I

V

FB

R

SET

TPS61165

SLVS790A – NOVEMBER 2007 – REVISED MAY 2010

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DETAILED DESCRIPTION

OPERATION

The TPS61165 is a high efficiency, high output voltage boost converter in small package size, The device 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/1.2A switch FET and operates in pulse width modulation (PWM) with 1.2MHz 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 40%. 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, each step takes 213ms. 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 700mA (typical). These two features ensure smooth start-up and minimize

the inrush current. See the start-up waveform of a typical example (Figure 10).

OPEN LED PROTECTION

Open LED protection circuitry prevents IC damage as the result of white LED disconnection. The TPS61165

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 TPS61165 has different open lamp protection thresholds to prevent

the internal 40V FET from breaking down. The threshold is set at 38V.

SHUTDOWN

The TPS61165 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 1mA (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.

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 RSET is calculated using Equation 1 .

(1)

Where:

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

8

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