Rev. 1.00

5

April 13, 2012

HT7945

Typical Application Circuit

LED1

LED2

LED4

LED5

LED7

LED8

LED3

LED6

OSC

AVCC

VCC5

EN

AGND1

AGND2

DIM

COMP

OVP

PGND2

LX2

LX1

PGND1

HT7945

ISET

VIN

10uH

L1

1N5819

D1

4.7uF

C1

IN

0.1uF

C2

C3

4.7uF/50V

R2

33kW

R1

1MW

RSET

10uF

C4

C5

R3

24kW

PWM

EP

VOUT

1.5nF

PGND3

Figure 1

Functional Description

VIN Under-Voltage Lockout – UVLO

The device contains an Input Under Voltage Lockout

(UVLO) circuit. The purpose of the UVLO circuit

is to ensure that the input voltage is high enough for

reliable operation. For low input voltage operation

of 4.5V to 6V, the VREF pin is connected to the VIN

pin (input voltage) to bypass the voltage regulator

in which the inherent voltage drop can degrade low

voltage operation. When input voltage level is below

this range (4.5V to 6V) to 4.0V, then proper use is

not possible. When the input voltage falls below the

under voltage threshold, the internal MOSFET switch

will be turned off. If the input voltage rises beyond

the under voltage lockout hysteresis, it can return to

the original operating situation and does not required

to be powered on again. The UVLO threshold is set

below the minimum input voltage of 3.8V to avoid

any transient VIN drops under the UVLO threshold

causing the converter to turn off.

Current Limit Protection

The device has a cycle-by-cycle current limit to

protect the internal power MOSFET. If the inductor

current reaches the current limit threshold of 2.2A,

the MOSFET will be turned off. It is import to note

that this current limit will not protect the output from

excessive current if the output is short circuited. If an

output short circuit has occurred, excessive current

can damage both the inductor and diode.

Output Voltage Protection

Over-Voltage Protection

The device includes an over-voltage protection

function. If the one of ISEN pins is shorted to

ground or an LED is disconnected from the circuit,

the voltage on the ISEN pin will fall to zero and the

internal power MOSFET will switch with its full duty

cycle. This may cause the output voltage to exceed

its maximum voltage rating, possibly damaging

the device and external components. The internal

over-voltage protection circuitry turns off the power

MOSFET and shuts down the device as soon as the

the output voltage falls to the level of the input supply

voltage. The device remains in this shutdown mode