AAT1232
24V 100mA Step-Up Converter
D1
Schottky
L1
2.2µH
R4
10k
VOUT
VIN
JP1
U1
1
2
3
4
5
6
12
11
10
9
8
7
3
2
1
VP
LIN
FB1
EN/SET
SEL
VIN
R1
576k
FB2
C2
2.2µF
GND
PGND
SW
Enable
JP2
N/C
SW
C1
2.2µF
R2
10k
3
2
1
R3
20k
Select
U1 AAT1232 TSOPJW12
C1 10V 0805 2.2µF
C2 25V 0805 2.2µF
D1 30V 0.5A MBR0530T SOD-123
L1 2.2µH NR4018T2R2
R1 576k 0603
R2, R4 10k 0603
R3 20k 0603
Figure 1: AAT1232 Demo Board Schematic.
Minimum 6.3V rated ceramic capacitors are required
at the input. Ceramic capacitors sized as small as
0603 are available which meet these requirements.
Application Information
Selecting DC/DC Boost Capacitors
The high output ripple inherent in the boost con-
verter necessitates low impedance output filtering.
Multi-layer ceramic (MLC) capacitors provide small
size and adequate capacitance, low parasitic
equivalent series resistance (ESR) and equivalent
series inductance (ESL), and are well suited for
use with the AAT1232 boost regulator. MLC capac-
itors of type X7R or X5R are recommended to
ensure good capacitance stability over the full
operating temperature range.
Large capacitance tantalum or solid-electrolytic
capacitors may be necessary to meet stringent out-
put ripple and transient load requirements. These
can replace (or be used in parallel with) ceramic
capacitors. Both tantalum and OSCON-type capac-
itors are suitable due to their low ESR and excellent
temperature stability (although they exhibit much
higher ESR than MLC capacitors). Aluminum-elec-
trolytic types are less suitable due to their high ESR
characteristics and temperature drift. Unlike MLC
capacitors, these types are polarized and proper
orientation on input and output pins is required.
30% to 70% voltage derating is recommended for
tantalum capacitors.
The output capacitor is sized to maintain the output
load without significant voltage droop during the
power switch ON interval, when the output diode is
not conducting. A ceramic output capacitor from
2.2µF to 4.7µF is recommended. Typically, 25V
rated ceramic capacitors are required for the 24V
boost output. Ceramic capacitors sized as small as
0805 are available which meet these requirements.
Selecting the Output Diode
To ensure minimum forward voltage drop and no
recovery, high voltage Schottky diodes are consid-
ered the best choice for the AAT1232 boost convert-
er. The AAT1232 output diode is sized to maintain
acceptable efficiency and reasonable operating
junction temperature under full load operating condi-
tions. Forward voltage (VF) and package thermal
resistance (θJA) are the dominant factors to consider
in selecting a diode. The diode's published current
rating may not reflect actual operating conditions
and should be used only as a comparative measure
between similarly rated devices. 20V rated Schottky
MLC capacitors exhibit significant capacitance
reduction with applied voltage. Output ripple
measurements should confirm that output voltage
droop is acceptable.
The boost converter input current flows during both
ON and OFF switching intervals. The input ripple
current is less than the output ripple and, as a result,
less input capacitance is required. A ceramic output
capacitor from 1µF to 3.3µF is recommended.
10
1232.2006.12.1.3
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
