eorex
Buck Regulator Design Procedure
Given:
V
OUT
= Regulated Output Voltage (3.3V, 5V or 12V)
V
IN(max)
= Maximum DC Input Voltage
I
LOAD(max)
= Maximum Load Current
F = Switching Frequency (Fixed at a nominal 150KHz)
EP3101
1. Output Capacitor Selection (C
OUT
)
A low ESR (Equivalent Series Resistance) electrolytic capacitors between 82μF and 820μF and low ESR
solid tantalum capacitors between 10μF and 470μF provide the best results. This capacitor should be
located close to the IC using short capacitor leads and short copper traces. Do not use capacitors larger
than 820μF. The capacitor voltage rating for electrolytic capacitors should be at least 1.5 times greater
than the output voltage.
2. Catch Diode Selection (D1)
The catch diode current rating must be at least 1.3 times greater than the maximum load current. Also, if
the power supply design must withstand a continuous output short, the diode should have a current
rating equal to the maximum current limit of the EP3101. The reverse voltage rating of the diode should
be at least 1.25 times the maximum input voltage. This diode must have short reverse recovery time and
must be located close to the EP3101 using short leads and short printed circuit traces. Because of their
fast switching speed and low forward voltage drop, Schottky diodes provide the best performance and
efficiency, and should be the first choice, especially in low output voltage applications. Ultra-fast recovery
diodes typically have reverse recovery times of 50ns or less.
3. Input Capacitor (C
IN
)
A low ESR aluminum or tantalum bypass capacitor is needed to prevent large voltage transients from
appearing at the input. In addition, the RMS current rating of the input capacitor should be selected to be
at least 1⁄2 the DC load current. The capacitor manufacturer’s data sheet must be checked to assure that
this current rating is not exceeded. For aluminum electrolytic, the capacitor voltage rating should be
approximately 1.5 times the maximum input voltage.
4. Programming Output Voltage
(Selecting R1 and R2, as shown in Adjustable Type Circuit)
Use the following formula to select the appropriate resistor values.
V
OUT
=V
REF
×
⎜
1
+
⎛
⎝
R1
⎞
R2
⎠
⎟
where V
REF
=1.23V
Select a value for R2 between 240Ω and 1.5KΩThe lower resistor values minimize noise pickup in the
sensitive feedback pin. (For the lowest temperature coefficient and the best stability with time, use 1%
metal film resistors.)
5. Inductor Selection (L1)
Calculate the inductor Volt • microsecond constant E • T (V •μs), from the following formula:
E•T = (V
IN
-V
OUT
-V
SAT
) •
V
OUT
+
V
D
V
IN
−
V
SAT
+
V
D
150KHz
•
1000
(V•μs)
where V
SAT
= internal switch saturation voltage = 1.16V and V
D
= diode forward voltage drop = 0.5V
On the horizontal axis selects the maximum load current.
Jul. 2006
9/13
www.eorex.com
EOREX [ EOREX CORPORATION ]
