5

NR092005

HV9921/HV9922/HV9923

Functional Description

The HV9921/22/23 are PWM peak current controllers

for controlling a buck converter topology in continuous

conduction mode (CCM). The output current is internally

preset at 20mA (HV9921), 50mA (HV9922), or 30mA

(HV9923).

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

DRAIN pin, the internal high-voltage linear regulator seeks

to maintain a voltage of 7VDC at the V

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 duration of the off-state

(10.5

µS typ.). 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.

Application Information

The HV9921/22/23 is a low-cost off-line buck converter IC

specifically designed for driving multi-LED strings. It can be

operated from either universal AC line range of 85 to 264VAC,

or 20 to 400VDC, and drives up to tens of high brightness

LEDs. All LEDs can be run in series, and the HV9921/22/23

regulates at constant current, yielding uniform illumination.

The HV9921/22/23 is compatible with triac dimmers. The

output current is internally fixed at 20mA for the HV9921,

50mA for the HV9922, and 30mA for HV9923. These parts

are available in space saving TO-92 and SOT-89 packages.

Selecting L1 and D1

There is a certain trade-off to be considered between

optimal sizing of the output inductor L1 and the tolerated

output current ripple. The required value of L1 is inversely

proportional to the ripple current

∆I

(1)

V

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

string (I

(2)

where I

TH

is the current sense comparator threshold.

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 the

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 permitting a

reduced value of L1. However, one must keep in mind that

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

of T

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 capacitance of L1, junction

capacitance and reverse recovery of the rectifier diode D1,

capacitance of the printed circuit board traces C

capacitance C

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.

Coil capacitance of inductors is typically provided in the

manufacturer’s data books either directly or in terms of the

self-resonant frequency (SRF).

where L is the inductance value, and C

L

is the coil

capacitance.) Charging and discharging this capacitance

every switching cycle causes high-current spikes in the LED

string. Therefore, connecting a small capacitor C

recommended to bypass these spikes.

Using an ultra-fast rectifier diode for D1 is recommended

to achieve high efficiency and reduce the risk of false

triggering of the current sense comparator. Using diodes

with shorter reverse recovery time t

capacitance C

voltage rating V

maximum input voltage of the LED lamp.

The total parasitic capacitance present at the DRAIN pin of

the HV9921/22/23 can be calculated as:

(3)

When the switching MOSFET turns on, the capacitance C

P

is discharged into the DRAIN pin of the IC. The discharge

current is limited to about 150mA typically. However, it

may become lower at increased junction temperature. The

duration of the leading edge current spike can be estimated

/(

)

L

SRF

1 2

L C

π

=⋅

PDRAIN

PCB

L

J

CC

C

C

C

=+

+

+

OOFF

O

VT

L1

I

∆

⋅

=

OTH

O

1

II

I

2

∆

=− ⋅