1MHz, All-Ceramic, 2.6V to 5.5V Input,

2A PWM Step-Down DC-to-DC Regulators

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9

Design Procedure

Output Voltage Selection: Adjustable

(MAX1951) or Preset (MAX1952)

The MAX1951 provides an adjustable output voltage

output. To set the output voltage of the MAX1951 to a

to FB and GND using a resistive divider, as shown in

Figure 2a. Choose R2 between 2k

Ω and 20kΩ, and set

R3 according to the following equation:

The MAX1951 PWM circuitry is capable of a stable min-

imum duty cycle of 18%. This limits the minimum output

The MAX1952 provides a preset output voltage.

Connect the output to FB, as shown in Figure 2b.

Output Inductor Design

Use a 2µH inductor with a minimum 2A-rated DC cur-

rent for most applications. For best efficiency, use an

inductor with a DC resistance of less than 20m

Ω and a

saturation current greater than 3A (min). See Table 2

for recommended inductors and manufacturers. For

most designs, derive a reasonable inductor value

oscillator. Keep the inductor current ripple percentage

LIR between 20% and 40% of the maximum load cur-

rent for the best compromise of cost, size, and perfor-

mance. Calculate the maximum inductor current as:

Check the final values of the inductor with the output

ripple voltage requirement. The output ripple voltage is

given by:

where ESR is the equivalent series resistance of the

output capacitors.

Input Capacitor Design

The input filter capacitor reduces peak currents drawn

from the power source and reduces noise and voltage

ripple on the input caused by the circuit’s switching.

The input capacitor must meet the ripple current

defined by the following equation:

For duty ratios less than 0.5, the input capacitor RMS

current is higher than the calculated current. Therefore,

use a +20% margin when calculating the RMS current

at lower duty cycles. Use ceramic capacitors for their

low ESR, equivalent series inductance (ESL), and lower

cost. Choose a capacitor that exhibits less than 10°C

temperature rise at the maximum operating RMS cur-

rent for optimum long-term reliability.

After determining the input capacitor, check the input

ripple voltage due to capacitor discharge when the

high-side MOSFET turns on. Calculate the input ripple

voltage as follows:

Keep the input ripple voltage less than 3% of the input

voltage.

Output Capacitor Design

The key selection parameters for the output capacitor

are capacitance, ESR, ESL, and the voltage rating

requirements. These affect the overall stability, output

ripple voltage, and transient response of the DC-to-DC

converter. The output ripple occurs due to variations in

the charge stored in the output capacitor, the voltage

drop due to the capacitor’s ESR, and the voltage drop

due to the capacitor’s ESL. Calculate the output voltage

ripple due to the output capacitance, ESR, and ESL as:

where the output ripple due to output capacitance,

ESR, and ESL is:

whichever is greater

Use these equations for initial capacitor selection, but

determine final values by testing a prototype or evalua-

tion circuit. As a rule, a smaller ripple current results in

less output voltage ripple. Since the inductor ripple

current is a factor of the inductor value, the output

voltage ripple decreases with larger inductance. Use

ceramic capacitors for their low ESR and ESL at the

switching frequency of the converter. The low ESL of

ceramic capacitors makes ripple voltages negligible.

Load transient response depends on the selected

output capacitor. During a load transient, the output

can respond, the output deviates further, depending on

the inductor and output capacitor values. After a short

time (see the Load Transient Response graph in the

IV

I

V

V

V

RMS

IN

OUT

OUT

IN

OUT

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1

2