MAX16909 Datasheet, PDF (13/18 Pages) Maxim Integrated Products – 36V, 220kHz to 1MHz Step-Down Converter with Low Operating Current

MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
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
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:
∆I_INDUCTOR
=
V_OUT (V_SUP − V_OUT )
V_SUP × f_SW × L
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):
I_PEAK
= I_LOAD(MAX)
+
∆I_INDUCTOR
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:
I_RMS = I_LOAD(MAX)
V_OUT (V_SUP − V_OUT )
V_SUP
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
=
∆V_ESR
I_OUT
+
∆I_L
2
where
∆I_L
=
(V_SUP − V_OUT ) × V_OUT
V_SUP × f_SW × L
and
C_IN
=
I_OUT × D(1− D)
∆V_Q × f_SW
and D = ^VOUT
V_SUPSW
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.
The output capacitance must be high enough to absorb
the inductor energy while transitioning from full-load
to no-load conditions without tripping the overvoltage
fault protection. When using high-capacitance, low-ESR
capacitors, the filter capacitor’s ESR dominates the
output-voltage ripple. So the size of the output capaci-
tor depends on the maximum ESR required to meet the
output-voltage ripple (V_RIPPLE(P-P)) specifications:
V_RIPPLE(P-P) = ESR × I_LOAD(MAX) × LIR
The actual capacitance value required relates to the
physical size needed to achieve low ESR, as well as
to the chemistry of the capacitor technology. Thus, the
capacitor is usually selected by ESR and voltage rating
rather than by capacitance value.
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