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If ZESR > fC, it can be ignored, as is typically the case

with ceramic output capacitors. If ZESR is less than fC,

it should be cancelled with a pole set by capacitor CP

connected from CCSU to GND:

CP = COUT x RESR / RC

If CP is calculated to be <10pF, it can be omitted.

Step-Down Component Selection

Step-Down Inductor

The external components required for the step-down

are an inductor, input and output filter capacitors, and

a compensation RC network.

The MAX1585/1585 step-down converter provides best

efficiency with continuous inductor current. A reason-

able inductor value (LIDEAL) can be derived from the

following:

LIDEAL = [2(VIN) x D(1 - D)] / IOUT x fOSC

which sets the peak-to-peak inductor current at half the

DC inductor current. D is the duty cycle:

D = VOUT / VIN

Given LIDEAL, the peak-to-peak inductor current is 0.5 x

IOUT. The absolute peak inductor current is 1.25 x IOUT.

Inductance values smaller than LIDEAL can be used to

reduce inductor size; however, if much smaller values

are used, inductor current rises and a larger output

capacitance may be required to suppress output ripple.

Larger values than LIDEAL can be used to obtain higher

output current, but with typically larger inductor size.

Step-Down Compensation

The relevant characteristics for step-down compensa-

tion are as follows:

• Transconductance (from FBSD to CCSD), gMEA

(135µS)

• Current-sense amplifier transresistance, RCS (0.6V/A)

• Feedback regulation voltage, VFB (1.25V)

• Step-down output voltage, VSD, in V

• Output load equivalent resistance, RLOAD, in

Ω = VSD / ILOAD

The key steps for step-down compensation are as fol-

lows:

1) Set the compensation RC zero to cancel the RLOAD

COUT pole.

2) Set the loop crossover below 1/10 the switching fre-

quency.

If we assume VIN = 3.5V, VOUT = 1.5V, and IOUT =

250mA, then RLOAD = 6

Ω.

If we select fOSC = 500kHz and L = 22µH,

choose fC = 24kHz and calculate CC:

CC = (VFB / VOUT)(RLOAD / RCS)(gM / 2

π x fC)

= (1.25 / 1.5)(6 / 0.6) x (135µS / (6.28 x 40kHz))

= 4.5nF

Choose 4.7nF.

Now select RC so transient-droop requirements are met.

For example, if 4% transient droop is allowed, the input

to the error amplifier moves 0.04 x 1.25V, or 50mV. The

error-amp output drives 50mV x 135µS, or 6.75µA across

RC to provide transient gain. Since the current-sense

transresistance is 0.6V/A, the value of RC that allows the

required load step swing is as follows:

RC = 0.6 x IIND(PK) / 6.75µA

In a step-down DC-DC converter, If LIDEAL is used, out-

put current relates to inductor current by the following:

IIND(PK) = 1.25 x IOUT

So for a 250mA output load step with VIN = 3.5V and

VOUT = 1.5V:

RC = (1.25 x 0.6 x 0.25) / 6.75µA = 27.8k

Ω

Choose 27k

Ω.

The inductor does somewhat limit the response in this

case since it ramps at (VIN - VOUT) / 22µH, or (3.5 - 1.5)

/ 22µH = 90mA/µs.

The output filter capacitor is then chosen so the COUT

RLOAD pole cancels the RC CC zero:

COUT x RLOAD = RC x CC

For the example:

COUT = 27k

Ω x 4.7nF / 6Ω = 21µF

Choose 22µF or greater.

If the output filter capacitor has significant ESR, a zero

occurs at:

ZESR = 1 / (2

π x COUT x RESR)

If ZESR > fC, it can be ignored, as is typically the case

with ceramic output capacitors. If ZESR is less than fC,

it should be cancelled with a pole set by capacitor CP

connected from CCSD to GND:

CP = COUT x RESR / RC

If CP is calculated to be <10pF, it can be omitted.

AUX Controller Component Selection

External MOSFET

MAX1584/MAX1585 AUX1(step-up) controllers drive

external logic-level N-channel MOSFETs. AUX3 (step-

down) controllers drive P-channel MOSFETs. AUX2

(step-up) on the MAX1584 drives an N channel, while

AUX2 (inverting) on the MAX1585 drives a P channel.

5-Channel Slim DSC Power Supplies

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