AAT1156
1MHz 700mA Step-Down DC-DC Converter
Enable
Vin+
12
R1
100
R2
C1
10µF
100K
C2
0.1
µF
11
10
7
9
6
R6
8
100k
5
VP
VP
VP
EN
VCC
LL
N/C
SGND
U1
AAT1156
FB
LX
LX
LX
N/C
PGND
PGND
PGND
4
15
14
13
16
3
2
1
R4
59k
L1
4.7µH
R3
200k
Vout+
C3, C4
2 x 22µF
LL
C1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3
C3, C4 MuRata 22µF 6.3V GRM21BR60J226ME396 X5R 0805
L1 Sumida CDRH3D16-4R7NC
Figure 1: AAT1156 Evaluation Board Schematic—Lithium-Ion to 2.5V Converter.
Current Limit and Over-Temperature
Protection
For overload conditions, the peak input current is lim-
ited. As load impedance decreases and the output
voltage falls closer to zero, more power is dissipated
internally, raising the device temperature. Thermal
protection completely disables switching when inter-
nal dissipation becomes excessive, protecting the
device from damage. The junction over-temperature
threshold is 140°C with 15°C of hysteresis.
L=
L=
⎛
V
⎞
V
OUT
⋅
1 -
OUT
I
O
⋅
k
⋅
F
⎝
V
IN
⎠
1.5V
⎛
1.5V
⎞
⋅
1-
0.7A
⋅
0.4
⋅
1MHz
⎝
4.2V
⎠
L = 3.44µH
The factor "k" is the fraction of full load selected for
the ripple current at the maximum input voltage.
For ripple current at 40% of the full load current, the
peak current will be 120% of full load. Selecting a
standard value of 3.3µH gives 42% ripple current.
A 3.3µH inductor selected from the Sumida
CDRH3D16 series has a 63mΩ DCR and a 1.1A
DC current rating. At full load, the inductor DC loss
is 31mW which amounts to less than 3% loss in
efficiency for a 0.7A, 1.5V output.
Inductor
The output inductor is selected to limit the ripple cur-
rent to a predetermined value, typically 20% to 40%
of the full load current at the maximum input voltage.
Manufacturer's specifications list both the inductor
DC current rating, which is a thermal limitation, and
the peak current rating, which is determined by the
saturation characteristics. The inductor should not
show any appreciable saturation under normal load
conditions. Some inductors may meet the peak and
average current ratings yet result in excessive losses
due to a high DCR. Always consider the losses asso-
ciated with the DCR and its effect on the total con-
verter efficiency when selecting an inductor.
For a 0.7A, 1.5V output with the ripple set to 40%
at a maximum input voltage of 4.2V, the maximum
peak-to-peak ripple current is 280mA. The induc-
tance value required is 3.44µH.
Input Capacitor
The primary function of the input capacitor is to pro-
vide a low impedance loop for the edges of pulsed
current drawn by the AAT1156. A low ESR/ESL
ceramic capacitor is ideal for this function. To mini-
mize stray inductance, the capacitor should be
placed as closely as possible to the IC. This keeps
the high frequency content of the input current local-
ized, minimizing radiated and conducted EMI while
facilitating optimum performance of the AAT1156.
Ceramic X5R or X7R capacitors are ideal for this
function. The size required will vary depending on
1156.2005.11.1.2
8
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