FAN5330 Datasheet, PDF (6/10 Pages) Fairchild Semiconductor – High Efficiency Serial LED Driver with 30V Integrated Switch

Block Diagram
SHDN
V_IN
4
5
FB 3
Shutdown
Circuitry
FB
1.15 x V_REF
+_Over
Voltage
- ^Comp
Reference
-
Error
Amp
+
Thermal
Shutdown
Σ
+
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 FAN5330 is a pulse-width modulated (PWM) current-mode
boost converter. The FAN5330 improves the performance of bat-
tery powered equipment by significantly 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 cir-
cuit). The error amplifier amplifies the difference between the
feedback voltage and the internal bandgap reference. The
amplified error voltage serves as a reference 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 oscillator. The oscilla-
tor sets the latch, and the latch turns on the FET switch. Under
normal operating conditions, the PWM comparator resets the
latch and turns off the FET, thus terminating the pulse. Since
the comparator input contains information about the output volt-
age and the control loop is arranged to form a negative feed-
back 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 FAN5330, 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). 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_R---^E----^F-
6
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FAN5330 Rev. 1.0.1