MAX20004 datasheet(14/20 Pages) MAXIM

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Compensation Network

The devices use a transconductance amplifier for external

frequency compensation. The compensation network in

conjunction with the output capacitance primarily deter-

mine the loop stability and response. The inductor and the

output capacitor are chosen based on performance, size,

and cost. The compensation network is used to optimize

the loop stability and response.

The converter uses a peak current mode control scheme

that regulates the output voltage by forcing the required

peak current through the external inductor. The devices

use the voltage drop across the high-side MOSFET to

sense inductor current. Current-mode control eliminates

the double pole in the feedback loop caused by the induc-

tor and output capacitor, resulting in a smaller phase shift

and requiring less elaborate error-amplifier compensation

than voltage-mode control.

The final control system can be modeled according to

Figure 2 from which the following transfer function is

derived:

OUT

REF

(s)

C(s)M(s)

1 F(s)C(s)M(s)

where M(s), C(s) and F(s) are the modulator, compensator

and feedback transfer functions, respectively, VOUT is the

regulated output voltage and VREF is the internal voltage

reference. The product of the modulator, compensator and

feedback transfer functions is typically referred to as the

loop transfer function.

A simplified condition for stability is that the denominator

of the transfer function never equals zero. Accordingly,

the loop transfer function should never equal -1, which

correspondingly means that the phase must not equal

-180 degrees when the magnitude equals 1. In addition,

the loop gain should be much less than zero when the

phase equals -180 degrees. The frequency at which the

magnitude of the loop gain equals 1 (or 0dB) is defined as

the crossover frequency (fc). The difference between the

loop phase at the crossover frequency and -180 degrees

is defined as the phase margin. The phase margin rep-

resents the additional loop phase lag that must occur at

the crossover frequency for the system to be unstable.

In addition to stability, phase margin is also related to

the transient response of the system. Insufficient phase

margin causes overshoot and ringing, whereas excessive

phase margin causes slow response.

The goal of the system is to have a high crossover fre-

quency, so there is adequate gain to regulate against load

transients and other variations in the relevant frequency

range, while maintaining adequate phase margin to guard

against instability, overshoot, and ringing. In practice,

these are fundamentally conflicting criteria that must be

managed along with other design goals. According to

sampling theory, the crossover frequency cannot exceed

one half the switching frequency. In practice, noise and

phase margin considerations limit crossover frequency to

below one tenth the switching frequency with a practical

limit of approximately 100kHz.

Figure 2. Control System

C(s)

M(s)

F(s)

VFB

VERR

VCOMP

VOUT

VREF

MAX20004/MAX20006/

MAX20008

36V, 220kHz to 2.2MHz, 4A/6A/8A

Fully Integrated Automotive

Step-Down Converters

www.maximintegrated.com

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