AAT1106
600mA Step-Down Converter
Below 50% duty cycle, the slope compensation
requirement is:
V
O
V
V
O
· 1 -
V
IN
IN
C =
IN
V
PP
- ESR ·F
S
1.25
I
O
m =
= 0.284A/µs
2 · L
V
O
V
1
O
· 1 -
=
for V = 2 · V
IN
O
V
IN
V
IN
4
Therefore:
1
C
=
IN(MIN)
V
I
0.625
PP
- ESR ·4 · F
L =
= 2.2µH
= 1.136A/µs
= 2.2µH
S
m
O
Always examine the ceramic capacitor DC voltage
coefficient characteristics when selecting the proper
value. For example, the capacitance of a 10ꢀF, 6.3V,
X5R ceramic capacitor with 5.0V DC applied is actu-
ally about 6ꢀF.
Above 50% duty cycle,
5
m =
2 · L
The maximum input capacitor RMS current is:
Therefore:
V
O
V
V
O
I
= I ·
O
· 1 -
RMS
V
IN
2.5
IN
L =
m
The input capacitor RMS ripple current varies with
the input and output voltage and will always be less
than or equal to half of the total DC load current
With these adaptive settings, a 2.2µH inductor can
be used for all output voltages from 0.6V to 5V.
Input Capacitor Selection
V
O
V
1
2
D · (1 - D) = 0.5 =
O
· 1 -
=
The input capacitor reduces the surge current drawn
from the input and switching noise from the device.
The input capacitor impedance at the switching fre-
quency shall be less than the input source imped-
ance to prevent high frequency switching current
passing to the input. A low ESR input capacitor sized
for maximum RMS current must be used. Ceramic
capacitors with X5R or X7R dielectrics are highly
recommended because of their low ESR and small
temperature coefficients. A 4.7µF ceramic capacitor
is sufficient for most applications.
V
IN
V
IN
2
for VIN = 2 x VO
I
O
I
=
RMS(MAX)
2
VO
VO
·
1 -
VIN
VIN
The term
appears in both the input volt-
age ripple and input capacitor RMS current equa-
tions and is at maximum when VO is twice VIN. This
is why the input voltage ripple and the input capaci-
tor RMS current ripple are a maximum at 50% duty
cycle. The input capacitor provides a low impedance
loop for the edges of pulsed current drawn by the
AAT1106. Low ESR/ESL X7R and X5R ceramic
capacitors are ideal for this function. To minimize
stray inductance, the capacitor should be placed as
To estimate the required input capacitor size, deter-
mine the acceptable input ripple level (VPP) and
solve for C. The calculated value varies with input
voltage and is a maximum when VIN is double the
output voltage.
1106.2007.07.1.0
13
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
