Datasheet

V
OVP
=
2.0 x (R19+ R18)
R18
R19 = R18 x =
V
OUT(MAX)
- 1.85V
1.85V
VSS/SH
1V/Div
ILED
100 mA/Div
Vout
10V/Div
VEN
5V/Div
4 ms/DIV
C6 =
t
ss
x 19 PA
1.85V
LM3431
www.ti.com SNVS547G –NOVEMBER 2007–REVISED MAY 2013
Soft-Start
The SS/SH pin is a dual function softstart and sample/hold pin. The SH function is described in sections below.
When the EN pin is pulled above the programmable UVLO threshold and VCC rises above the internal UVLO
threshold, the SS/SH pin begins sourcing current. This charges the SS cap (C6) and the SS pin voltage in turn
controls the output voltage ramp-up, sensed via the AFB pin. The softstart capacitor is calculated as shown
below, where 20 µA is the typical softstart source current:
(3)
The LED current regulators are held off until softstart is completed. During softstart, current limit is active and the
CFB pin is monitored for a cathode short fault (See LED Protection section). When the SS/SH voltage reaches
1.85V, the current regulators are activated, LED current begins flowing, and output voltage control is transferred
to the CFB pin. Typical startup is shown below in Figure 15.
Figure 15. Typical Startup Waveforms (From power-on, DIM = high)
Output Voltage, OVP, and SH
The LM3431 boost controls the LED cathode voltage in order to drive the LED strings with sufficient headroom at
optimum efficiency. When the LED strings are on, voltage is regulated to 1.7V (typical) at the CFB pin, which is
one diode Vf above the LED cathode voltage. Therefore, when the LED strings are on, the output voltage (LED
anodes, shown as VA in Figure 14) will vary according to the Vf of the LED string, while the LED cathode voltage
will be regulated via the CFB pin.
The AFB pin is used to regulate the output voltage when the LED strings are off, which is during startup and
dimming off cycles. During LED-off times, the cathode voltage is not regulated.
AFB should set the initial output voltage to at least 1.0V (CFB voltage minus one diode drop) above the
maximum LED string forward voltage. This ensures that there is enough headroom to drive the LED strings at
startup and keeps the SS/SH voltage below its maximum. The AFB pin voltage at the end of softstart is 1.85V
typically, which determines the ratio of the feedback resistors according to the following equation:
(4)
The AFB resistors also set the output over-voltage (OVP) threshold. The OVP threshold is monitored during both
LED on and LED off states and protects against any over voltage condition, including all LEDs open (See Open
LED section).The OVP threshold at Vout can be calculated as follows:
(5)
Copyright © 2007–2013, Texas Instruments Incorporated Submit Documentation Feedback 11
Product Folder Links: LM3431