HV9910B
Current Sense
The current sense input of the HV9910B goes to the non-
inverting inputs of two comparators. The inverting terminal
of one comparator is tied to an internal 250mV reference
whereas the inverting terminal of the other comparator is
connected to the LD pin. The outputs of both these compara-
tors are fed into an OR gate and the output of the OR gate
is fed into the reset pin of the flip-flop. Thus, the comparator
which has the lowest voltage at the inverting terminal deter-
mines when the GATE output is turned off.
The outputs of the comparators also include a 150-280ns
blanking time which prevents spurious turn-offs of the exter-
nal FET due to the turn-on spike normally present in peak
current mode control. In rare cases, this internal blanking
might not be enough to filter out the turn-on spike. In these
cases, an external RC filter needs to be added between the
external sense resistor (R
CS
) and the CS pin.
Please note that the comparators are fast (with a typical
80ns response time). Hence these comparators are more
susceptible to be triggered by noise than the comparators
of the HV9910. A proper layout minimizing external induc-
tances will prevent false triggering of these comparators.
Linear Dimming
The Linear Dimming pin is used to control the LED current.
There are two cases when it may be necessary to use the
Linear Dimming pin.
►
In some cases, it may not be possible to find the exact
R
CS
value required to obtain the LED current when the
internal 250mV is used. In these cases, an external volt-
age divider from the VDD pin can be connected to the LD
pin to obtain a voltage (less than 250mV) corresponding to
the desired voltage across R
CS
.
►
Linear dimming may be desired to adjust the current
level to reduce the intensity of the LEDs. In these cases,
an external 0-250mV voltage can be connected to the LD
pin to adjust the LED current during operation.
To use the internal 250mV, the LD pin can be connected to
VDD.
Note: Although the LD pin can be pulled to GND, the output
current will not go to zero. This is due to the presence of a
minimum on-time (which is equal to the sum of the blanking
time and the delay to output time) which is about 450ns.
This will cause the FET to be on for a minimum of 450ns and
thus the LED current when LD = GND will not be zero. This
current is also dependent on the input voltage, inductance
value, forward voltage of the LEDs and circuit parasitics. To
get zero LED current, the PWMD pin has to be used.
Oscillator
The oscillator in the HV9910B is controlled by a single re-
sistor connected at the RT pin. The equation governing the
oscillator time period t
OSC
is given by:
PWM Dimming
PWM Dimming can be achieved by driving the PWMD pin
with a low frequency square wave signal. When the PWM
signal is zero, the gate driver is turned off and when the
PWMD signal if high, the gate driver is enabled. Since the
PWMD signal does not turn off the other parts of the IC,
the response of the HV9910B to the PWMD signal is almost
instantaneous. The rate of rise and fall of the LED current is
thus determined solely by the rise and fall times of the induc-
tor current.
To disable PWM dimming and enable the HV9910B perma-
nently, connect the PWMD pin to VDD.
If the resistor is connected between RT and GND, HV9910B
operates in a constant frequency mode and the above equa-
tion determines the time-period. If the resistor is connected
between RT and GATE, the HV9910B operates in a constant
off-time mode and the above equation determines the off-
time.
GATE Output
The gate output of the HV9910B is used to drive and external
FET. It is recommended that the gate charge of the external
FET be less than 25nC for switching frequencies ≤ 100kHz
and less than 15nC for switching frequencies > 100kHz.
6
SUPERTEX [ Supertex, Inc ]
