AAT2504
Adjustable 3-Channel Regulator
sation terminates the transconductance voltage
error amplifier output. The internal error amplifier
reference is fixed at 0.9V.
Applications Information
Step-Down Converter Inductor Selection
A logic low on the EN pin shuts the converter down
and makes it consume less than 1µA of current.
The step-down converter uses peak current mode
control with slope compensation to maintain stabil-
ity for duty cycles greater than 50%. The output
inductor value must be selected so the inductor
current down slope meets the internal slope com-
pensation requirements. The internal slope com-
pensation for the AAT2504 step-down converter is
0.51A/µsec. This equates to a slope compensation
that is 75% of the inductor current down slope for a
1.5V output and 2.2µH inductor.
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
limited. As load impedance decreases and the out-
put voltage falls closer to zero, more power is dis-
sipated internally, raising the device temperature.
Thermal protection completely disables switching
when internal dissipation becomes excessive, pro-
tecting 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
m =
=
L
2.2μH
μsec
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
saturation 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 loss-
es 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 inde-
pendent input voltages operating from 1.8V to 5.5V.
An external feedback pin for each LDO allows pro-
gramming the output voltage from 3.6V to 0.6V.
The regulators have thermal protection 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 compara-
tor which indicates when the output is out of regu-
lation. 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 capac-
itor for the input of the step-down converter. To esti-
mate 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.
Over-Temperature Protection
Thermal protection completely disables switching
when internal dissipation becomes excessive. The
junction over-temperature threshold is 140°C with
15°C of hysteresis. Once an over-temperature or
over-current fault condition is removed, the output
voltage automatically recovers.
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2504.2007.06.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
