Datasheet

HV9910C
DS20005323A-page 8  2014 Microchip Technology Inc.
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 determines 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 external 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 instances, an external RC fil-
ter needs to be added between the external sense
resistor (RCS) and the CS pin.
Please note that the comparators are fast (with a typi-
cal 80ns response time). A proper layout minimizing
external inductances will prevent false triggering of
these comparators.
3.3 Oscillator
The oscillator in HV9910C is controlled by a single
resistor connected at the RT pin. The equation govern-
ing the oscillator time period T
osc
is given by:
If the resistor is connected between RT and GND,
HV9910C operates in a constant frequency mode and
the above equation determines the time period. If the
resistor is connected between RT and GATE,
HV9910C operates in a constant off-time mode and the
above equation determines the off-time.
3.4 Gate Output
The gate output of the HV9910C is used to drive an
external FET. It is recommended that the GATE charge
of the external FET be less than 25nC for switching fre-
quencies  100kHz and less than 15nC for switching
frequencies > 100kHz.
3.5 Linear Dimming
The Linear Dimming pin is used to control the LED cur-
rent. There are two cases when it may be necessary to
use the Linear Dimming pin.
1. In some cases, when using the internal 250mV,
it may not be possible to find the exact R
CS
value required to obtain the LED current. In
these cases, an external voltage divider from the
V
DD
pin can be connected to the LD pin to obtain
a voltage (less than 250mV) corresponding to
the desired voltage across RCS.
2. 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 con-
nected to V
DD
.
3.6 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; when
the PWMD signal if high, the GATE driver is enabled.
The PWMD signal does not turn off the other parts of
the IC, therefore, the response of HV9910C 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 inductor current.
To disable PWM Dimming and enable the HV9910C
permanently, connect the PWMD pin to V
DD
.
3.7 Over-Temperature Protection
The auto-recoverable thermal shutdown at 140°C (typ.)
junction temperature with 20°C hysteresis is featured
to avoid thermal runaway. When the junction tempera-
ture reaches T
SD
= 140°C (typ.), HV9910C enters a low
power consumption shut-down mode with I
IN
<350µA.
T
OSC
s
R
OSC
k
25
-------------------------- -=
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,
or about 450ns. This minimum on-time
causes the FET to be on for a minimum of
450ns, and thus the LED current when LD
= GND is not zero. This current is also
dependent on the input voltage, induc-
tance value, forward voltage of the LEDs,
and circuit parasitics. To get zero LED cur-
rent, the PWMD pin has to be used.