Datasheet

I
LED
+
V
FB
R
SET
TPS61160
TPS61161
SLVS791C –NOVEMBER 2007–REVISED APRIL 2012 www.ti.com
DETAILED DESCRIPTION
OPERATION
The TPS61160/1 is a high efficiency, high output voltage boost converter in a small package size. The device is
ideal for driving white LED in series. The serial LED connection provides even illumination by sourcing the same
output current through all LEDs, eliminating the need for expensive factory calibration. The device integrates
40V/0.7A switch FET and operates in pulse width modulation (PWM) with 600kHz fixed switching frequency. For
operation see the block diagram. The duty cycle of the converter is set by the error amplifier output and the
current signal applied to the PWM control comparator. The control architecture is based on traditional current-
mode control; therefore, a slope compensation is added to the current signal to allow stable operation for duty
cycles larger than 50%. The feedback loop regulates the FB pin to a low reference voltage (200mV typical),
reducing the power dissipation in the current sense resistor.
SOFT START-UP
Soft-start circuitry is integrated into the IC to avoid a high inrush current during start-up. After the device is
enabled, the voltage at FB pin ramps up to the reference voltage in 32 steps with each step taking 213μs. This
ensures that the output voltage rises slowly to reduce the input current. Additionally, for the first 5msec after the
COMP voltage ramps, the current limit of the switch is set to half of the normal current limit spec. During this
period, the input current is kept below 400mA (typical). See the start-up waveform of a typical example,
Figure 11.
OPEN LED PROTECTION
Open LED protection circuitry prevents IC damage as the result of white LED disconnection. The TPS61160/1
monitors the voltage at the SW pin and FB pin during each switching cycle. The circuitry turns off the switch FET
and shuts down the IC when both of the following conditions persist for 8 switching clock cycles: (1) the SW
voltage exceeds the V
OVP
threshold and (2) the FB voltage is less than half of regulation voltage. As a result, the
output voltage falls to the level of the input supply. The device remains in shutdown mode until it is enabled by
toggling the CTRL pin logic. To allow the use of inexpensive low-voltage output capacitor, the TPS61160/1 has
different open lamp protection thresholds. The threshold is set at 26V for the TPS61160 and 38V for the
TPS61161. Select the appropriate device so that the product of the number of external LEDs and each LED's
maximum forward voltage plus the 200mV reference voltage does not exceed the minimum OVP threshold or
(n
LEDS
X V
LED(MAX)
) + 200 mV ≤ V
OVP(MIN).
SHUTDOWN
The TPS61160/1 enters shutdown mode when the CTRL voltage is logic low for more than 2.5ms. During
shutdown, the input supply current for the device is less than 1μA (max). Although the internal FET does not
switch in shutdown, there is still a DC current path between the input and the LEDs through the inductor and
Schottky diode. The minimum forward voltage of the LED array must exceed the maximum input voltage to
ensure that the LEDs remain off in shutdown. However, in the typical application with two or more LEDs, the
forward voltage is large enough to reverse bias the Schottky and keep leakage current low.
CURRENT PROGRAM
The FB voltage is regulated by a low 0.2V reference voltage. The LED current is programmed externally using a
current-sense resistor in series with the LED string. The value of the R
SET
is calculated using Equation 1:
(1)
Where
I
LED
= output current of LEDs
V
FB
= regulated voltage of FB
R
SET
= current sense resistor
The output current tolerance depends on the FB accuracy and the current sensor resistor accuracy.
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