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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 V FB = V REF .
Use 30k ? for R1. R2 is given by the equation:
L =
V OUT × t OFF
I OUT × LIR
R2 = R1 ? OUT ? 1 ?
? V ?
? V REF ?
where V REF 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
where:
I OUT = 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 I OUT if
the above equation is used; otherwise, the peak current
is calculated by:
and output voltage and the value of R TOFF , connected
from TOFF to GND. R TOFF sets the PMOS power switch
off-time in PWM mode. Use the following equation to
I PEAK = I OUT +
V OUT × t OFF
2 × L
select the off-time according to your desired switching
frequency in PWM mode:
Choose an inductor with a saturation current at least as
high as the peak-inductor current. The inductor you
t OFF =
( V IN – V OUT ? V PMOS )
f PWM ( V IN ? V PMOS + V NMOS )
select should exhibit low losses at your chosen operat-
ing frequency.
Capacitor Selection
The input-filter capacitor reduces peak currents and
where:
t OFF = the programmed off-time
V IN = the input voltage
V OUT = the output voltage
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:
V OUT ( IN ? V OUT
V PMOS = the voltage drop across the internal PMOS
power switch
V NMOS = the voltage drop across the internal NMOS
I RIPPLE = I LOAD
V
V IN
)
synchronous-rectifier switch
f PWM = switching frequency in PWM mode
Select R TOFF according to the formula:
R TOFF = (t OFF - 0.07μs) (110k ? / 1.00μs)
Recommended values for R TOFF range from 36k ? to
430k ? for off-times of 0.4μs to 4μs.
where I RIPPLE = 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 V RIPPLE ≥ 1%
? V OUT .
The minimum ESR of the output capacitor should be:
Inductor Selection
The key inductor parameters must be specified: induc-
tor value (L) and peak current (I PEAK ). The following
ESR > 1 % ×
L
t OFF
C OUT ≥ OFF 79 μ FV / μ s
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:
Stable operation requires the correct output-filter
capacitor. When choosing the output capacitor, ensure
that:
t
V OUT
10
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