Supertex inc.
High Current LED Driver Demoboard
The HV9918DB1 demoboard is a high current LED driver designed
to drive one or two LEDs at 700mA from a 9.0 - 16VDC input. The
demoboard uses Supertex’s HV9918 hysteretic buck LED driver
IC.
The HV9918DB1 includes digital control of PWM dimming, which
allows the user to dim the LEDs using an external, TTL-compatible
square wave source applied between
DIM
and
GND.
In this case,
the PWM dimming frequency and duty ratio are set by the external
square wave source.
The demoboard is protected against short circuit and open LED
conditions. It also includes thermal derating of the LED current us-
ing an external NTC resistor to prevent over-heating.
The bottom of the HV9918DB1 is an exposed copper plane (con-
nected to the input ground) which can be connected to a 1” square
heatsink (eg: 374324B00035G from Aavid Thermalloy) to allow
for operation in higher ambient temperatures without tripping the
HV9918’s built-in over temperature protection.
HV9918DB1
Specifications
Parameter
Input voltage
Output voltage
Output current
Line and load regulation
Output current ripple
Full load efficiency
(@12.0V input)
Open LED protection
Output short circuit protection
Dimensions
Value
9.0 - 16VDC
2.0 - 7.0V
700mA ± 5%
< ±2%
5% (peak to peak)
87%
Yes
Yes
25.4mm X 29.2mm
General Description
Connection Diagram
Silk Screen
+
-
NTC
Supertex
Connections
Input Connection:
Connect the input DC voltage between
VIN
and
GND
terminals.
Output Connection:
Connect the LED(s) between
LED+
(anode of LED string) and
LED-
(cathode of LED string) ter-
minals.
PWM Dimming Connection:
1. If no PWM dimming is required, short
PWMD
and
VDD
terminals.
2. If dimming using an external PWM dimming source,
connect the PWM source between the
PWMD
and
GND
terminals.
NTC Thermistor Connection:
1. If no thermal derating is required, the
NTC
terminal
can be left open.
2. If thermal derating of the LED current is required, the
NTC thermistor can be connected between
NTC
and
GND
terminals as shown.
Doc.# DSDB-HV9918DB1
A032913
Supertex inc.
www.supertex.com
HV9918DB1
Operation of the Board
The HV9918DB1 uses Supertex’s HV9918 hysteretic buck
LED driver IC to control the LED current. Since the regula-
tion method controls both the peak and the valley current
in the inductor, the demoboard has excellent line and load
regulation.
The LED current can be controlled in by PWM dimming.
PWM dimming can be achieved in one of two ways:
1. Analog control of PWM dimming where a 0-2V source
can be applied between
NTC
and
GND
terminals (the
NTC
terminal can also be used for thermal derating of
the LED current as explained in the next section).
2. Direct control of PWM dimming by applying a TTL com-
patible square wave source between
PWMD
and
GND
terminals.
Thermal derating in the HV9918DB1 uses the analog control
of PWM dimming function to limit the LED current when the
LED temperature rises. During normal operating mode (con-
stant LED current; no PWM dimming), the LED current will
be PWM dimmed at 1.0kHz. During PWM dimming mode,
the thermal derating function limits the maximum PWM dim-
ming duty cycle so that the LED current does not exceed
the maximum allowable current determined by the thermal
derating circuit.
Direct Control of PWM Dimming
In the direct control method, the PWM dimming of the LEDs
is achieved by driving the
PWMD
terminal using an external
square wave source. In this case, PWM dimming frequency
and duty cycle are set by the external source.
In this mode, if the thermal derating function is not desired,
NTC
terminal should be left open. In this case, the recom-
mended PWM dimming frequency can be anything up to
10kHz.
If thermal derating is desired, then the NTC thermistor should
be connected between the
NTC
and
GND
terminals. In this
case, the PWM dimming frequency should be greater than
1.2kHz.
Analog Control of PWM Dimming / Thermal De-
rating
Analog Control of PWM dimming can be accomplished by
applying a 0 – 2.0V DC voltage between
NTC
and
GND
(the
DC voltage must have a 500μA source/sink capability). In
this case,
PWMD
is connected to
VDD
and the LEDs are
dimmed at 1.0kHz (as set by the capacitor at the RAMP pin
of the IC). The duty cycle of the LED current can be adjusted
by varying the external voltage at
NTC
(0V = 0% LED cur-
rent and 2.0V = 100% LED current).
The same
NTC
terminal can instead be used to de-rate the
LED current based on the LED temperature, if desired. This
would reduce the LED current as the LED temperature rises
and prevents over-heating of the LED. An NTC resistor can
be used to sense the temperature of the LED and this resis-
tor can be connected between the
NTC
and
GND
terminals
of the HV9918DB1. The demoboard is designed to operate
with a 100k NTC thermistor which has a B-constant of 4250
(eg: NCP18WF104 from Murata). With this NTC thermistor,
the LED current will start dropping at 85ºC and will reach
about 350mA at 125ºC.
Doc.# DSDB-HV9918DB1
A032913
2
Supertex inc.
www.supertex.com
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