XRP7704 Datasheet, PDF (28/31 Pages) Exar Corporation – Quad-Output Digital PWM Buck Controller

REV 1.1.0
XRP7704
Quad-Output Digital PWM Buck Controller
load current forces the energy surplus in the inductor to be absorbed by Cout. This causes an overshoot in output
voltage that is corrected by power switch reduced duty cycle. Use the following equation to calculate Cout:
Where:
C_out
=
L
⋅
(I₂
V_os ²
− I₁)²
− V_out ²
L is the output inductance
I2 is the step load high current
I1 is the step load low current
Vos is output voltage including the overshoot
VOUT is the steady state output voltage
Or it can be expressed approximately as:
C_out
=
L⋅
(I₂ − I₁)²
2V_out ⋅ ΔV
Here, ΔV = V_os − V_out is the overshoot voltage deviation.
Select ESR such that output voltage ripple (Vrip) specification is met. There are two components in Vrip. First
component arises from the charge transferred to and from Cout during each cycle. The second component of Vrip
is due to the inductor ripple current flowing through the output capacitor’s ESR. It can be calculated for Vrip:
Where:
Irip is the inductor ripple current
fs is the switching frequency
Cout is the output capacitance
Note that a smaller inductor results in a higher Irip, therefore requiring a larger Cout and/or lower ESR in order to
meet Vrip.
Input Capacitor Selection
Select the input capacitor for Voltage, Capacitance, ripple current, ESR and ESL. Voltage rating is nominally
selected to be at least twice the input voltage. The RMS value of input capacitor current, assuming a low inductor
ripple current, can be approximated as:
Where:
I_in = I_out ⋅ D(1 − D)
Iin is the RMS input current
Iout is the DC output current
D is the duty cycle
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