0

500

1000

1500

2000

2500

3000

3500

10

100

1000

ExternalResistor-kW

TPS61175

www.ti.com

SLVS892A – DECEMBER 2008 – REVISED OCTOBER 2010

Figure 13. Switching Frequency vs External Resistor

Alternatively, the TPS61175 switching frequency will synchronize to an external clock signal that is applied to the

SYNC pin. The logic level of the external clock is shown in the specification table. The duty cycle of the clock is

recommended in the range of 10% to 90%. The resistor also must be connected to the FREQ pin when IC is

switching by the external clock. The external clock frequency must be within ±20% of the corresponding

frequency set by the resistor. For example, if the corresponding frequency as set by a resistor on the FREQ pin

is 1.2-MHz, the external clock signal should be in the range of 0.96-MHz to 1.44-MHz.

If the external clock signal is higher than the frequency per the resistor on the FREQ pin, the maximum duty

SOFT START

The TPS61175 has a built-in soft start circuit which significantly reduces the start-up current spike and output

voltage overshoot. When the IC is enabled, an internal bias current (6-mA typically) charges a capacitor (C3) on

the SS pin. The voltage at the capacitor clamps the output of the internal error amplifier that determines the duty

cycle of PWM control, thereby the input inrush current is eliminated. Once the capacitor reaches 1.8-V, the soft

start cycle is completed and the soft start voltage no longer clamps the error amplifier output. Refer to Figure 13

for the soft start waveform. See Table 2 for C3 and corresponding soft start time. A 47-nF capacitor eliminates

the output overshoot and reduces the peak inductor current for most applications.

Table 2. Soft Start Time vs C3

Vin (V)

Vout (V)

Load (A)

Cout (mF)

C3 (nF)

Overshot (mV)

47

4

none

5

24

0.4

10

1.2

10

0.8

210

100

6.5

none

12

35

0.6

10

2

10

0.4

300

When the EN is pulled low for 10-ms, the IC enters shutdown and the SS capacitor discharges through a 5k

Ω

resistor for the next soft start.

OVERCURRENT PROTECTION

The TPS61175 has a cycle-by-cycle overcurrent limit protection that turns off the power switch once the inductor

current reaches the overcurrent limit threshold. The PWM circuitry resets itself at the beginning of the next switch

cycle. During an overcurrent event, the output voltage begins to droop as a function of the load on the output.

When the FB voltage drops lower than 0.9-V, the switching frequency is automatically reduced to 1/4 of the set

value. The switching frequency does not reset until the overcurrent condition is removed. This feature is disabled

during soft start.

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