Datasheet

2V/div

VOUT

5V/div

500mA/div

t-1ms/div

TPS61175

SLVS892B –DECEMBER 2008–REVISED FEBRUARY 2012

www.ti.com

Soft Startup

Figure 12.

DETAILED DESCRIPTION

OPERATION

The TPS61175 integrates a 40-V low side switch FET for up to 38-V output. The device regulates the output with

current mode PWM (pulse width modulation) control. The PWM control circuitry turns on the switch at the

beginning of each switching cycle. The input voltage is applied across the inductor and stores the energy as

inductor current ramps up. During this portion of the switching cycle, the load current is provided by the output

capacitor. When the inductor current rises to the threshold set by the error amplifier output, the power switch

turns off and the external Schottky diode is forward biased. The inductor transfers stored energy to replenish the

output capacitor and supply the load current. This operation repeats each every switching cycle. As shown in the

block diagram, the duty cycle of the converter is determined by the PWM control comparator which compares the

error amplifier output and the current signal. The switching frequency is programmed by the external resistor or

synchronized to an external clock signal.

A ramp signal from the oscillator is added to the current ramp to provide slope compensation. Slope

compensation is necessary to avoid subharmonic oscillation that is intrinsic to the current mode control at duty

cycle higher than 50%. If the inductor value is lower than 4.7μH, the slope compensation may not be adequate.

The feedback loop regulates the FB pin to a reference voltage through a transconductance error amplifier. The

output of the error amplifier is connected to the COMP pin. An external RC compensation network is connected

to the COMP pin to optimize the feedback loop for stability and transient response.

SWITCHING FREQUENCY

The switch frequency is set by a resistor (R4) connected to the FREQ pin of the TPS61175. Do not leave this pin

open. A resistor must always be connected for proper operation. See Table 1 and Figure 13 for resistor values

and corresponding frequencies.

Table 1. Switching Frequency vs External Resistor

R4 (kΩ) f

(kHz)

443 240

256 400

176 600

80 1200

51 2000

Product Folder Link(s): TPS61175