FAN5333A Datasheet, PDF (7/11 Pages) Fairchild Semiconductor – High Efficiency, High Current Serial LED Driver with 30V Integrated Switch

Block Diagram
SHDN
V_IN
4
5
FB 3
Shutdown
Circuitry
FB ₊
Over
Voltage
Comp
-
1.15 x V_REF
Thermal
Shutdown
-
Error
Amp
+
S
Reference
+
Comp
-
Ramp
Generator
Current Limit
Comparator _{- +}
Oscillator
R
RQ
RS
SW
1
n
Driver
+
Amp
-
30mΩ
2
GND
Figure 4. Block Diagram
Circuit Description
The FAN5333A/FAN5333B is a pulse-width modulated (PWM)
current-mode boost converter. The FAN5333A/FAN5333B
improves the performance of battery powered equipment by signif-
icantly minimizing the spectral distribution of noise at the input
caused by the switching action of the regulator. In order to facilitate
effective noise filtering, the switching frequency was chosen to be
high, 1.5MHz. The device architecture is that of a current mode
controller with an internal sense resistor connected in series
with the N-channel switch. The voltage at the feedback pin
tracks the output voltage at the cathode of the external Schottky
diode (shown in the test circuit). The error amplifier amplifies the
difference between the feedback voltage and the internal band-
gap reference. The amplified error voltage serves as a refer-
ence voltage to the PWM comparator. The inverting input of the
PWM comparator consists of the sum of two components: the
amplified control signal received from the 30mΩ current sense
resistor and the ramp generator voltage derived from the oscilla-
tor. The oscillator sets the latch, and the latch turns on the FET
switch. Under normal operating conditions, the PWM compara-
tor resets the latch and turns off the FET, thus terminating the
pulse. Since the comparator input contains information about
the output voltage and the control loop is arranged to form a
negative feedback loop, the value of the peak inductor current
will be adjusted to maintain regulation.
Every time the latch is reset, the FET is turned off and the cur-
rent flow through the switch is terminated. The latch can be
reset by other events as well. Over-current condition is moni-
tored by the current limit comparator which resets the latch and
turns off the switch instantaneously within each clock cycle.
Over-Voltage Protection
The voltage on the feedback pin is sensed by an OVP Compar-
ator. When the feedback voltage is 15% higher than the nominal
voltage, the OVP Comparator stops switching of the power tran-
sistor, thus preventing the output voltage from going higher.
Open-circuit protection
As in any current regulator, if the feedback loop is open, the out-
put voltage increases until it is limited by some additional exter-
nal circuitry. In the particular case of the FAN5333, the output
voltage is limited by the switching transistor breakdown at
around 45V, typically (assuming that C_OUT and the Schottky
diode rating voltage are higher). Since at such high output volt-
age the output current is inherently limited by the discontinuous
conduction mode, in most cases, the switching transistor enters
non-destructive breakdown and the IC survives.
However, to ensure 100% protection for LED disconnection, we
recommend limiting V_OUT with an external Zener diode or stop-
ping the boost switching with an external voltage supervisory
circuit.
Applications Information
Setting the Output Current
The internal reference (V_REF) is 110mV (Typical) for FAN5333A
and 315mV (Typical) for FAN5333B. The output current is set by
a resistor divider R connected between FB pin and ground. The
output current is given by
I_LED = -V---R-^F---^B-
7
FAN5333A/FAN5333B Rev. 1.0.1
www.fairchildsemi.com