GM19371 데이터 시트보기 (PDF) - Gamma Microelectronics Inc.

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GM19371

BOOST WHITE LED DRIVER WITH OVP

Gamma Microelectronics Inc. 

GM19371

BOOST WHITE LED DRIVER

WITH OVP

Application Information

General Operation

The GM19371 uses a constant frequency, current mode control scheme to provide excellent line and load

regulations. See Fig 2 for GM19371operation. At the start of each oscillator cycle, the RS 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 RS 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.

Over Voltage Protection and Open Circuit Protection

In case of output open circuit, while the LED strings are disconnected from the circuit or the LEDs fail

open, the feedback voltage, VFB, will be zero. The GM19371 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 voltage

rating. In order to prevent this condition occur, the GM19317 builds a latch-mode open-circuit protection

circuit. Based on the result from the sensing pin, OVP pin, the chip can detect the output open condition

when the OVP voltage is higher than 30V. The chip will shutdown until VIN reset

Inductor Selection

A 10uH inductor is recommended for most GM19371 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).

Capacitor Selection

The small size of ceramic capacitors makes them ideal for GM19371 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 4.7uF input capacitor and a 4.7uF output capacitor are sufficient for

most GM19371 applications.

Diode Selection

Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for

GM19371 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 GM19371. A Schottky diode rated at

1000mA is sufficient for most GM19371 applications.

Soft Start and Current Limit

The internal soft start circuit minimizes the inrush current during turning on GM19371. The Typical switch

current is limited to about 650mA by the chip.

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