FAN5333B Fairchild Semiconductor, FAN5333B Datasheet - Page 7

• Manufacturer Part Number: FAN5333B
• Description: The FAN5333B is a general-purpose LED driver that features Fixed-Frequency Mode operation and an integrated FET switch
• Manufacturer: Fairchild Semiconductor

FAN5333A/FAN5333B Rev. 1.0.1
Block Diagram
Circuit Description
The FAN5333A/FAN5333B is a pulse-width modulated (PWM)
current-mode
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 30m
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.
boost
FB
3
converter.
Reference
Shutdown
Circuitry
SHDN
4
1.15 x V
The
Amp
Generator
-
Error
+
Oscillator
Ramp
REF
FAN5333A/FAN5333B
FB
Shutdown
Thermal
+
-
Over
Voltage
Comp
current sense
Figure 4. Block Diagram
S
Current Limit
Comparator
Comp
V
+
-
5
IN
7
-
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
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
ping the boost switching with an external voltage supervisory
circuit.
Applications Information
Setting the Output Current
The internal reference (V
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
+
R
R
R
S
Q
Driver
OUT
I
LED
REF
with an external Zener diode or stop-
) is 110mV (Typical) for FAN5333A
Amp
=
+
-
n
V
--------- -
GND
R
FB
SW
2
1
30m
OUT
and the Schottky
www.fairchildsemi.com