5

HV9925

Functional Description

The HV9925 is a PWM peak current control IC for driving

a buck converter topology in continuous conduction mode

(CCM). The HV9925 controls the output current (rather than

output voltage) of the converter that can be programmed by

a single external resistor (R

string of light emitting diodes (LED). An external enable input

(PWMD) is provided that can be utilized for PWM dimming of

an LED string. The typical rising and falling edge transitions

of the LED current when using the PWM dimming feature of

the HV9925 are shown in Fig. 6 and Fig. 7.

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

pin, the internal linear regulator seeks to maintain a voltage

of 7.5VDC at the V

internally programmed under-voltage threshold, no output

switching occurs. When the threshold is exceeded, the

integrated high-voltage switch turns on, pulling the DRAIN

low. A 200mV hysteresis is incorporated with the under-

voltage comparator to prevent oscillation.

When the voltage at R

off and the DRAIN output becomes high impedance. At the

same time, a one-shot circuit is activated that determines

the off-time of the switch (10.5µs typ.).

A “blanking” delay of 300ns is provided upon the turn-on of

the switch that prevents false triggering of the current sense

comparator due to the leading edge spike caused by circuit

parasitics.

Application Information

Selecting L1 and D1

The required value of L1 is inversely proportional to the ripple

current ∆I

20~30% is a good practice to ensure noise immunity of the

current sense comparator.

L1 = (V

(1)

V

time of the HV9925. The output current in the LED string (I

is calculated then as:

I

(2)

where V

R

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 HV9925, the ripple

current is nearly independent of the input AC or DC 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

HV9925 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

HV9925.

Coil capacitance of inductors is typically provided in the

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

self-resonant frequency (SRF).

SRF = 1 / (2π√(L • C

where Lis the inductance value, and C

Charging and discharging this capacitance every switching

cycle causes high-current spikes in the LED string. Therefore,

connecting a small capacitor C

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

achieves better performance. The reverse voltage rating V

R

of the diode must be greater than the maximum input voltage

of the LED lamp.

The total parasitic capacitance present at the DRAIN output

of the HV9925 can be calculated as:

C

(3)

When the switch turns on, the capacitance C

into the DRAIN output 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 as:

T

(4)