SS

SS

I

t

C

(Farads)

V

´ D

=

D

SW

9

SW

X

f

(X

24 k

MHz)

RT

10

f

24

RT

=

=

W ´

=

´

TPS43337-Q1

www.ti.com

SLVSBC2A – AUGUST 2013 – REVISED SEPTEMBER 2013

DETAILED DESCRIPTION

BUCK CONTROLLERS: NORMAL-MODE PWM OPERATION

Frequency Selection and External Synchronization

The buck controllers operate using constant-frequency peak-current-mode control for optimal transient behavior

and ease of component choices. The switching frequency is programmable between 150 kHz and 600 kHz,

depending upon the resistor value at the RT pin. A short circuit to ground at this pin sets the default switching

frequency to 400 kHz. the frequency is also set by a resistor at RT according to Equation 1.

(1)

For example,

600 kHz requires 40 k

Ω.

150 kHz requires 160 k

Ω.

Synchronizing to an external clock at the SYNC pin in the same frequency range of 150 to 600 kHz is also

possible. The device detects clock pulses at this pin, and an internal PLL locks on to the external clock within the

specified range. The device also detects a loss of clock at this pin, and on detecting this loss, the device sets the

switching frequency to the internal oscillator. The two buck controllers operate at identical switching frequencies,

180 degrees out of phase.

Enable Inputs

Independent enable inputs from the ENA and ENB pins enable the buck controllers. These are high-voltage pins,

with a threshold of 1.5 V for high level, and with direct connection directly to the battery for self-bias. The low

threshold is 0.7 V. Both these pins have internal pullup currents of 0.5 µA (typical). As a result, an open circuit on

these pins enables the respective buck controllers. When both buck controllers are disabled, the device shuts

down and consumes a current less than 4 µA.

Feedback Inputs

An internal voltage divider presets the output voltage. Connect each FBx pin to the output of the respective

regulator of the pin.

Soft-Start Inputs

In order to avoid large inrush currents, each buck controller has an independent, programmable soft-start timer.

The voltage at the SSx pins acts as the soft-start reference voltage. A 50-µA pullup current available at the SSx

pins, in combination with a suitably chosen capacitor, generates a ramp of the desired soft-start speed. After

start-up, the pullup current ensures that this node is higher than the internal reference of 0.8 V; 0.8 V then

becomes the reference for the buck controllers. Equation 2 calculates the soft-start ramp time.

where,

•

•

∆V = 0.8 V

•

(2)

Alternatively, the soft-start pins are used as tracking inputs. In this case, connect these pins to the supply to be

tracked via a suitable resistor-divider network.

Copyright © 2013, Texas Instruments Incorporated

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