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I NTEGRATED C IRCUITS D IVISION
It is a good practice to select a power rating that is at
least twice the calculated value. This will give proper
margins, and make the design more reliable.
Figure 4 Resistor Selection
Oscillator Frequency, f S , vs. R T
(T A =27oC)
MXHV9910
250
2.2.3 Current Sense Blanking
The MXHV9910 has an internal current-sense
blanking circuit. When the power MOSFET is turned
on, the external inductor can cause an undesired
spike at the current sense pin, CS, initiating a
premature termination of the gate pulse. To avoid this
condition, a typical 400ns internal leading edge
blanking time is implemented. This internal feature
eliminates the need for external RC filtering, thus
200
150
100
50
simplifying the design. During the current sense
blanking time, the current limit comparator is disabled,
preventing the gate-drive circuit from terminating the
0
0
200
400
600
R T (k Ω )
8 00
1000
1200
D max = --------------------------
t ONmax = -------------
? V in – V LEDstring ? ? t ONmax
r iout ? I LED
gate-drive signal.
2.2.4 Enable/Disable
Connecting the PWMD pin to V DD enables the gate
driver. Connecting PWMD to G N D disables the gate
driver and sets the device into the shut-down mode. In
the shut-down mode, the gate output drive is disabled
while all other functions remain active. The maximum
quiescent current in the shut-down mode is 0.6mA.
2.2.5 Oscillator
The MXHV9910 operates in a constant frequency
mode. Setting the oscillator frequency is achieved by
connecting an external resistor between R T and G N D.
In general, switching frequency selection is based on
the inductor size, controller power dissipation, and the
input filter capacitor.
The typical off-line LED driver switching frequency, f S ,
is between 30kHz and 120kHz. This operating range
gives designers a reasonable compromise between
switching losses and inductor size. The internal RC
shows the R T resistor selection for the desired f S .
2.2.6 Inductor Design
The inductor value is determined based on LED ripple
current, maximum on-time, the forward voltage drop of
all LEDs in a string at the desired current, and the
minimum input voltage, which is based on design
requirements. The maximum on-time is determined by
the duty cycle and switching frequency. The maximum
duty cycle is given by:
V LEDstring
V in
Where:
? V LEDstring is the LED string voltage at desired
average LED current.
? V in is the minimum input voltage to V I N
The maximum duty cycle must be restricted to less
than 50% in order to prevent sub-harmonic oscillations
and open loop instability.
The converter maximum O N -time is given by:
D max
f s
Where f s is the switching frequency of the internal
oscillator.
The inductor value for the given ripple is:
L min = ---------------------------------------------------------------------
R03
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