HV9925SG-G Supertex, HV9925SG-G Datasheet - Page 5

• Manufacturer Part Number: HV9925SG-G
• Description: LED Drivers Programmable Current LED Lamp
• Manufacturer: Supertex

Specifications of HV9925SG-G

• Operating Supply Voltage: 20 V to 400 V
• Maximum Supply Current: 500 uA
• Maximum Power Dissipation: 800 mW
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Package / Case: SOIC N EP
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
V
time of the HV9925. The output current in the LED string (I
is calculated then as:
I
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.
O
O
SENSE
= (V
is the forward voltage of the LED string. T
TH
O
is the current sense resistor. The ripple current
TH
/ R
• T
O
is the current sense comparator threshold, and
OFF
SENSE
in it. Setting the relative peak-to-peak ripple to
) / ΔI
) - 1/2ΔI
O
DD
SENSE
pin. Until this voltage exceeds the
O
SENSE
exceeds 0.47V, the switch turns
), for the purpose of driving a
●
1235 Bordeaux Drive, Sunnyvale, CA 94089
OFF
is the off-
(2)
(1)
O
)
5
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).
where L is 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
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
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
SRF = 1 / (2π√(L • C
SPIKE
P
= C
OFF
= ((V
. Therefore, the initial output current accuracy might
DRAIN
●
IN
+ C
• C
Tel: [phone]
DRAIN
PCB
P
) / I
+ C
of the controller itself. These parasitic
SAT
L
L
))
) + t
rr
+ C
, and lower junction capacitance C
J
rr
●
O
www.supertex.com
(~10nF) is recommended to
L
is the coil capacitance.
P
PCB
is discharged
HV9925
and output
(3)
(4)
J
R
,