Datasheet

MAX16977
36V, 2A, 2.2MHz Step-Down Converter
with Low Operating Current
13Maxim Integrated
ratio (LIR = 0.3). The switching frequency, input voltage,
output voltage, and selected LIR then determine the
inductor value as follows:
OUT SUP OUT
SUP SW OUT
V(V V)
L
V f I LIR
−
=
where V
SUP
, V
OUT
, and I
OUT
are typical values (so that
efficiency is optimum for typical conditions). The switch-
ing frequency is set by R
FOSC
(see the Internal Oscillator
section). The exact inductor value is not critical and can
be adjusted to make trade-offs among size, cost, efficien-
cy, and transient response requirements. Table 1 shows
a comparison between small and large inductor sizes.
The inductor value must be chosen so that the maximum
inductor current does not reach the device’s minimum
current limit. The optimum operating point is usually
found between 25% and 35% ripple current. When pulse
skipping (FSYNC low and light loads), the inductor value
also determines the load-current value at which PFM/
PWM switchover occurs.
Find a low-loss inductor having the lowest possible
DC resistance that fits in the allotted dimensions. Most
inductor manufacturers provide inductors in standard
values, such as 1.0FH, 1.5FH, 2.2FH, 3.3FH, etc. Also
look for nonstandard values, which can provide a bet-
ter compromise in LIR across the input voltage range. If
using a swinging inductor (where the no-load inductance
decreases linearly with increasing current), evaluate
the LIR with properly scaled inductance values. For
the selected inductance value, the actual peak-to-peak
inductor ripple current (DI
INDUCTOR
) is defined by:
OUT SUP OUT
INDUCTOR
SUP SW
V(V V)
I
V fL
−
∆=
××
where DI
INDUCTOR
is in A, L is in H, and f
SW
is in Hz.
Ferrite cores are often the best choices, although pow-
dered iron is inexpensive and can work well at 200kHz.
The core must be large enough not to saturate at the
peak inductor current (I
PEAK
):
INDUCTOR
PEAK LOAD(MAX)
I
II
2
∆
= +
Input Capacitor
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 RMS current requirement (I
RMS
) is
defined by the following equation:
OUT SUP OUT
RMS LOAD(MAX)
SUP
V(V V)
II
V
−
=
I
RMS
has a maximum value when the input voltage
equals twice the output voltage (V
SUP
= 2V
OUT
), so
I
RMS(MAX)
= I
LOAD(MAX)
/2.
Choose an input capacitor that exhibits less than 10NC
self-heating temperature rise at the RMS input current for
optimal long-term reliability.
The input-voltage ripple is composed of DV
Q
(caused
by the capacitor discharge) and DV
ESR
(caused by the
equivalent series resistance (ESR) of the capacitor). Use
low-ESR ceramic capacitors with high ripple-current
capability at the input. Assume the contribution from the
ESR and capacitor discharge equal to 50%. Calculate
the input capacitance and ESR required for a specified
input-voltage ripple using the following equations:
ESR
IN
L
OUT
V
ESR
I
I
2
∆
=
∆
+
where
SUP OUT OUT
L
SUP SW
(V V ) V
I
V fL
−×
∆=
××
and
OUT
IN
Q SW
I D(1 D)
C
Vf
×−
=
∆×
and
OUT
SUPSW
V
D
V
=
where I
OUT
is the maximum output current, and D is the
duty cycle.
Output Capacitor
The output filter capacitor must have low enough ESR to
meet output ripple and load-transient requirements, yet
have high enough ESR to satisfy stability requirements.
Table 1. Inductor Size Comparison
INDUCTOR SIZE
SMALLER LARGER
Lower price Smaller ripple
Smaller form factor Higher efficiency
Faster load response
Larger fixed-frequency
range in skip mode