MAX16930 Datasheet, PDF (20/29 Pages) Maxim Integrated Products – 2MHz, 36V, Dual Buck with Preboost and 20μA Quiescent Current

MAX16930/MAX16931
2MHz, 36V, Dual Buck with Preboost and
20µA Quiescent Current
Output Capacitor in Buck Converters
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. The capacitor
is usually selected by ESR and the voltage rating rather
than by capacitance value.
When using low-capacity filter capacitors, such as
ceramic capacitors, size is usually determined by the
capacity needed to prevent V_SAG and V_SOAR from caus-
ing problems during load transients. Generally, once
enough capacitance is added to meet the overshoot
requirement, undershoot at the rising load edge is no
longer a problem (see the Transient Considerations sec-
tion). However, low-capacity filter capacitors typically
have high-ESR zeros that can affect the overall stability.
The total voltage sag (V_SAG) can be calculated as follows:
V_SAG
=
L(∆I_LOAD(MAX) )²
2C_OUT ((V_IN × D_MAX ) − V_OUT
)
+ ^{∆ILOAD(MAX) (t − ∆t)}
C_OUT
The amount of overshoot (V_SOAR) during a full-load to
no-load transient due to stored inductor energy can be
calculated as:
V_SOAR
≈
(∆I_LOAD(MAX) )²
2C_OUT V_OUT
L
ESR Considerations
The output filter capacitor must have low enough
equivalent series resistance (ESR) to meet output rip-
ple and load-transient requirements, yet have high
enough ESR to satisfy stability requirements. When using
high-capacitance, low-ESR capacitors, the filter capaci-
tor’s ESR dominates the output-voltage ripple. So the
output capacitor’s size depends on the maximum ESR
required to meet the output-voltage ripple (V_RIPPLE(P-P))
specifications:
V_{RIPPLE(P−P)} = ESR x I_LOAD(MAX) x LIR
In standby mode, the inductor current becomes discon-
tinuous, with peak currents set by the idle-mode current-
sense threshold (V_CS,SKIP = 26mV (typ)).
Transient Considerations
The output capacitor must be large enough to absorb
the inductor energy while transitioning from no-load to
full-load condition without tripping the overvoltage fault
protection. The total output-voltage sag is the sum of
the voltage sag while the inductor is ramping up and the
voltage sag before the next pulse can occur. Therefore:
( )²
L ∆I_LOAD(MAX)
C_OUT = _{2VSAG (VIN x DMAX − VOUT )}
+ ^{∆ILOAD(MAX) (t − ∆t)}
V_SAG
where D_MAX is the maximum duty factor (approximately
95%), L is the inductor value in µH, C_OUT is the output
capacitor value in µF, t is the switching period (1/f_SW) in
µs, and Dt equals (V_OUT/V_IN) x t.
The MAX16930/MAX16931 use a current-mode control
scheme that regulates the output voltage by forcing
the required current through the external inductor, so
the controller uses the voltage drop across the DC
resistance of the inductor or the alternate series current-
sense resistor to measure the inductor current. Current-
mode control eliminates the double pole in the feedback
loop caused by the inductor and output capacitor result-
ing in a smaller phase shift and requiring less elaborate
error-amplifier compensation than voltage-mode control.
A single series resistor (R_C) and capacitor (C_C) is all
that is required to have a stable, high-bandwidth loop in
applications where ceramic capacitors are used for out-
put filtering (see Figure 2). For other types of capacitors,
due to the higher capacitance and ESR, the frequency
of the zero created by the capacitance and ESR is
lower than the desired closed-loop crossover frequency.
To stabilize a non-ceramic output capacitor loop, add
another compensation capacitor (C_F) from COMP to
AGND to cancel this ESR zero.
Maxim Integrated
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