MAX20004E datasheet(16/21 Pages) MAXIM

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∆ IL =

(VSUP−VOUT)×VOUT

VSUP × fSW × L

and:

CIN =

IOUT × D(1 − D)

∆ VQ × fSW

D =

VOUT

VSUPSW

where: IOUT is the maximum output current and D is the duty cycle.

Output Capacitor

Output capacitance is selected to satisfy the output load-transient, output-voltage ripple, and closed-loop stability

requirements. During a load step, the output current changes almost instantaneously, whereas the inductor is slow to

react. During this transition time, the load-charge requirements are supplied by the output capacitor, which causes an

undershoot/overshoot in the output voltage. For a buck converter that is controlled by peak-current, as employed in

MAX20004E/MAX20006E/MAX20008E, output capacitance also affects the control-loop stability.

The output ripple comprises ΔVQ (caused by the capacitor discharge) and ΔVESR(caused by the ESR of the output

capacitor). Use low-ESR ceramic or aluminum electrolytic capacitors at the output. For aluminum electrolytic capacitors,

the entire output ripple is contributed by ΔVESR. Use Equation 4 to calculate the ESR requirement and choose the

capacitor accordingly. If using ceramic capacitors, assume the contribution to the output-ripple voltage from the ESR and

the capacitor discharge to be equal. The following equations show the output capacitance and ESR requirement for a

specified output-voltage ripple.

Equation 9:

ESR =

ΔVESR

ΔIp−p

COUT =

ΔIp−p

8 × ΔVQ × fSW

where

ΔIp − p =

(VIN − VOUT) × VOUT

VIN × fSW × L

VOUT_RIPPLE = ΔVESR + ΔVQ

ΔIP-P is the peak-to-peak inductor current as calculated above, and fSW is the converter’s switching frequency.

The output capacitor supplies the step-load current until the converter responds with a greater duty cycle. The resistive

drop across the output capacitor’s ESR and the capacitor discharge causes a voltage droop during a step load. Use a

combination of low-ESR tantalum and ceramic capacitors for better transient-load and ripple/noise performance. Keep

the maximum output-voltage deviations below the tolerable limits of the electronics being powered. When using a ceramic

capacitor, assume an 80% and 20% contribution from the output-capacitance discharge and the ESR drop, respectively.

Use the following equations to calculate the required ESR and capacitance value:

Equation 10:

ESROUT =

ΔVESR

ISTEP

COUT ≥

(

ISTEP

2 × L

2 × (VIN − VOUT) × DMAX × ΔVQ

)+(ISTEP×tDELAY

ΔVQ

)

where ISTEP is the load step and tDELAY is the delay for the PWM mode, the worst-case delay would be (1-D) tSW when

MAX20004E/MAX20006E/

MAX20008E

Automotive, 36V, 4A/6A/8A Integrated Step-Down

Converters with Integrated Compensation

www.maximintegrated.com

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