CPC9909 Datasheet, PDF (6/12 Pages) IXYS Corporation – High Effciency,Off-line, High Brightness LED Driver

INTEGRATED CIRCUITS DIVISION
2. Functional Description
Figure 1 Typical Application Circuit
8 - 550V
V_DD
V_DD
V_IN
Voltage
Regulator
R_T
Voltage
Reference
-
+
LD
-
+
C_VDD
PW MD
R_T
CPC9909
RT Minimum Off-Time
Q
One Shot ^TRIG
SQ
R
GND
CPC9909
D1
L1
Q1
GATE
CS
R_SENSE
Buck Configuration
2.1 Overview
The CPC9909 drives the LEDs via a minimum
off-time, peak-current-limited, pulse-frequency
modulation scheme. This control scheme is inherently
stable, and the driver can be operated above a 50%
duty cycle without any open-loop instability or
sub-harmonic oscillations. Since the switching
frequency depends on the LED load current, it results
in a high efficiency operation.
2.2 LED Driver Theory of Operation
The typical application circuit is as shown in Figure 1
When PWMD is high, the control circuit is enabled and
the gate driver turns on the external power MOSFET
(Q1), causing the inductor (L1) current to ramp up until
the voltage across the current sense resistor (R_SENSE)
exceeds V_CS(high). When the voltage at the CS pin
exceeds this threshold, the gate driver turns Q1 off.
Q1 remains off for the duration of the fixed minimum
off-time. While the switch is off, the inductor continues
to deliver the current to the load though the diode
(D1). When the off-time expires, Q1 turns on again
until the peak current limit is reached, and the process
repeats.
The peak current limit threshold is set by the external
sense resistor, R_SENSE, and the internal voltage
threshold, V_CS(high). This internal voltage threshold can
also be set externally via the LD pin. The lower of
these two thresholds and R_SENSE set the peak current
in the inductor.
A soft start function can be implemented by ramping
up the DC voltage at the LD pin from 0V to V_CS(high) at
the desired rate. To utilize the soft start function,
connect a resistor divider from V_DD to ground and a
capacitor from the LD pin to ground, as shown in
Figure 2 Soft-Start Circuit
CPC9909
V_IN
CS
GND
GATE
R_T
LD
V_DD
PWMD
51kΩ
2.2kΩ 0.1μF
6
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