MAX38800 datasheet(21/28 Pages) MAXIM

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Thus, when the ramp injection components (RR, CR1,

CR2), lead compensation components (CLEAD, RLEAD)

and lag compensation components (RLAG, CLAG) are not

used, the approximate loop gain and bandwidth (BW) are

given by the following equations.

Equation 9

DIV

SENSE(GAIN)

OUT

DIV

SENSE(GAIN)

OUT

GAIN_EFF

OUT

Loop_Gain(f)

or BW

× π×

where RGAIN_EFF = RSENSE(GAIN)/KDIV

For stability, COUT should be chosen so that BW < fSW/3.

Designing with no loop compensation can result in fairly

large COUT; compensation schemes such as lead, lag

and ramp injection can be used to allow COUT reduction.

These compensations impact the transient performance

as they change the BW of the system. This should be

included in design analysis.

Integrator

The IC has an integrator included in its error amplifier to

improve load regulation. The integrator only adds gain at

low frequencies, so it does not affect the loop BW; there-

fore, it was not considered in previous equations. With

integrator, the loop gain from Equation 9 is multiplied by a

factor of (1/tREC + s)/s

where tREC is 20µs.

Step Response

RGAIN_EFF determines the small-signal transient

response of the regulator. When a load step is applied

that does not exceed the slew rate capability of the inductor

current, the regulator responds linearly and VOUT

temporarily changes by the amount of VOUT_ERROR (see

Equation 10a). If the load step applied exceeds the slew

rate capability of the inductor current, the voltage deviation

(VOUT_ERROR) is solely determined by output filter

values (See Equation 10a).

The actual voltage deviation (VOUT_ERROR) is given by

the largest of the values calculated using Equation 10a

and Equation 10b.

Equation 10

( )

OUT_ERROR

STEP

GAIN_EFF

OUT_ERROR

OUT

≈

××

After a transient event, VOUT returns to the nominal value

with a 20µs time constant, due to the integrator circuit. A

first order average small-signal model of the regulator is

shown in Figure 7. VEQ is an ideal voltage source equal

to VOUT, REQ (RGAIN_EFF) is an emulated lossless resistance

created by the control loop action and LEQ (tREC ×

RGAIN_EFF) is an emulated inductance. Note that LEQ

is not the same as the actual LOUT inductor which has

been absorbed into the model. COUT is the actual output

capacitance.

Figure 7. Averaged Small-Signal Equivalent Circuit of Regulator

REQ

LEQ

COUT

VEQ

MAX38800

Integrated, Step-Down Switching Regulator With

Selectable Applications Configurations

www.maximintegrated.com

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