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I NTEGRATED C IRCUITS D IVISION
MXHV9910
Time Scale: 5 ? s/div
CH1:
50mA/div
F S 65kHz
Max 77mA
CH2:
10V/div
CH3:
5mV/div x 10
R sense = -------------------------------------------------------------
P = I LED ? R sense
2.2.1 Input Voltage Regulator
The MXHV9910 has an internal voltage regulator that
can work with input voltages ranging from 12V DC to
450 V DC . When the input voltage applied at the V I N pin
is greater than 12V DC , the internal voltage regulator
regulates this voltage down to a typical 7.8V. The V DD
pin is the internal regulator output pin and must be
bypassed by a low ESR capacitor, typically 0.1 ? F, to
provide a low impedance path for high frequency
switching noise.
The MXHV9910 driver does not require the bulky
start-up resistors typically needed for off-line
controllers. An internal voltage regulator provides
sufficient voltage and current to power the internal IC
circuits. This voltage is also available at the V DD pin,
and can be used as bias voltage for external circuitry.
The internal voltage regulator can by bypassed by
applying an external DC voltage to the V DD pin that is
slightly higher than the internal regulator’s maximum
output voltage. This feature reduces power dissipation
of the integrated circuit and is more suitable in isolated
applications where an auxiliary transformer winding
could be used to supply V DD .
The total input current drawn by the V I N pin is equal to
the integrated circuit quiescent current, which is
0.6mA maximum, plus the gate driver current. The
gate driver current is dependant on the switching
frequency and the gate charge of the external power
MOSFET.
The following equation can be used to approximate
the V I N input current:
I IN ? 0.6 mA + ? Q GATE ? f S ?
Where Q GATE is the total gate charge of the external
power MOSFET, and f S is the switching oscillator
frequency.
2.2.2 Current Sense Resistor
The peak LED current is set by an external current
sense resistor connected from the CS pin to ground.
The value of the current sense resistor is calculated
based on the desired average LED current, the current
sense threshold, and the inductor ripple current.
The inductor is typically selected to be large enough to
keep the ripple current (the peak-to-peak difference in
the inductor current waveform) to less than 30% of the
average LED current. Factoring in this ripple current
requirement, the current sense resistor can be
determined by:
V csth
? 1 + ? 0.5 ? r iout ? ? ? I LED
Where:
? V csth = nominal current sense threshold = 0.25V
? r iout = inductor ripple = 0.3
? I LED = average LED current
The power dissipation rating of the sense resistor can
be found with the following formula:
2
6
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