HT7937 datasheet(4/13 Pages) HOLTEK

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HT7937

Rev 1.10

January 18, 2010

Functional Description

· Shutdown

The shutdown pin, SHDN, must not be allowed to

float. When the SHDN pin voltage is taken below 0.3V,

the internal MOSFET, voltage reference, error ampli-

fier, comparators and biasing circuitry will all be

switched off reducing the quiescent supply current to

less than 1

mA. If the SHDN pin has a value greater

than 1.5V, then the device will be fully enabled and op-

erational. This pin also can be used as a PWM signal

from 100Hz to 1kHz to allow brightness control.

· Over voltage protection - OVP

With an open circuit output, such as when no LEDs

are connected, the FB pin will be pulled down to

ground via the sense resistor RFB. As the device will

now react by trying to increase the output voltage by

generating a maximum duty cycle signal, this may

cause the SW pin to exceed its maximum rated volt-

age, which may damage the internal N-MOS switch-

ing transistor.

The OVP function is designed to prevent damage to

the internal NMOS switching transistor. When the out-

put voltage rises above the OVP threshold voltage,

typically 28V, the converter will clamp the output volt-

age to this level. When the output voltage returns to a

value below the OVP threshold, it will automatically re-

sume normal switching operation.

· Dimming control

There are three methods to control the LEDs bright-

ness as listed below:

¨ A PWM signal on the SHDN pin

A PWM signal is applied to the SHDN pin as shown

in Figure 1.

The magnitude of the PWM signal should be higher

than the enable voltage of the SHDN pin, the LEDs

operate with either zero or full current. The average

LED current is proportional to the duty cycle of the

applied PWM signal with a duty cycle increase re-

sulting in higher LEDs brightness. Typical PWM fre-

quencies should be between 100Hz and 1kHz.

¨ A DC signal on the FB pin

This method of dimming control uses a DC voltage

circuit as shown in Figure 2.

The LED brightness is directly proportional to the

LED current which is given by the following equation:

Where

VFB = Feedback voltage is 95mV

VDC = DC voltage

R1 and R2 >> RFB

¨ A filtered PWM signal on the FB pin

For frequencies greater than 1kHz, dimming can be

implemented by using the circuit shown in Figure 3.

The PWM control circuitry is connected to the FB

pin. Reducing the duty cycle on the PWM signal re-

sults in increased LEDs brightness levels. The LED

brightness is directly proportional to the LED cur-

rent which is given by the following equation:

Where

VFB = Feedback voltage is 95mV

VPWM = PWM high level voltage

D = PWM duty cycle

R1 and R2 >> RFB

PWM frequency >>

H T 7 9 3 7

V I N

S H D N

G N D

S W

O V P

F B

1 0 m H

C 1

1 m F

1 N 5 8 1 9

D 1

C 2

1 m F

R F B

4 . 7 W

V I N

P W M

f = 1 0 0 H z ~ 1 k H z

U p t o

6 W L E D s

Figure 1. Dimming Control with PWM Signal

H T 7 9 3 7

V I N

S H D N

G N D

S W

O V P

F B

1 0 m H

C 1

1 m F

1 N 5 8 1 9

D 1

C 2

1 m F

R F B

4 . 7 W

V I N

1 k W

5 1 k W

R 1

R 2

V D C

U p t o

6 W L E D s

V F B

Figure 2. Dimming Control Using a DC Voltage

H T 7 9 3 7

V I N

S H D N

G N D

S W

O V P

F B

1 0 m H

C 1

1 m F

1 N 5 8 1 9

D 1

C 2

1 m F

R F B

4 . 7 W

V I N

1 k W

5 1 k W

R 1

R 2

V P W M

R 3

5 . 1 k W

C 3

0 . 1 m F

U p t o

6 W L E D s

V F B

0 V

Figure 3. Dimming Control Using a Filter PWM Signal

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