FAN5340 Datasheet, PDF (9/12 Pages) Fairchild Semiconductor – Synchronous Constant-Current Series Boost LED Driver with PWM Brightness Control and Integrated Load Disconnect

Circuit Description
Overview
The FAN5340 is an inductive current-mode boost serial LED
driver that achieves LED current regulation by maintaining
0.5V across RSET. The current through the LED string (ILED)
is therefore:
I _LED
=
0.5
R_SET
(1)
While the forward-voltage across the LEDs determines VOUT,
the FAN5340’s boost regulator output can also support
additional loads on VOUT (see Figure 21) provided its input
current limit is not exceeded.
Over-Voltage Protection
If the LED string is open circuit, FB remains at 0V and the
output voltage continues to increase in the absence of an
Over-Voltage Protection (OVP) circuit. The FAN5340’s OVP
circuit disables the boost regulator when VOUT exceeds
19.0V and continues to keep the regulator off until VOUT
drops below 18.2V.
Thermal Shutdown
If the die temperature exceeds 150°C, a reset occurs and
remains in effect until the die cools to 125°C, at which time
the circuit is allowed to begin the soft-start sequence.
Applications
Figure 21. Maximum Output Current
vs. Input Voltage
UVLO and Soft-Start
If EN has been LOW for more than 20ms, the IC initiates a “cold
start” soft-start cycle when EN rises, provided VIN is above the
UVLO threshold. The soft-start circuit ramps the voltage
reference to the error amplifier to control inrush current.
PWM Dimming
When EN goes LOW, the IC turns off a MOSFET (Q3 in
Figure 2), which disconnects the LED load, preventing COUT
from being discharged when EN is LOW. As long as EN is
low for less than 20ms, the regulator’s main regulation loop
quickly regains control when EN returns to a HIGH state.
Short-Circuit Detection
If VOUT falls below VIN – 1.5V, Q3 turns off and remains off
until VOUT recovers to at least VIN – 1.3V.
Using VOUT to Drive Additional LED Strings
The VOUT pin can be used as a supply for simple current
sources (shown in Figure 22 using the FAN5640) or discrete
current sinks. To avoid dragging VOUT down when the EN pin
is LOW, the auxiliary strings should not be enabled unless
the EN pin is HIGH. The auxiliary strings can therefore be
PWM dimmed using either the same line as the EN line as
shown below or enabled separately, but within the on-time of
the FAN5340.
IOUT1 ₁
6 ^IOUT2
GND ₂
R1 _{RSET 3}
FAN5640
5 ^VIN
4 ^EN
GND ₁
EN ₂
8 ^VIN
7 ^VOUT
C_OUT
C_IN
VLED ₃ ^FAN5340 ₆ SW
FB ₄
5 ^PGND
R2
Figure 22. Driving Additional LED Strings
If using VOUT to drive additional loads, care should be taken
not to exceed the input current limit. This limitation is shown in
Figure 21 for a typical IC. The total load (IOUT1 + IOUT2 + ILED)
should always remain below 70% of the value in Figure 21.
© 2010 Fairchild Semiconductor Corporation
FAN5340 • Rev. 1.0.1
9
www.fairchildsemi.com