ANP007 Datasheet, PDF (10/11 Pages) Anachip Corp – Dual Buck Converter

Application Note
AP2001 Dual Buck Converter
ANP007
ΔIL = 2 x 10% x Io = 2 x 0.1 x 3 = 0.6A (For 5V and 3.3V)
The inductor “L” value is:
L
≧
(Vin - Vds(sat) – Vo) x Dmin
ΔIL x fs
=
(13.2 – 0.1 – 3.3) x 0.29
0.6 x (200 x 10^3)
= 23.7μH
For 3.3V
L
≧
(Vin - Vds(sat) – Vo) x Dmin
ΔIL x fs
=
(13.2 – 0.1 – 5) x 0.42
0.6 x (200 x 10^3)
= 28.4μH For 5V
So we can choose 33μH for output voltage “3.3V” and “5V”.
3.3.2 Selection of the output capacitor (Cout)
Assuming that all of the inductor ripple current flows through the capacitor and the effective series
resistance (ESR) is zero, the capacitance needed is:
Cout
≧
ΔIL
8 x fs x ΔVo
=
0.6
8 x (200 x 10^3) x 0.05
=
7.5μF
Assuming the capacitance is very large, the ESR needed to limit the ripple to 50 mV is:
ESR ≦
ΔVo
ΔIo
=
0.05
0.6
=
0.083Ω
The output filter capacitor should be rated at least ten times the calculated capacitance and 30–50
percent lower than the calculated ESR. This design used a 470μF/25V OS-Con capacitor in parallel
with a ceramic to reduce ESR.
3.3.3 Selection of the power switch (MOSFET)
Based on the preliminary estimate, RDS(on) should be less than 0.10 V ÷ 3A = 33mΩ. The
CEM4435(CET) is a -30V p-channel MOSFET with RDS(on) = 35mΩ. Power dissipation (conduction +
switching losses) can be estimated as:
PMOSFET = Io^2 x Rds(on) x Dmax + [0.5 x Vin x Io x (tr + tf) x fs]
Assuming total switching time (tr + tf) is 150 ns, a 55°C maximum ambient temperature, and
thermal impedance RθJA = 50°C/W , thus:
PMOSFET = (3 x 3 x 0.035 x 0.36) + [0.5 x 12 x 3 x (0.15 x 10^(-6)) x (200 x 10^3) = 0.65W
Anachip Corp.
www.anachip.com.tw
10/11
Rev. A.0 Feb. 20, 2003