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

Application Note
AP2001 Dual Buck Converter
ANP007
TJ = TA+ (RθJA x PMOSFET) = 55 + (50 x 0.65) = 87.5°C…………………………………………(For 3.3V)
PMOSFET = (3 x 3 x 0.035 x 0.51) + [0.5 x 12 x 3 x (0.15 x 10^(-6)) x (200 x 10^3) = 0.7W
TJ = TA+ (RθJA x PMOSFET) = 55 + (50 x 0.7) = 90°C……………………………………………(For 5V)
3.3.4 Selection of the Rectifier (D)
The catch rectifier conducts during the time interval when the MOSFET is off. The B340 (DIODES)
is a 3A, 40V schottky rectifier in a SMC power surface-mount package. The power dissipation is:
PD = Io x Vd x (1 – Dmin) = 3 x 0.5 x (1 – 0.29) = 1.065W……………………………………(For 3.3V)
PD = Io x Vd x (1 – Dmin) = 3 x 0.5 x (1 – 0.42) = 0.87W……………………………………..(For 5V)
Assuming a 55°C maximum ambient temperature, and thermal impedance RθJA = 15°C/W , thus:
TJ = TA+ (RθJA x PD) = 55 + (15 x 1.065) = 70.975°C……………………………………….. (For 3.3V)
TJ = TA+ (RθJA x PD) = 55 + (15 x 0.87) = 68.05°C……………………………………….. (For 5V)
3.3.5 Selection of the input capacitor (Cin)
The RMS current rating of the input capacitor can be calculated from the following formula. The
capacitor manufacturers data sheet must be checked to assure that this current rating is not exceeded.
ΔI Iin(rms) = 2√[D x (Io(max) + Io(min)) x (Io(max) - Io(min)) + ( L^2)/3] = √[0.36 x (3 + 0.3) x (3 – 0.3) + 0.36/3]
= 2 x 1.8A = 3.6A
This capacitor should be located close to the IC using short leads and the voltage rating should be
approximately 2 times the maximum input voltage. We select input capacitor value “470uF/25V”.
4. Voltage monitor by AP434
In some applications, the output voltage is concerned too high to damage the IC. To avoid the IC
being damaged, comparing output voltage with a reference could monitor the output voltage. AP434 is a
monolithic IC that includes one independent OP-Amp and another OP-Amp, which the non-inverting
input is wired to a fixed voltage reference. AP434 data sheet provides the low cost and space saving of
voltage monitoring function.
Anachip Corp.
www.anachip.com.tw
Written by Cheng-Yu Chen(陳政佑)
11/11
Rev. A.0 Feb. 20, 2003