L6725 datasheet(18/27 Pages) STMICROELECTRONICS

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Application details

L6725

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6.4

Compensation network

The loop is based on a voltage mode control (Figure 11). The output voltage is regulated to the

internal/external reference voltage and scaled by the external resistor divider. The error

amplifier output VCOMP is then compared with the oscillator triangular waveform to provide a

pulse-width modulated (PWM) with an amplitude of VIN at the PHASE node. This waveform is

filtered by the output filter. The modulator transfer function is the small signal transfer function

of VOUT/VCOMP. 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’s ESR. The DC Gain of the

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

The compensation network consists in the internal error amplifier, the impedance networks ZIN

(R3, R4 and C20) and ZFB (R5, C18 and C19). The compensation network has to provide a

closed loop transfer function with the highest 0dB crossing frequency to have fastest transient

response (but always lower than fSW/10) and the highest gain in DC conditions to minimize the

load regulation error. A stable control loop has a gain crossing the 0dB axis with -20dB/decade

slope and a phase margin greater than 45°. To locate poles and zeroes of the compensation

networks, the following suggestions may be used:

●

Modulator singularity frequencies:

●

Compensation network singularity frequencies:

Figure 11. Compensation Network

Cout

⋅

Cout

ESR

⋅

(8)

(9)

⎟⎟⎠

⎞

⎜⎜⎝

⎛

⋅

(10)

(11)

⋅

()

⋅

(12)

(13)

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