Datasheet

2
I ×L
TRAN(MAX)
C =
OUT(MIN)
V × V
OUT OVER
( )
2
I ×L
TRAN(MAX)
C =
OUT(MIN)
V - V × V
IN(min) OUT UNDER
V - V
V
1
IN(max) OUT
OUT
L = × ×
I V f
RIPPLE IN(max) SW
TPS40140
www.ti.com SLUS660H –SEPTEMBER 2005–REVISED JUNE 2013
6 DESIGN EXAMPLES
6.1 Example 1: Dual-Output Configuration from 12 V to 3.3 V and 1.5 V DC-to-DC
Converter Using a TPS40140
The following example illustrates the design process and component selection for a dual output
synchronous buck converter using TPS40140. The design goal parameters are given in Table 6-1. Only
the calculated numbers for the 1.5-V output are shown, however, the equations are suitable for both
channel design. A list of symbol definitions is found at the end of this section.
Table 6-1. Design Goal Parameters
PARAMETER TEST CONDITION MIN TYP MAX UNIT
V
IN
Input voltage 10.8 12 13.2 V
V
OUT1
Output voltage 1 1.5 V
V
OUT2
Output voltage 2 3.3 V
V
RIPPLE
Output ripple I
O
= 20 A 2%Vo V
I
OUT1
Output current 1 20 A
I
OUT2
Output current 2 20 A
f
sw
Switching frequency 500 kHz
6.1.1 Step 1: Inductor Selection
The inductor is determined by the desired ripple current. The required inductor is calculated by:
(21)
Typically the peak-to-peak inductor current, I
RIPPLE
is selected to be around 20% of the rated output
current. In this design, I
RIPPLE
is targeted at 15% of I
OUT1
. The calculated inductor is 0.89 μH and in
practical a 1-μH, 32-A inductor from Vishay is selected. So, the inductor ripple current is 2.66 A.
6.1.2 Step 2: Output Capacitor Selection
The output capacitor is typically selected by the output load transient response requirement. Equation 22
estimates the minimum capacitor to reach the undervoltage requirement with load step up. Equation 23
estimates the minimum capacitor for overvoltage requirement with load step down. When V
IN(min)
< 2
×V
OUT
, the minimum output capacitance can be calculated using Equation 22. Otherwise, Equation 23 is
used.
(22)
when V
IN(min)
< 2 × V
OUT
(23)
when V
IN(min)
> 2×V
OUT
In this design, V
IN(min)
is much larger than 2 × V
OUT
, so Equation 23 is used to determine the minimum
capacitance. Based on a 10-A load transient with a maximum of 80-mV deviation, a minimum 833-μF
output capacitor is required. In the design, four 220-μF, 4-V, SP capacitor are selected to meet this
requirement. Each capacitor has an ESR of 5 mΩ.
Another criterion for capacitor selection is the output ripple voltage. The output ripple is determined mainly
by the capacitance and the ESR.
Copyright © 2005–2013, Texas Instruments Incorporated DESIGN EXAMPLES 43
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