MAX15112 Datasheet, PDF (14/23 Pages) Maxim Integrated Products – High-Efficiency, 12A, Current-Mode Synchronous Step-Down Regulator with Integrated Switches

MAX15112
High-Efficiency, 12A, Current-Mode Synchronous
Step-Down Regulator with Integrated Switches
Applications Information
Setting the Output Voltage
The MAX15112 output voltage is adjustable from 0.6V
up to 94% of VIN by connecting FB to the center tap of
a resistor-divider between the output and GND (see the
Typical Operating Circuits). Choose R1 and R2 values
so that the DC errors due to the FB input bias current
(Q500nA) do not affect the output volta­ ge accuracy.
With lower value resistors the DC error is reduced, but
the amount of power consumed in the resistor-divider
increases. R2 values between 1kI and 20kI are accept-
able (see Table 1 for typical values). Once R2 is chosen,
calculate R1 using:
R1=R2 × (V_OUT /V_FB) -1
where the feedback threshold voltage VFB = 0.6V (typ).
When regulating for an output of 0.6V in skip mode, short
FB to OUT and keep R2 connected from FB to GND.
Inductor Selection
A high-valued inductor results in reduced inductor-ripple
current, leading to a reduced output-ripple voltage.
However, a high-valued inductor results in either a larger
physical size or a high series resistance (DCR) and a
lower saturation current rating. Typically, choose an
inductor value to produce a current ripple, DIL, equal to
30% of load current. Choose the inductor with the follow-
ing formula:
L
=
f_SW
V_OUT
× LIR × I_LOAD
×

^1-

V_OUT
V_IN



where fSW is the fixed 1MHz switching frequenc­ y, and
LIR is the desired inductor current ratio (typically 0.3). In
addition, the peak inductor current, IL_PK, must always
be below the 24A high-side current-limit and the inductor
saturation current rating, IL_SAT. Ensure that the following
relationship is satisfied:
where:
I_{L_PK}
=
I_LOAD
+
1
2
∆I_L(P-P)
<
min
(24A,
I_{L_SAT} )
∆I_L(P-P)
=
(V_IN − V_OUT) x
L x f_SW
V_OUT
V_IN
Input Capacitor Selection
For a step-down converter, the input capacitor, CIN,
helps to keep the DC input voltage steady, in spite of
discontinuous input AC current. Use low-ESR capacitors
to minimize the voltage ripple due to ESR.
Size CIN using the following formula:
C_IN
=
f_SW
×
I_LOAD
∆V_{IN_RIPPLE}
×
V_OUT
V_IN
where DVIN_RIPPLE is the maximum-allowed input-ripple
voltage across the input capacitors and is recommend­
ed to be less than 2% of the minimum input voltage,
fSW is the switching frequency (1MHz), and ILOAD is the
output load. The impedance of the input capacitor at
the switching frequency should be less than that of the
input source so high-frequency switching currents do not
pass through the input source, but are instead shunted
through the input capacitor.
Ensure that the input capacitor can accommodate the
input-ripple current require­ment imposed by the switch-
ing currents. The RMS input-ripple current is given by:
I_RMS
=




V_OUT
×
(V_IN -
V_IN
V_OUT
)¹
2




×
I_LOAD
where IRMS is the input RMS ripple current.
Use multiple capacitors in parallel to meet the RMS cur-
rent rating requirement.
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