Datasheet

SLUU182 − March 2004
7
High-Performance Dual Synchronous Buck Conversion Using the TPS5124
4 Design Procedure
4.1 Frequency Setting
Many factors influence frequency selection. Higher switching frequency leads to smaller output
inductor and capacitor, reducing the size of the converter. However, higher switching frequencies
increase switching losses, and lower the converter’s efficiency. A frequency of 300 kHz is
chosen for this design for reasonable efficiency and size.
Capacitor C16, which is connected from CT (pin 5) to ground, programs the oscillator frequency.
A C16 value of 47 pF yields a switching frequency of 300 kHz at 25°C.
4.2 Inductance Value
The inductance value can be calculated using equation (1).
L +
V
OUT
f (min) I
RIPPLE
ǒ1 *
V
OUT
V
IN(max)
Ǔ
where
• I
RIPPLE
is the ripple current flowing through the inductor
The ripple current affects the output voltage ripple and core losses. Based on 20% ripple current
and 300 kHz, the inductance value is calculated as 2.2 µH. An off-the-shelf 2.2-µH inductor from
Vishay is chosen. The part number is IHLP−5050CE−01−2R2M01. The DCR is 7 mΩ and the
DCR related conduction loss is 1.6 W, which is about 3.3% of output power.
The same procedure is followed to choose the inductor for Channel 2. The same inductor is
chosen.
4.3 Output Capacitors
The required output capacitance and its ESR can be calculated using equations (2) and (3).
C
OUT(min)
+
I
RIPPLE
8 f V
RIPPLE
ESR
OUT
+
V
RIPPLE
I
RIPPLE
With 1% output voltage ripple, the required minimum output capacitance is 54 µF and its ESR
should be less than 7.7 mΩ.
From the load transient point of view, the capacitance needed for 6% overshoot can be
calculated using equation (4).
C
OUT
+
ǒ
I
OUT(max)
Ǔ
2
L
ǒ
V
OUT2
Ǔ
2
*
ǒ
V
OUT1
Ǔ
2
(1)
(2)
(3)
(4)