MAXM17536 datasheet(18/22 Pages) MAXIM

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Output-Capacitor Selection

X7R ceramic output capacitors are preferred due to their

stability over temperature in industrial applications. The

output capacitors are usually sized to support a step load

of 50% of the maximum output current in the application,

so the output-voltage deviation is contained to 3% of the

output-voltage change. The minimum required output

capacitance can be calculated as follows:

COUT =

2 ×

ISTEP × tRESPONSE

∆ VOUT

tRESPONSE

≅ (0.33f

fSW

)

where:

ISTEP = Load-current step,

tRESPONSE = Response time of the controller,

VOUT = Allowable output-voltage deviation,

fC = Target closed-loop crossover frequency,

fSW = Switching frequency.Select fC to be 1/10th of fSW if

the swtiching frequency is less than or equal to 400kHz.

Select fC to be 40kHz if the switching frequency is more

than 400kHz.

Soft-Start Capacitor Selection

The MAXM17536 implements adjustable soft-start operation

to reduce inrush current. A capacitor connected from

the SS pin to SGND programs the soft-start time. The

selected output capacitance (CSEL) and the output voltage

(VOUT) determine the minimum required soft-start

capacitor as follows:

CSS ≥ 28 × 10−6 × CSEL × VOUT

The soft-start time (tSS) is related to the capacitor

connected at SS (CSS) by the following equation:

tSS =

CSS

5.55

where tSS is in milliseconds and CSS is in nanofarads.

For example, to program a 4ms soft-start time, a 22nF

capacitor should be connected from the SS pin to SGND.

Setting the Input Undervoltage-Lockout Level

The MAXM17536 offers an adjustable input undervoltage

lockout level. Set the voltage at which MAXM17536 turns

on. Calculate R3 as follows:

R3 = 3.32 × 1.215

(VINU−1.215)

where R3 is in MΩ and VINU is the voltage at which the

MAXM17536 is required to turn on. Ensure that VINU is

higher than 0.8 x VOUT.

Loop Compensation

The MAXM17536 is internally loop-compensated. Connect

a 2.2pF capacitor from CF to FB for stable operation.

Typically, designs with crossover frequency (fC) less than

fSW/10 and less than 40kHz offers good phase margin

and transient response. For other choices of fC, the

design should be carefully evaluated according to user

requirements.

Adjusting Output Voltage

Set the output voltage with a resistive voltage-divider

connected from the positive terminal of the output capacitor

(VOUT) to SGND (see Figure 2). Connect the center

node of the divider to the FB pin. To choose the resistive

voltage-divider values calculate for resistor R1, then R2.

First, calculate resistor R1 from the output to FB as follows:

where:

R1 is in kΩ

fC = Desired crossover frequency (kHz)

COUT = Derated value of the capacitor (µF)

Then, calculate resistor R2 from FB to SGND as follows:

R2 = R1 × 0.9

(VOUT−0.9)

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