GM1937 查看數據表（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

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

DCR (Ω)

Current Rating

(mA)

LQH3C220

0.71

250

Manufacturer

Murata

ELJTC220KF

4.0

160

Panasonic

CDRH3D16-220

0.53

350

Sumida

LB2023B220M

1.7

75

Taiyo Yuden

LEM2520-220

5.5

125

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 1µF input capacitor and a 0.22µF output capacitor are sufficient for

most GM1937 applications.

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