AAT2500B Datasheet, PDF (12/21 Pages) Advanced Analogic Technologies – Adjustable 3-Channel Regulator

SystemPower^TM
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 internally dissipated,
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.
Linear Regulators
The two linear regulators are high performance LDOs
where LDOA sources 500mA and LDOB sources 150mA
of current. For added flexibility, both regulators have
independent input voltages operating from 1.8V to 5.5V.
An external feedback pin for each LDO allows program-
ming the output voltage from 3.6V to 0.6V. The regula-
tors have short-circuit and thermal protection in case of
adverse operating conditions.
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
AAT2500B is in shutdown mode.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VIN pin.
Under-voltage lockout guarantees sufficient V_IN 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 hys-
teresis. Once an over-temperature or over-current fault
conditions is removed, the output voltage automatically
recovers.
PRODUCT DATASHEET
AAT2500B
Adjustable 3-Channel Regulator
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 AAT2500B 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.
m
=
0.75 ⋅
L
V_O
=
0.75 ⋅ 1.5V
2.2μH
=
0.51
A
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 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.
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.
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 (V_PP) and solve for C_IN. The calculated
value varies with input voltage and is a maximum when
V_IN is double the output voltage.
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
C_IN =
⎛ V_PP
⎝ I_O
- ESR^⎞⎠ · F_S
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
=
1
4
for
V_IN
=
2
·
V_O
1
C_IN(MIN) = _{⎛ VPP}
⎝ I_O
- ESR^⎞⎠ · 4 · F_S
12
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2500B.2008.03.1.0