Datasheet

O
IN
IN SW
I
0.25
C
V RIPPLE f
æ ö
³ ´
ç ÷
è ø
0.5
2
CIN L
D1 D1
I rms = I peak
3 4
æ ö
æ ö æ ö
ç ÷
´ -
ç ÷ ç ÷
ç ÷
è ø è ø
è ø
RIPPLE
C
L
V
R
I peak
£
O
O
CO
I
1
C 3
V f
³
D
( )
( )
2 2
O O
2
2
O
O
Io 0
C 2 L
V + V V
-
³ ´
D -
L
O
RIPPLE SW
I peak
D1 + D2
C 1
V 8 f
æ ö
£ ´
ç ÷
´
è ø
0.5
2
CO L
D1 + D2 D1 + D2
I rms = I peak
3 4
æ ö
æ ö æ ö
ç ÷
´ -
ç ÷ ç ÷
ç ÷
è ø è ø
è ø
0.5
L L
D1 + D2
I rms = I peak
3
æ ö
´
ç ÷
è ø
S O
O
V V
D2 = D1
V
æ ö
-
´
ç ÷
è ø
( )
0.5
O O O SW
S S O
2 V I L
D1 =
V V V
f
æ ö
´ ´ ´ ´
ç ÷
ç ÷
´ -
è ø
( )
0.5
O O S O
L
S O SW
2 V I max V max V
I peak =
V max L f
æ ö
´ ´ ´ -
ç ÷
ç ÷
´ ´
è ø
S O O
O
S SW O
V min V V
1
L max
2 V min If
æ ö
-
æ ö
£ ´ ´
ç ÷
ç ÷
´
è ø
è ø
2
S O S O
O SW
O O
V max V V max t nmin
L min
V 2 I min
x f
æ ö
-
æ ö
³ ´ ´
ç ÷
ç ÷
è ø
è ø
TPS54062
www.ti.com SLVSAV1B –MAY 2011–REVISED AUGUST 2012
Use Equation 28, to make sure the minimum current limit on the high side power switch is not exceeded at the
maximum output current. The peak current is calculated as 23.9mA and is lower than the 134 mA current limit.
To determine the rms current for the inductor and output capacitor, it is necessary to calculate the duty cycle.
The duty cycle, D1, for a step down regulator in DCM is calculated in Equation 29. D1 is the portion of the
switching cycle the high side power switch is on, and is calculated to be 0.1153. D2 is the portion of the switching
cycle the low side power switch is on, and is calculated to be 0.7253.
Using the Equation 31 and Equation 32, the rms current of the inductor and output capacitor are calculated, to be
12.8mA and 7.6mA respectively. Select components that ratings exceed the calculated rms values. Calculate the
output capacitance using the Equation 33 to Equation 35 and use the largest value, Vripple is the steady state
voltage ripple and deltaV is voltage change during a transient. A minimum of 1.5 µF capacitance is calculated.
Additional capacitance de-ratings for aging, temperature and dc bias should be factored in which increases this
minimum value. For this example, a 22 µF 6.3 V X7R ceramic capacitor with 5mΩ ESR is used. To have a low
output ripple power supply use a low esr capacitor. Use Equation 36 to estimate the maximum esr for the output
capacitor. Equation 37 and Equation 38 estimate the rms current and capacitance for the input capacitor. An rms
current of 3.7 mA and capacitance of 0.2 µF is calculated. A 1 µF 100V/X7R ceramic is used for this example.
(26)
(27)
(28)
(29)
(30)
(31)
(32)
(33)
(34)
(35)
(36)
(37)
(38)
Copyright © 2011–2012, Texas Instruments Incorporated Submit Documentation Feedback 23
Product Folder Link(s): TPS54062