XRP7704 Datasheet, PDF (29/31 Pages) Exar Corporation – Quad-Output Digital PWM Buck Controller

REV 1.1.0
XRP7704
Quad-Output Digital PWM Buck Controller
In general, the total input voltage ripple should be kept below 1.5% of VIN. The input voltage ripple also has two
major components: the voltage drop on the main capacitor ΔV_Cin and the voltage drop due to ESR - ΔV_ESR . The
contribution to Input voltage ripple by each term can be calculated from:
Δ V_Cin
=
I V _{out out} (V_in − V_out )
f _s C_inV_in²
ΔV_ESR = ESR ⋅ (I_out + 0.5I_rip )
Total input voltage ripple is the sum of the above:
ΔV_Tot = ΔV_Cin + ΔV_ESR
Power MOSFETs Selection
Selecting MOSFETs with lower Rdson reduces conduction losses at the expense of increased switching losses. A
simplified expression for conduction losses is given by:
P_cond
=
I2
out
⋅
R_dson
⋅ ^Vout
V_in
MOSFET’s junction temperature can be estimated from:
T_j = 2P_cond R_thja + T_ambient
This assumes that the switching loss is the same as the conduction loss. R_thja is the total MOSFET thermal
resistance from junction to ambient.
EXAR CONFIDENTIAL. PROPRIETARY. DO NOT DISTRIBUTE OR COPY.
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