HV9921/HV9922/HV9923

DS20005311A-page 6

 2014 Microchip Technology Inc.

2.0

PIN DESCRIPTION

See Pin Diagram on page 3 for the figures.

3.0

FUNCTIONAL DESCRIPTION

The HV9921/22/23 are PWM peak current controllers

designed to control a buck converter topology in contin-

uous conduction mode (CCM). The output current is

internally preset at 20mA for HV9921, 50mA for

HV992, and 30mA for HV9923.

When the input voltage of 20 to 400V appears at the

DRAIN pin, the internal high-voltage linear regulator

Until this voltage exceeds the internally programmed

under-voltage threshold, the output switching MOSFET

is non-conductive. When the threshold is exceeded,

the MOSFET turns on. The input current begins to flow

into the DRAIN pin. Hysteresis is provided in the under-

voltage comparator to prevent oscillation.

When the input current exceeds the internal preset

level, a current sense comparator resets an RS flip-

flop, and the MOSFET turns off. At the same time, a

one-shot circuit is activated that determines the dura-

tion of the off-state (10.5μs typical). As soon as this

time is over, the flip-flop sets again. The new switching

cycle begins.

A “blanking” delay of 300ns is provided that prevents

false triggering of the current sense comparator due to

the leading edge spike caused by circuit parasitics.

4.0

APPLICATION INFORMATION

HV9921/22/23 are low-cost off-line buck converter ICs

specifically designed for driving multi-LED strings.

They can be operated from either universal AC line

range of 85 to 264VAC, or 20 to 400VDC, and drive up

to tens of high-brightness LEDs. All LEDs can be run in

series, and the HV9921/22/23 regulate at constant cur-

rent, yielding uniform illumination. HV9921/22/23 are

compatible with triac dimmers. The output current is

internally fixed at 20mA for HV9921, 50mA for HV9922,

and 30mA for HV9923. These parts are available in

space saving TO-92 and SOT-89 packages.

4.1

Selecting L1 and D1

There is a certain trade-off to be considered between

optimal sizing of the output inductor L1 and the toler-

ated output current ripple. The required value of L1 is

off-time of HV9921/22/23. The output current in the

The ripple current introduces a peak-to-average error

in the output current setting that needs to be accounted

for. Due to the constant off-time control technique used

in HV9921/22/23, the ripple current is independent of

the input AC or DC line voltage variation. Therefore, the

output current will remain unaffected by the varying

input voltage.

Adding a filter capacitor across the LED string can

reduce the output current ripple even further, thus per-

mitting a reduced value of L1. However, keep in mind

that the peak-to-average current error is affected by the

accuracy might be sacrificed at large ripple current in

L1.

Another important aspect of designing an LED driver

with the HV9921/22/23 is related to certain parasitic

elements of the circuit, including distributed coil capac-

itance of L1, junction capacitance and reverse recovery

of the rectifier diode D1, capacitance of the printed cir-

of the controller itself. These parasitic elements affect

the efficiency of the switching converter and could

potentially cause false triggering of the current sense

comparator if not properly managed. Minimizing these

parasitics is essential for efficient and reliable operation

of the HV9921/22/23.

TABLE 2-1:

PIN DESCRIPTION

Pin #

Name

Description

1

Drain

Drain terminal of the output switching MOSFET and a linear regulator input

2

GND

Common connection for all circuits

3

VDD

Power Supply pin for all control circuits. By pass this pin with a 0.1 μF low-impedance

capacitor

L1

VO TOFF

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IO

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IO ITH

1

2

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IO

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