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
£
OUT
OUT co
I
1
Co3
V
³ ´
D f
( )
2 2
O O
2
2
OUT OUT
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
IN OUT
OUT
V V
D2 = D1
V
æ ö
-
´
ç ÷
è ø
( )
0.5
OUT O O sw
IN IN OUT
2 V I L
D1 =
V V V
æ ö
´ ´ ´ ´
ç ÷
ç ÷
´ -
è ø
f
( )
0.5
OUT IN OUT
L
IN O sw
2 V Iomax V max V
I peak =
V max L
æ ö
´ ´ ´ -
ç ÷
ç ÷
´ ´
è ø
f
IN OUT OUT
O
IN sw O
V min V V
1
L max
2 V min I
æ ö
-
æ ö
£ ´ ´
ç ÷
ç ÷
´
è ø
è ø
f
2
IN OUT O
IN
o
OUT O
V max V t nmin
V max
L min sw
V 2 I min
æ ö
-
æ ö
³ ´ ´
ç ÷
ç ÷
è ø
è ø
x f
TPS54061
SLVSBB7C –MAY 2012–REVISED JANUARY 2014 www.ti.com
Use Equation 29, 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 244 mA and is lower than the 350 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 30. D1 is the portion of the
switching cycle the high side power switch is on, and is calculated to be 0.1345. D2 is the portion of the switching
cycle the low side power switch is on, and is calculated to be 0.5111.
Using the Equation 32 and Equation 33, the rms current of the inductor and output capacitor are calculated, to be
0.1078 A and 0.0774 A respectively. Select components that ratings exceed the calculated rms values. Calculate
the output capacitance using the Equation 34 to Equation 36 and use the largest value, Vripple is the steady
state voltage ripple and deltaV is voltage change during a transient. A minimum of 7.5 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 10 V X7R ceramic capacitor with 5mΩ ESR is used. To
have a low output ripple power supply use a low esr capacitor. Use Equation 37 to estimate the maximum esr for
the output capacitor. Equation 38 and Equation 39 estimate the rms current and capacitance for the input
capacitor. An rms current of 38.7 mA and capacitance of 1.56 µF is calculated. A 2.2 µF 100V/X7R ceramic is
used for this example.
(27)
(28)
(29)
(30)
(31)
(32)
(33)
(34)
(35)
(36)
(37)
(38)
(39)
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