SLMA002 datasheet(17/46 Pages) TI1

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Output Voltage Regulation

REF

OUT

- VREF

R2=R1´

(2)

PVDD2

BOOT2

SW2

PVDD1

BOOT1

SW1

GND

TPS5538x

SEQ

ILIM2

FB2

EN1

EN2

FB1

OUTPUT1

UDG-08041

COMP2

COMP1

Feedback Loop Compensation Component Selection

TPS55383

,, TPS55386

www.ti.com ......................................................................................................................................................................................... SLUS818 – SEPTEMBER 2008

Each output has a dedicated feedback loop comprised of a voltage setting divider, an error amplifier, a pulse

width modulator, and a switching MOSFET. The regulation output voltage is determined by a resistor divider

connecting the output node, the FBx pin, and GND (see Figure 21). Assuming the value of the upper voltage

setting divider is known, the value of the lower divider resistor for a desired output voltage is calculated by

Equation 2.

where

• V

REF is the internal 0.8-V reference voltage

Figure 21. Voltage Setting Divider Network for Channel 1

DESIGN HINT

There is a leakage current of up to 12

µA out of the SW pin when a single output of

the TPS5538x is disabled. Keeping the series impedance of R1 + R2 less than 50 k

Ω

prevents the output from floating above the referece voltage while the controller output

is in the OFF state.

In the feedback signal path, the output voltage setting divider is followed by an internal gM-type error amplifier

with a typical transconductance of 315

µS. An external series connected R-C circuit from the g

M amplifier output

(COMPx pin) to ground serves as the compensation network for the converter. The signal from the error amplifier

output is then buffered and combined with a slope compensation signal before it is mirrored to be referenced to

the SW node. Here, it is compared with the current feedback signal to create a pulse-width-modulated (PWM)

signal-fed to drive the upper MOSFET switch. A simplified equivalent circuit of the signal control path is depicted

in Figure 22.

NOTE:

Noise coupling from the SWx node to internal circuitry of BOOTx may impact narrow

pulse width operation, especially at load currents less than 1 A. See SW Node

Ringing for further information on reducing noise on the SWx node.

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