66%
ISW = 250mA VSW = 5V
GM1937V2.00
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GM1937
WHITE LED STEP-UP CONVERTER
Application Information Operation
GM1937 is designed with a constant frequency, current mode control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the block diagram in Figure 2. At the start of each oscillator cycle, the SR latch is set, which turns on the power switch Q1. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator A2. When this voltage exceeds the level at the negative input of A2, the SR latch is reset turning off the power switch. The level at the negative input of A2 is set by the error amplifier A1, and is simply an amplified version of the difference between the feedback voltage and the reference voltage of 95mV. In this manner, the error amplifier sets the correct peak current level to keep the output in regulation. If the error amplifier's output increases, more current is delivered to the output; if it decreases, less current is delivered.
Minimum Output Current
GM 1937 can regulate three series LEDs connected a low output current. down to approximately 4mA from a 4.2V supply, without pulse skipping, using the same external components as specified for 15mA operation. As current is further reduced, the device will begin skipping pulses.This will result in some low frequency ripple, although the LED current remains regulated on an average basis down to zero.
Inductor Selection
A 22mH inductor is recommended for most GM1937 applications. Although small size and high efficiency are major concerns, the inductor should have low core losses at 1.2MHz and low DCR (copper wire resistance). Some inductors in this category with small size are listed in Table 1. Table 1. Recommended Inductors
Part Number LQH3C220 ELJTC220KF CDRH3D16-220 LB2023B220M LEM2520-220 DCR () 0.71 4.0 0.53 1.7 5.5 Current Rating (mA) 250 160 350 75 125 Manufacturer Murata Panasonic Sumida Taiyo Yuden Taiyo Yuden
Capacitor Selection
The small size of ceramic capacitors makes them ideal for GM1937 applications. X5R and X7R types are recommended because they retain their capacitance over wider voltage and temperature ranges than other types such as Y5V or Z5U. A 1F input capacitor and a 0.22F output capacitor are sufficient for most GM1937 applications.
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GM1937V2.00
GM1937
WHITE LED STEP-UP CONVERTER
Application Information - Cont'd Diode Selection
Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for GM1937 applications. The forward voltage drop of a Schottky diode represents the conduction losses in the diode, while the diode capacitance (CT or CD) represents the switching losses. For diode selection, both forward voltage drop and diode capacitance need to be considered. Schottky diodes with higher current ratings usually have lower forward voltage drop and larger diode capacitance, which can cause significant switching losses at the 1.2MHz switching frequency of the LT1937K. A Schottky diode rated at 100mA to 200mA is sufficient for most GM1937 applications. Some recommended Schottky diodes are listed in Table 2. Table 2. Recommended Schottky Diodes
Part Number CMDSH-3 CMDSH2-3 BAT54 Forward Current (mA) 100 200 200 Voltage Drop (V) 0.58 @ 100mA 0.49 @ 200mA 0.53 @ 100mA Diode Capacitance (pF) 7.0 @10V 15 @10V 10 @25V Manufacturer Central Central Zetex
LED Current Control
The LED current is controlled by the feedback resistor (R1 in Figure 1). The feedback reference is 95mV. The LED current is 95mV/R1. In order to have accurate LED current, precision resistors are preferred (1% is recommended). The formula and table for R1 selection are shown below.R1 = 95mV/ILED Table 3. R1 Resistor Value Selection
ILED (mA) 5 10 12 15 20 R1 () 19.1 9.53 7.87 6.34 4.75
Open-Circuit Protection
In the cases of output open circuit, when the LEDs are disconnected from the circuit or the LEDs fail, the feedback voltage will be zero. The GM1937 will then switch at a high duty cycle resulting in a high output voltage, which may cause the SW pin voltage to exceed its maximum 36V rating. A zener diode can be used at the output to limit the voltage on the SW pin (Figure 3). The zener voltage should be larger than the maximum forward voltage of the LED string. The current rating of the zener should be larger than L 0.1mA.
22H D VIN
GM1937V2.00
CIN 1F
VIN GM1937
SW R2 1K R1 6.34
COUT 0.22F
FB CE GND
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Figure 3. LED Driver with Open-Circuit Protection
GM1937
WHITE LED STEP-UP CONVERTER
Application Information - Cont'd Dimming Control
There are four different types of dimming control: 1. Using a PWM Signal to SHDN Pin With the PWM signal applied to the CE pin, the GM1937 is turned on or off by the PWM signal. The LEDs operate at either zero or full current. The average LED current increases proportionally with the duty cycle of the PWM signal. A 0% duty cycle will turn off the GM1937 and corresponds to zero LED current. A 100% duty cycle corresponds to full current. The typical frequency range of the PWM signal is 1kHz to 10kHz. The magnitude of the PWM signal should be higher than the minimum CE voltage high. 2. Using a DC Voltage For some applications, the preferred method of brightness control is a variable DC voltage to adjust the LED current. The dimming control using a DC voltage is shown in Figure 4. As the DC voltage increases, the voltage drop on R2 increases and the voltage drop on R1 decreases. Thus, the LED current decreases. The selection of R2 and R3 will make the current from the variable DC source much smaller than the LED current and much larger than the FB pin bias current. For VDC range from 0V to 2V, the selection of resistors in Figure 4 gives dimming control of LED current from 0mA to 15mA.
GM1937 R3 90K FB R2 5K R1 6.34 R3 90K 0.1F
GM1937 FB R2 5K R1 6.34
VDC
PWM
10K
Figure 4. Dimming Control Using a DC Voltage
Figure 5. Dimming Control Using a Filtered PWM Signal
3. Using a Filtered PWM Signal The filtered PWM signal can be considered as an adjustable DC voltage. It can be used to replace the variable DC voltage source in dimming control. The circuit is shown in Figure 5. 4. Using a Logic Signal For applications that need to adjust the LED current is discrete steps, a logic signal can be used as shown in Figure 6. R1 sets the minimum LED current (when the NMOS is OFF). RINC sets how much the LED current increases when the NMOS is turned on. The selection of R1 and RINC can be found in table 3
GM1937 FB RINC
Logic Signal
R1
Figure 5. Dimming Control Using a Logic Signal
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GM1937V2.00
GM1937
WHITE LED STEP-UP CONVERTER
Package Outline Dimensions - SOT 25
2.9 0.1 1.9 0.05 0.95 0.038 0.13
Pad Layout
0~0.1 2.8 0.1 1.5 0.05
0.028 0.7 0.039 1.0
0.65 0.05
0.094 2.4
0.35 0.03
0.8 0.05
0.037 0.95 0.037 0.95 0.074 1.9
5X
5X
(Inches ) mm
1.10 0.1
Unit:mm
GM1937V2.00
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GM1937
WHITE LED STEP-UP CONVERTER
Ordering Number
GM 1937 ST25 R
APM Gamma Micro Circuit Type Package Type ST25: SOT 25 Shipping Type
G
Blank: Pb-free G:Green
R: Tape & Reel
Note: Pb-free products: RoHS compliant and compatible with the current require-ments of IPC/JEDEC J-STD-020. Suitable for use in SnPb or Pb-free soldering processes with 100% matte tin (Sn) plating. Lead-free (RoHS compliant) Halogen free(Br or Cl does not exceed 900ppm by weight in homogeneous material and total of Br and Cl does not exceed 1500ppm by weight)
Green products:
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GM1937V2.00

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