GM1937 Просмотр технического описания (PDF) - Gamma Microelectronics Inc.

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GM1937

WHITE LED STEP-UP CONVERTER

Gamma Microelectronics Inc. 

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

Forward

Current (mA)

Voltage Drop (V)

CMDSH-3

100

0.58 @ 100mA

Diode

Capacitance

(pF)

7.0 @10V

Manufacturer

Central

CMDSH2-3

200

0.49 @ 200mA 15 @10V

Central

BAT54

200

0.53 @ 100mA 10 @25V

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

R1 (Ω)

19.1

10

9.53

12

7.87

15

6.34

20

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

0.1mA.

L

22µH

D

VIN

CIN

1µF

VIN

SW

COUT

0.22µF

GM1937

FB

CE

GND

R2

1K

R1

6.34Ω

6

Figure 3. LED Driver with Open-Circuit Protection

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