ss

ref

ss

ss

C

(nF)

V

(V)

t

(ms)

I

( A)

´

=

m

TPS5403

www.ti.com

SLVSBK5 – SEPTEMBER 2012

Pulse Skipping Mode

The TPS5403 is designed to operate in pulse skipping mode at light load currents to boost light load efficiency.

When the peak inductor current is lower than 300 mA typically, the COMP pin voltage falls to 0.5 V typically and

the device enters pulse skipping mode. When the device is in pulse skipping mode, the COMP pin voltage is

clamped at 0.5 V internally which prevents the high side integrated MOSFET from switching. The peak inductor

current must rise above 300 mA for the COMP pin voltage to rise above 0.5 V and exit pulse skipping mode.

Since the integrated current comparator catches the peak inductor current only, the average load current

entering pulse skipping mode varies with the applications and external output filters.

The voltage reference system produces a ±3% accuracy voltage reference by scaling the output of a temperature

stable 3.3-V output voltage, a minimum 15-mA current is needed to apply at the output.

Bootstrap Voltage (BOOT)

The TPS5403 has an integrated boot regulator and requires a 0.1-µF ceramic capacitor between the BOOT and

LX pins to provide the gate drive voltage for the high-side MOSFET. A ceramic capacitor with an X7R or X5R

grade dielectric is recommended because of the stable characteristics over temperature and voltage. To improve

drop out, the TPS5403 is designed to operate at 100% duty cycle as long as the BOOT to LX pin voltage is

greater than 2.1 V typically.

Programmable Slow Start Using SS Pin

It is recommended to program the slow start time externally because no slow start time is implemented internally.

The TPS5403 effectively uses the lower voltage of the internal voltage reference or the SS pin voltage as the

power supply’s reference voltage fed into the error amplifier and will regulate the output accordingly. A capacitor

of 2

μA that charges the external slow start capacitor. The equation for the slow start time (10% to 90%) is

(2)

The slow start time should be set between 1 ms to 10 ms to ensure good start-up behavior. The slow start

capacitor should be no more than 27 nF.

If during normal operation, the input voltage drops below the VIN UVLO threshold, or a thermal shutdown event

occurs, the TPS5403 stops switching.

Error Amplifier

The TPS5403 has a transconductance amplifier for the error amplifier. The error amplifier compares the

VSENSE voltage to the internal effective voltage reference presented at the input of the error amplifier. The

transconductance of the error amplifier is 92

μA/V during normal operation. Frequency compensation

components are connected between the COMP pin and ground.

Slope Compensation

To prevent the sub-harmonic oscillations when operating the device at duty cycles greater than 50%, the

TPS5403 adds a built-in slope compensation which is a compensating ramp to the switch current signal.

Overcurrent Protection and Frequency Shift

The TPS5403 implements current mode control that uses the COMP pin voltage to turn off the high-side

MOSFET on a cycle by cycle basis. Every cycle the switch current and the COMP pin voltage are compared;

when the peak inductor current intersects the COMP pin voltage, the high-side switch is turned off. During

overcurrent conditions that pull the output voltage low, the error amplifier responds by driving the COMP pin high,

causing the switch current to increase. The COMP pin has a maximum clamp internally, which limits the output

current.

Copyright © 2012, Texas Instruments Incorporated

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