HV9903K6 데이터 시트보기 (PDF) - Supertex Inc

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HV9903K6

White LED Driver

Supertex Inc 

Functional Block Diagram

VDD

VDD

SHDN

OVP

R

Q

S

L

D

CDD

SW

COUT

1. 2MHz

PWM

enable Soft

Start

HV9903

err

amp

LED

22.5V

ILED

RSET

VDD

100mV

VDD

current

mirror

1: 3

7 5R

Fault

RSET

RSET

R

GND

HHVV99990033

Note: This drawing is a generalized representation of the HV9903. Actual internal circuitry may differ.

Operation

The HV9903 operates as a boost converter that

regulates output current rather than output voltage.

To maintain constant output current, LED current is

monitored via the LED pin and the boost converter’s

PWM duty cycle is adjusted accordingly to maintain

the desired current level. LED current is controlled

100% via the PWM boost converter – the MOSFET

connected to the LED pin is fully turned on during

normal operation and is not regulated to maintain

constant LED current. This minimizes voltage drop at

the LED pin, maximizing overall efficiency.

LED current is set by the value of the resistor

connected to the RSET pin. The voltage at the RSET pin

is maintained at 100mV and the resulting current

through the RSET resistor is used as a reference for

LED current control. LED current is regulated at 225

times RSET current.

I LED

=

22.5V

RSET

Current through the RSET pin is monitored. If it falls

below 1.5µA, both the PWM boost converter switch

and LED switch are turned off. Soft-start is not reset

and the IC does not go into low power standby. Such

a condition can occur two ways: 1) if RSET is greater

than about 66kΩ, or 2) an external voltage greater

than 100mV is applied to the RSET pin. Internal

blocking prevents reverse current flow into the RSET

pin if the externally applied voltage exceeds 100mV.

However, applied voltage must not exceed VDD.

The control loop is designed for discontinuous mode

operation. That is, inductor current is allowed to

return to zero between PWM conversion cycles. To

assure discontinuous mode operation, the inductor

value must be below a certain value for given

conditions of supply voltage and LED string voltage

drop. The Inductor Selection section provides further

information.

The PWM boost converter is a current mode controller

operating at an internally fixed 1.2MHz.

A soft-start circuit minimizes inrush current when

power is initially applied or the device is enabled via

the SHDN input. Inrush current is typically limited to

130% of steady-state current. Although the soft-start

period is short (~1ms), it means that if using SHDN for

PWM dimming, the PWM dimming signal should be

4

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