PRODUCT DATASHEET
AAT2608A
TM
BatteryManager
8 Channel LDO with DC/DC Converter
The step-down converter offers soft-start to limit the
current surge seen at the input and eliminate output
voltage overshoot. The current across the internal
P-channel power switch is sensed and turns off when the
current exceeds the current limit. Also, thermal protec-
tion completely disables switching if internal dissipation
becomes excessive, thus protecting the device from
damage. The junction over-temperature threshold is
145°C with 25°C of hysteresis.
Application Information
Synchronous Step-
Down (Buck) Converter
The step-down converter uses peak current mode con-
trol with slope compensation to maintain stability for
duty cycles greater than 50%. The output inductor value
must be selected so the inductor current down slope
meets the internal slope compensation requirements.
Table 1 displays suggested inductor values for various
output voltages.
The Buck converter is designed for a peak continuous
output current of 800mA. It was designed to maintain
over 80% efficiency at its maximum rated output current
load of 800mA with a 1.2V output. Peak efficiency is
above 90%. It also has excellent transient response,
load and line regulation. Transient response time is typi-
cally less than 60ns.
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 satura-
tion 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 associated with the DCR and
its effect on the total converter efficiency when selecting
an inductor.
LDO Regulators
The AAT2608A includes eight LDO regulators. The regu-
lators operate from the 2.7V to 5.5V input voltage to a
regulated output voltage. Each LDO regulator has its
own independent enable pin. LDO regulators 1, 7 and 8
have adjustable output voltages set by resistors. LDO
regulators 2 thru 6 have a fixed output programmed dur-
ing manufacturing. Each LDO consumes 30μA of quies-
cent current and is stable with a small 1.0ꢀF ceramic
output capacitor. These LDOs offer high power supply
rejection, over-current protection and over-temperature
protection.
Input Capacitor
Select a 4.7ꢀF to 10ꢀF X7R or X5R ceramic capacitor for
the input; see Table 3 for suggested capacitor compo-
nents. To estimate the required input capacitor size,
determine the acceptable input ripple level (VPP) and solve
for CIN. The calculated value varies with input voltage and
is a maximum when VCC is double the output voltage.
VO
VIN
VO
VIN
Feedback Resistor Selection
· 1 -
The output voltage of the buck converter and LDOs 1, 7
and 8 is each set with an individual resistor divider feed-
back network. The voltage output of the buck converter
can be set between 0.6V and 80% of VIN. The voltage
output of the LDOs can be set between 0.6V and VIN
VDO.
CIN =
VPP
IO
- ESR · FS
VO
VIN
VO
VIN
1
4
· 1 -
=
for VIN = 2 · VO
-
1
CIN(MIN)
=
VOUT is defined by the following equation:
VPP
IO
- ESR · 4 · FS
R1
VOUT = 0.6V · 1 +
R2
Always examine the ceramic capacitor DC voltage coef-
ficient characteristics when selecting the proper value.
For example, the capacitance of a 10ꢀF, 6.3V, X5R ceram-
ic capacitor with 5.0V DC applied is actually about 6ꢀF.
To minimize the current consumed by the voltage divider
and to isolate the FB pin from noise, the resistor values
for R1 and R2 should be beween 5kΩ and 500kΩ.
w w w . a n a l o g i c t e c h . c o m
2608A.2010.05.1.2
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