AAT1230_07 Datasheet, PDF (12/21 Pages) Advanced Analogic Technologies – 18V 100mA Step-Up Converter

2.0
1.8
VIN = 3.0V
1.6
VOUT = 12V
VIN = 3.0V
VOUT = 10V
1.4
1.2
1.0
0.8
VIN = 2.7V VIN = 2.7V
0.6
VOUT = 10V VOUT = 12V
0.4
40
50
60
70
VIN = 3.6V VIN = 3.6V
VOUT = 12V VOUT = 10V
80
90
100
Output Current (mA)
I_PEAK =
I_OUT
(1 - D_MAX)
+
D_MAX · V_IN(MIN)
(2 · F_S · L)
At light load and low output voltage, the controller
reduces the operating frequency to maintain maxi-
mum operating efficiency. As a result, further
reduction in output load does not reduce the peak
current. Minimum peak current can be estimated
from 0.5A to 0.75A.
The RMS current flowing through the boost induc-
tor is equal to the DC plus AC ripple components.
Under worst-case RMS conditions, the current
waveform is critically continuous. The resulting
RMS calculation yields worst-case inductor loss.
The RMS current value should be compared
against the manufacturer's temperature rise, or
thermal derating, guidelines.
I_RMS =
I_PEAK
3
For a given inductor type, smaller inductor size leads
to an increase in DCR winding resistance and, in
most cases, increased thermal impedance. Winding
resistance degrades boost converter efficiency and
increases the inductor’s operating temperature.
P_{LOSS(INDUCTOR)} = I_RMS2 · DCR
To ensure high reliability, the inductor temperature
should not exceed 100ºC. In some cases, PCB
heatsinking applied to the AAT1230/1230-1 L_IN
node (non-switching) can improve the inductor's
thermal capability. PCB heatsinking may degrade
12
AAT1230/1230-1
18V 100mA Step-Up Converter
EMI performance when applied to the SW node
(switching) of the AAT1230/1230-1.
Shielded inductors provide decreased EMI and may
be required in noise sensitive applications.
Unshielded chip inductors provide significant space
savings at a reduced cost compared to shielded
(wound and gapped) inductors. In general, chip-
type inductors have increased winding resistance
(DCR) when compared to shielded, wound varieties.
Selecting the 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 AAT1230/1230-1 boost regulator.
MLC capacitors of type X7R or X5R are recom-
mended to ensure good capacitance stability over
the full operating temperature range.
The output capacitor is sized to maintain the output
load without significant voltage droop (ΔV_OUT) dur-
ing the power switch ON interval, when the output
diode is not conducting. A ceramic output capaci-
tor from 2.2µF to 4.7µF is recommended. Typically,
25V rated capacitors are required for the 18V boost
output. Ceramic capacitors sized as small as 0805
are available which meet these requirements.
MLC capacitors exhibit significant capacitance
reduction with applied voltage. Output ripple
measurements should confirm that output voltage
droop is acceptable. Voltage derating can mini-
mize this factor, but results may vary with package
size and among specific manufacturers.
Output capacitor size can be estimated at a switch-
ing frequency (F_SW) of 500kHz (worst-case).
C_OUT =
I_OUT · D_MAX
F_S · ΔV_OUT
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.
1230.2007.06.1.6