MAXM17543_V01 datasheet(14/20 Pages) MAXIM

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Design Procedure

Setting the Output Voltage

The MAXM17543 supports an adjustable output

voltage range of 0.9V to 12V from an input voltage range

of 4.5V to 42V by using a resistive feedback divider from

OUT to FB. Table 1 provides the feedback dividers for

desired input and output voltages. Other adjustable output

voltages can be calculated by following the procedure to

choose the resistive voltage-divider values:

Calculate resistor RU from the output to FB as follows:

OUT

216 1000

Where RU is in kΩ, crossover frequency (fC) is in kHz,

and output capacitor (COUT) is in μF. Choose fC to be

1/9th of the switching frequency (fSW) if the switching

frequency is less than or equal to 500kHz. If the switching

frequency is more than 500kHz, select fC to be 55kHz.

OUT

0.9

k , whereR isink .

0.9

ΩΩ

−

Input Voltage Range

The minimum and maximum operating input voltages for

a given output voltage should be calculated as follows:

(

)

(

) (

)

OUT

OUT(MAX)

IN(MIN)

OUT(MAX)

OFF_MIN(MAX)

IN(MIN)

OUT

IN(MAX)

ON_MIN(MAX)

0.22

0.175

1 1.12 f

FOR D 0.4, V

4.26 V

53900

1.12 f

+×

−

××

×−

××

where,

VOUT = Steady-state output voltage

IOUT(MAX) = Maximum load current

fSW = Selected operating switching frequency in Hz

tOFF_MIN(MAX) = Worst-case minimum switch off-time (160ns)

tON_MIN(MAX) = Worst-case minimum switch on-time (80ns)

Input Capacitor Selection

The input capacitor serves to reduce the current peaks

drawn from the input power supply and reduces switching

noise to the IC. The input capacitor values in Table 1 are

the minimum recommended values for desired input and

output voltages. Applying capacitor values larger than those

indicated in Table 1 are acceptable to improve the dynamic

response. For further operating conditions, the total input

capacitance must be greater than or equal to the value given

by the following equation in order to keep the input-voltage

ripple within specifications and minimize the high-frequency

ripple current being fed back to the input source:

IN_AVG

(1 D)

× −

∆

where:

IIN_AVG is the average input current given by:

OUT

IN_AVG

η×

D is the operating duty cycle, which is approximately

equal to VOUT/VIN.

∆VIN is the required input voltage ripple.

fSW is the operating switching frequency.

POUT is the out power, which is equal to VOUT x IOUT.

η is the efficiency.

The input capacitor must meet the ripple-current require-

ment imposed by the switching currents. The RMS input

ripple current is given by:

RMS

OUT

D (1 D)

× −

The worst-case RMS current requirement occurs when

operating with D = 0.5. At this point, the above equation

simplifies to IRMS = 0.5 x IOUT.

For the MAXM17543 system (IN) supply, ceramic capaci-

tors are preferred due to their resilience to inrush surge

currents typical of systems, and due to their low parasitic

inductance that helps reduce the high-frequency ring-

ing on the IN supply when the internal MOSFETs are

turned off. Choose an input capacitor that exhibits less

than +10°C temperature rise at the RMS input current for

optimal circuit longevity.

Figure 1. Adjustable Output Voltage

VOUT

OUT

MAXM17543

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MAXM17543

4.5V to 42V, 2.5A High-Efficiency, DC-DC Step-Down

Power Module with Integrated Inductor

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