HV9910B データシートの表示（PDF） - Unspecified
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HV9910B Datasheet PDF : 8 Pages
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HV9910C
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
comparators 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 determines when the GATE output is turned off.
► 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.
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.
Please note that the comparators are fast (with a typical
80ns response time). A proper layout minimizing external
inductances will prevent false triggering of these comparators.
Oscillator
The oscillator in the HV9910C is controlled by a single
resistor connected at the RT pin. The equation governing
the oscillator time period Tosc is given by:
Tosc (µs) =
Rosc (kΩ)
25
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, the HV9910C operates in a constant off-time
mode and the above equation determines the off-time.
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 frequencies ≤ 100kHz
and less than 15nC for switching frequencies > 100kHz.
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.
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 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 VDD.
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 temperature
reaches TSD = 140°C (typ.), the HV9910C enters a low power
consumption shut-down mode with IIN <350µA.
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
RCS value required to obtain the LED current when the
internal 250mV is used. 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)
corresponding to the desired voltage across RCS.
Doc.# DSFP-HV9910C
NR041813
Supertex inc.
5
www.supertex.com
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