PRODUCT DATASHEET
AAT2504178
SystemPowerTM
Adjustable 3-Channel Regulator
The output of the voltage error amplifier programs the
current mode loop for the necessary peak switch current
to force a constant output voltage for all load and line
conditions. Internal loop compensation terminates the
transconductance voltage error amplifier output. The
internal error amplifier reference is fixed at 0.9V.
Applications Information
Step-Down Converter Inductor Selection
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.
The internal slope compensation for the AAT2504 step-
down converter is 0.51A/μs. This equates to a slope
compensation that is 75% of the inductor current down
slope for a 1.5V output and 2.2μH inductor.
A logic low on the EN pin shuts the converter down and
makes it consume less than 1μA of current.
Soft start increases the inductor current limit point in
discrete steps when the input voltage or enable input is
applied. It limits the current surge seen at the input and
eliminates output voltage overshoot.
For overload conditions, the peak input current is limit-
ed. As load impedance decreases and the output voltage
falls closer to zero, more power is dissipated internally,
raising the device temperature. Thermal protection com-
pletely disables switching when internal dissipation
becomes excessive, protecting the device from damage.
The junction over-temperature threshold is 140°C with
15°C of hysteresis.
0.75 ⋅ VO 0.75 ⋅ 1.5V
= 0.51
A
µs
m =
=
L
2.2µH
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.
Linear Regulators
The two linear regulators are high performance LDOs
where each LDO sources 300mA of current. For added
flexibility, both regulators have independent input volt-
ages operating from 1.8V to 5.5V. An external feedback
pin for each LDO allows programming the output voltage
from 3.6V to 0.6V. The regulators have thermal protec-
tion in case of adverse operating conditions.
The 2.2μH CDRH2D14 series Sumida inductor has a
94mΩ DCR and a 1.5A DC current rating. At full 800mA
load, the inductor DC loss is 60mW which gives a 4.16%
loss in efficiency for a 800mA, 1.8V output.
LDOA features an integrated Power-OK comparator
which indicates when the output is out of regulation. The
POK is an open drain output and it is held low when the
AAT2504 is in shutdown mode.
Input Capacitor
Select a 4.7μF to 10μF X7R or X5R ceramic capacitor for
the input of the step-down converter. 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
VIN is double the output voltage.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VIN pin.
Under-voltage lockout guarantees sufficient VIN bias and
proper operation of all internal circuits prior to activation.
VO
VIN
⎛
VO ⎞
VIN ⎠
· 1 -
⎝
CIN =
Over-Temperature Protection
⎛ VPP
⎝ IO
⎞
⎠
- ESR ·FS
Thermal protection completely disables switching when
internal dissipation becomes excessive. The junction
over-temperature threshold is 140°C with 15°C of hys-
teresis. Once an over-temperature fault condition is
removed, the output voltage automatically recovers.
VO
VIN
⎛
VO ⎞
VIN ⎠
1
· 1 -
⎝
=
for VIN = 2 · VO
4
1
CIN(MIN)
=
⎛ VPP
⎝ IO
⎞
⎠
- ESR · 4 · FS
w w w . a n a l o g i c t e c h . c o m
12
2504.2008.02.1.2
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
