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MAX1830

3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches

Maxim Integrated 

3A, 1MHz, Low-Voltage, Step-Down Regulators with

Synchronous Rectification and Internal Switches

voltage and ground (Figure 4). Regulation is main-

tained for adjustable output voltages when VFB = VREF.

Use 30kΩ for R1. R2 is given by the equation:

R2

=

R1





VOUT



− 1

 VREF



where VREF is typically 1.1V.

Programming the Switching

Frequency and Off-Time

The MAX1830/MAX1831 feature a programmable PWM

mode-switching frequency, which is set by the input

and output voltage and the value of RTOFF, connected

from TOFF to GND. RTOFF sets the PMOS power switch

off-time in PWM mode. Use the following equation to

select the off-time according to your desired switching

frequency in PWM mode:

( ( ) ) tOFF

=

VIN – VOUT − VPMOS

fPWM VIN − VPMOS + VNMOS

where:

tOFF = the programmed off-time

VIN = the input voltage

VOUT = the output voltage

VPMOS = the voltage drop across the internal PMOS

power switch

VNMOS = the voltage drop across the internal NMOS

synchronous-rectifier switch

fPWM = switching frequency in PWM mode

Select RTOFF according to the formula:

RTOFF = (tOFF - 0.07µs) (110kΩ / 1.00µs)

Recommended values for RTOFF range from 36kΩ to

430kΩ for off-times of 0.4µs to 4µs.

Inductor Selection

The key inductor parameters must be specified: induc-

tor value (L) and peak current (IPEAK). The following

equation includes a constant, denoted as LIR, which is

the ratio of peak-to-peak inductor AC current (ripple

current) to maximum DC load current. A higher value of

LIR allows smaller inductance but results in higher loss-

es and ripple. A good compromise between size and

losses is found at approximately a 25% ripple-current

to load-current ratio (LIR = 0.25), which corresponds to

a peak-inductor current 1.125 times the DC load cur-

rent:

L = VOUT × tOFF

IOUT × LIR

where:

IOUT = maximum DC load current

LIR = ratio of peak-to-peak AC inductor current to DC

load current, typically 0.25

The peak-inductor current at full load is 1.125 x IOUT if

the above equation is used; otherwise, the peak current

is calculated by:

IPEAK =

IOUT +

VOUT ×

2×

tOFF

L

Choose an inductor with a saturation current at least as

high as the peak-inductor current. The inductor you

select should exhibit low losses at your chosen operat-

ing frequency.

Capacitor Selection

The input-filter capacitor reduces peak currents and

noise at the voltage source. Use a low-ESR and low-

ESL capacitor located no further than 5mm from IN.

Select the input capacitor according to the RMS input

ripple-current requirements and voltage rating:

IRIPPLE = ILOAD

( ) VOUT VIN − VOUT

VIN

where IRIPPLE = input RMS current ripple.

The output-filter capacitor affects the output-voltage rip-

ple, output load-transient response, and feedback-loop

stability. For stable operation, the MAX1830/MAX1831

require a minimum output ripple voltage of VRIPPLE ≥ 1%

✕ VOUT.

The minimum ESR of the output capacitor should be:

ESR > 1% × L

tOFF

Stable operation requires the correct output-filter

capacitor. When choosing the output capacitor, ensure

that:

COUT

≥

tOFF

VOUT

79µFV / µs

10 ______________________________________________________________________________________

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