DETAILED DESCRIPTION

OPERATION

SOFT START-UP

OPEN LED PROTECTION

SHUTDOWN

CURRENT PROGRAM

I

V

FB

R

SET

(1)

TPS61160A

TPS61161A

The TPS61160A/61A is a high efficiency, high output voltage boost converter in small package size that is ideal

for driving up to 10 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 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, each step takes 213

µs. 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 10.

Open LED protection circuitry prevents IC damage as the result of white LED disconnection. The

TPS61160A/61A 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 as soon as the SW voltage exceeds the Vovp threshold and the FB

voltage is less than half of regulation voltage for 8 clock cycles. 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 TPS61160A/61A has different open lamp

protection thresholds to prevent the internal 40V FET from breaking down. The threshold is set at 26V for the

TPS61160A and 38V for the TPS61161A. The devices can be selected according to the number of external

LEDs and their maximum forward voltage.

The TPS61160A/61A 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 1

µA (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.

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 RSET is calculated using Equation 1:

Where

The output current tolerance depends on the FB accuracy and the current sensor resistor accuracy.

8

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