AAT1217
600mA, 1.2MHz, Micropower
Synchronous Step-Up Converter
Part Number
L (µH)
Max DCR (mΩ)
Rated DC Current (A)
Size WxLxH (mm)
2.2
4.7
10
2.2
4.7
10
2.2
4.7
10
2.2
4.7
10
2.2
4.7
10
4.7
10
123
238
431
1.15
0.75
0.53
1.12
0.8
Sumida CDH28D11/S
3x3.3x1.2
140 (typ)
246 (typ)
446 (typ)
100
Coiltronics SD3112
TDK VLF3012A
Sumida CR43
3.1x3.1x1.2
2.8x2.6x1.2
4.3x4.8x3.5
0.55
1
190
410
0.74
0.49
1.75
1.15
1.04
2.04
1.32
1
71.2
108.7
182
31.3
72
128
45
90
Sumida CDRH4D28
Toko D53LC
5.0x5.0x3.0
5.0x5.0x3.0
1.87
1.33
Table 2. Typical Surface Mount Inductors.
Output Capacitor
Load Disconnect in Shutdown
The output capacitor limits the output ripple and pro-
vides holdup during large load transitions. A 4.7ꢀF to
10ꢀF, X5R or X7R, ceramic capacitor is suggested
for the output capacitor. Typically the recommended
capacitor range provides sufficient bulk capacitance
to stabilize the output voltage during large load tran-
sitions and has the ESR and ESL characteristics
necessary for low output voltage ripple.
In conventional synchronous step-up converters, a
conduction path exists from input to output through
the backgate (body diode) of the P-channel MOS-
FET during shutdown. Special application circuitry
can disconnect the load from the battery during
shutdown (see Figure 1).
PCB Layout Guidance
In addition, the output voltage droop during load
transient is dominated by the capacitance of the
ceramic output capacitor. During a step increase in
load current, the ceramic output capacitor alone
supplies the load current until the loop responds.
Within several switching cycles, the loop responds
and the inductor current increases to match the
load current demand. Larger output capacitor val-
ues help to reduce the voltage droop during large
load current transients.
The AAT1217 typically operates at 1.2MHz. This is a
considerably high frequency for DC-DC converters.
PCB layout is important to guarantee satisfactory
performance. It is recommended to make traces of
the power loop, especially where the switching node
is involved, as short and wide as possible. First of
all, the inductor, input and output capacitor should
be as close as possible to the device. Feedback and
shutdown circuits should avoid the proximity of large
AC signals involving the power inductor and switch-
ing node. The optional rectifier diode (D1 in Figure
1) can improve efficiency and alleviate the stress on
the integrated MOSFETs. The diode should also be
close to the inductor and the chip to form the short-
est possible switching loop. While the two-layer PCB
shown in Figures 2 and 3 is enough for most appli-
cations, large and integral multi-layer ground planes
External Diode Selection
An external Schottky diode is required when the
output voltage is above 4.5V. The Schottky diode is
optional for output voltages ≤ 4.5V, but can
improve efficiency by about 2% to 3%.
10
1217.2007.07.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
