MAX16930 Datasheet, PDF (24/29 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
Input Capacitor Selection in Boost Converter
The input current for the boost converter is continuous
and the RMS ripple current at the input capacitor is low.
Calculate the minimum input capacitor value and maxi-
mum ESR using the following equations:
C_BAT
=
∆I_L x D
4 x f_SW x ∆V_Q
ESR = ^∆VESR
∆I_L
where:
∆I_L
=^{( VBAT}
L
− V_DS
x f_SW
)xD
V_DS is the total voltage drop across the external MOSFET
plus the voltage drop across the inductor ESR. DI_L is
peak-to-peak inductor ripple current as calculated above.
DV_Q is the portion of input ripple due to the capacitor
discharge and DV_ESR is the contribution due to ESR of
the capacitor. Assume the input capacitor ripple contri-
bution due to ESR (DV_ESR) and capacitor discharge
(DV_Q) are equal when using a combination of ceramic
and aluminum capacitors. During the converter turn-on, a
large current is drawn from the input source especially at
high output-to-input differential.
Output Capacitor Selection in Boost Converter
In a boost converter, the output capacitor supplies the
load current when the boost MOSFET is on. The required
output capacitance is high, especially at higher duty
cycles. Also, the output capacitor ESR needs to be low
enough to minimize the voltage drop while supporting the
load current. Use the following equations to calculate the
output capacitor for a specified output ripple. All ripple
values are peak-to-peak.
ESR = ^∆VESR
I_OUT
C_OUT
=
I_OUT x D_MAX
∆V_Q x f_SW
I_OUT is the load current in A, f_SW is in MHz, C_OUT is µF,
DV_Q is the portion of the ripple due to the capacitor dis-
charge, and DV_ESR is the contribution due to the ESR of
the capacitor. D_MAX is the maximum duty cycle at the
minimum input voltage. Use a combination of low-ESR
ceramic and high-value, low-cost aluminum capacitors
for lower output ripple and noise.
Shunt Resistor Selection in Boost Converter
The current-sense resistor (R_CS), connected between the
battery and the inductor, sets the current limit. The CS
input has a voltage trip level (V_CS) of 120mV (typ).
Set the current-limit threshold high enough to accommo-
date the component variations. Use the following equa-
tion to calculate the value of R_CS:
R _CS
=
V_CS
I_{IN(MAX )}
where I_IN(MAX) is the peak current that flows through the
MOSFET at full load and minimum V_IN.
I_IN(MAX) = I_LOAD(MAX) /(1 - D_MAX)
When the voltage produced by this current (through
the current-sense resistor) exceeds the current-limit
comparator threshold, the MOSFET driver (DL3) quickly
terminates the on-cycle.
Maxim Integrated
  24