I
NTEGRATED
C
IRCUITS
D
IVISION
CPC9909
R05 www.ixysic.com 7
2.3 Input Voltage Regulator
The CPC9909 has an internal voltage regulator that
can work with input voltages ranging from 12V
DC
to
550V
DC
. When a DC voltage greater than 12V is
applied at the V
IN
pin, the internal voltage regulator
regulates the voltage down to a typical 7.8V. The V
DD
pin is the internal voltage regulator output pin and
must be bypassed by a low-ESR capacitor to provide
a low impedance path for high frequency switching
noise.
The CPC9909 driver does not require the bulky
start-up resistors typically needed for off-line
controllers. The internal voltage regulator provides
sufficient voltage and current to power internal IC
circuits. This voltage is also available at the V
DD
pin,
and can be used as a bias voltage for external
circuitry.
The internal voltage regulator can be bypassed by
applying an external DC voltage to the V
DD
pin that is
slightly higher than the internally generated regulator
voltage. This reduces the power dissipation of the
integrated circuit, and it is more suitable in isolated
applications where an auxiliary winding can be used to
drive the V
DD
pin.
The total input current drawn from the V
IN
pin is equal
to the quiescent current drawn by the internal circuitry
(which is specified at 0.6mA maximum) plus the gate
driver current. See “Shut-Down Mode Supply Current”
in Section 1.5 “Electrical Characteristics” on
page 5.
The current draw of the gate driver depends on the
switching frequency and the gate charge of the
external power MOSFET. The total input current can
be calculated by:
Where Q
GATE
is the total gate charge of the MOSFET
and f
S
is the oscillator external frequency.
2.4 Current Sense Resistor
The peak LED current is set by an external sense
resistor (R
SENSE
) connected from the CS pin to
ground.
The value of the current sense resistor is calculated
based on the average LED current desired, the current
sense threshold, and the inductor ripple current.
The peak-to-peak difference in the inductor current
waveform is referred to as inductor ripple current (the
inductor is typically selected to be large enough to
keep this ripple within 30% of the average). Factor in
the ripple current when calculating the sense resistor.
The current sense resistor value can be found by:
Where:
• V
CS(high)
= current sense threshold =0.25V (or V
LD
)
• I
LED
= average LED/inductor current
• I
L
= inductor ripple current = 0.3*I
LED
Combining terms:
2.5 Current Sense Blanking
The CPC9909 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, initiating a premature termination of
the gate pulse. To avoid this condition, a typical 400ns
internal leading edge blanking time is implemented,
thereby eliminating the need for external RC filtering,
and simplifying the design. During the current sense
blanking time, the current limit comparator is disabled,
preventing the gate-drive circuit from terminating the
gate-drive signal.
2.6 Enable/Disable Function
Connecting the PWMD pin to V
DD
enables the gate
driver. Connecting PWMD to GND disables the gate
driver and sets the device in standby mode. In standby
mode, the quiescent current is 0.6mA maximum.
2.7 Minimum Off-Time One-Shot
The CPC9909 uses a fixed off-time control scheme.
The minimum off-time is set by an external resistor
connected between the RT and GND terminals.
The off-time can be determined by:
Off-time selection indirectly determines the switching
frequency of the LED driver.
I
IN
0.6mA Q
GATE
f
S
+
R
SENSE
V
CS high
I
LED
0.5I
L
+
----------------------------------=
R
SENSE
V
CS high
1.15 I
LED
--------------------------- -=
t
off
s R
T
k660.8+=
