HV9921/22

NR040805

5

Functional Description

HV9921 and HV9922 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) or

50mA (HV9922).

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

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 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 regulates at

constant current, yielding uniform illumination. The

HV9921/22 is compatible with triac dimmers. The

output current is internally fixed at 20mA for HV9921

and 50mA for HV9922. Both 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

O

OFF

O

I

T

V

L

∆

⋅

=

1

(1)

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

O

TH

O

I

I

I

∆

⋅

−

=

2

1

,

(2)

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, 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

the initial output current accuracy might be sacrificed

at large ripple current in L1.

Another important aspect of designing an LED driver

with HV9921/22 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

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

HV9921/22.

Coil capacitance of inductors is typically provided in

the manufacturer’s data books either directly or in

terms of the self-resonant frequency (SRF).

)

2

/(

1

L

C

L

SRF

⋅

=

π

,

capacitance.)

Charging

and

discharging

this

capacitance every switching cycle causes high-

current spikes in the LED string. Therefore,

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

J

achieves better performance. The reverse voltage