MAX1951A データシートの表示（PDF） - Maxim Integrated
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MAX1951A Datasheet PDF : 12 Pages
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1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC
Step-Down Regulator with Enable
Design Procedure
Adjustable Output Voltage
The MAX1951A provides an adjustable output voltage
between 0.8V and VIN. Connect FB to output for 0.8V
output. To set the output voltage of the MAX1951A to a
voltage greater than VFB (0.8V typ), connect the output
to FB and GND using a resistive divider, as shown in
Figure 2. Choose R2 between 2kΩ and 20kΩ, and set
R3 according to the following equation:
R3 = R2 x [(VOUT/VFB) - 1]
The MAX1951A PWM circuitry is capable of a stable
minimum duty cycle of 18%. This limits the minimum
output voltage that can be generated to 0.18 VIN with
an absolute minimum of 0.8V. Instability may result for
VIN/VOUT ratios below 0.18.
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 20mΩ and a
saturation current greater than 3A (min). See Table 2
for recommended inductors and manufacturers. For
most designs, derive a reasonable inductor value
(LINIT) from the following equation:
LINIT = VOUT x (VIN - VOUT)/(VIN x LIR x IOUT(MAX) x fSW)
where fSW is the switching frequency (1MHz typ) of the
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:
IL(MAX) = (1 + LIR/2) x IOUT(MAX)
Check the final values of the inductor with the output
ripple voltage requirement. The output ripple voltage is
given by:
VRIPPLE = VOUT x (VIN - VOUT) x ESR/(VIN x LFINAL x fSW)
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
requirement (IRMS) imposed by the switching currents
defined by the following equation:
IRMS = (1 VIN) × (IOUT2 × VOUT × (VIN − VOUT ))
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 and equivalent series inductance (ESL).
Choose a capacitor that exhibits less than 10°C tem-
perature rise at the maximum operating RMS current 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:
VIN_RIPPLE = (IOUT x VOUT)/(fSW x VIN x CIN)
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-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:
VRIPPLE = VRIPPLE(C) + VRIPPLE(ESR) + VRIPPLE(ESL)
where the output ripple due to output capacitance,
ESR, and ESL is:
VRIPPLE(C) = IP-P/(8 x COUT x fSW)
VRIPPLE(ESR) = IP-P x ESR
VRIPPLE(ESL) = (IP-P/tON) x ESL or (IP-P/tOFF) x ESL,
whichever is greater
and IP-P the peak-to-peak inductor current is:
IP-P = [(VIN – VOUT )/fSW x L)] x VOUT/VIN
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
instantly changes by ESR x ∆ILOAD. Before the con-
troller can respond, the output deviates further,
depending on the inductor and output capacitor val-
ues. After a short time (see the Load Transient graph in
the Typical Operating Characteristics), the controller
responds by regulating the output voltage back to its
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