HV9910B
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 cases, an external RC filter
needs to be added between the external sense resistor
(RCS) and the CS pin.
To use the internal 250mV, the LD pin can be con-
nected 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,
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.
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 minimiz-
ing external inductances will prevent false triggering of
these comparators.
3.3
Oscillator
3.6
PWM Dimming
The oscillator in the HV9910B is controlled by a single
resistor connected at the RT pin. The equation govern-
ing the oscillator time period tOSC is given by:
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 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 inductor current.
RTk + 22
tOSCs= --------------------------------
25
If the resistor is connected between RT and GND,
HV9910B operates in a constant frequency mode and
the above equation 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.
To disable PWM dimming and enable the HV9910B
permanently, connect the PWMD pin to VDD.
3.4
Gate Output
The GATE output of the HV9910B 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 RCS
value required to obtain the LED current. In
these cases, an external voltage divider from the
VDD pin can be connected to the LD pin to
obtain a voltage (less than 250mV) correspond-
ing 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.
2015 Microchip Technology Inc.
DS20005344A-page 7
MICROCHIP [ MICROCHIP ]
