A6727 datasheet(16/29 Pages) STMICROELECTRONICS

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Application details

A6727

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DocID025003 Rev 1

Application details

9.1

Output voltage selection

The A6727 is capable of precisely regulating an output voltage as low as 0.8 V. In fact, the

device has a fixed 0.8 V internal reference that guarantees the output regulated voltage

within ±1% tolerance over line and temperature variations between 0 °C and +70 °C

(excluding output resistor divider tolerance, when present).

The output voltage higher than 0.8 V can be easily achieved by adding a resistor ROS

between FB pin and ground. Referring to Figure 1, the steady-state DC output voltage is:

Equation 4

where VREF is 0.8 V.

9.2

Compensation network

The control loop showed in Figure 8 is a voltage mode control loop. The error amplifier is a

voltage mode type. The output voltage is regulated to the internal reference (when present,

the offset resistor, between FB node and GND, can be neglected in control loop calculation).

Error amplifier output is compared with oscillator sawtooth waveform to provide the driver

section with the PWM signal. The PWM signal is then transferred to the switching node with

VIN amplitude. This waveform is filtered by the output filter.

The converter transfer function is the small signal transfer function between the output of the

EA and VOUT. This function has a double pole at frequency FLC depending on the L-COUT

resonance and a zero at FESR depending on the output capacitor ESR. The DC gain of the

modulator is simply the input voltage VIN divided by the peak-to-peak oscillator voltage

ΔVOSC.

The compensation network closes the loop joining VOUT and EA output with transfer

function ideally equal to -ZF/ZFB.

OUT

REF

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