AAT2784 Analogic Corporation, AAT2784 Datasheet - Page 12

• Manufacturer Part Number: AAT2784
• Manufacturer: Analogic Corporation

circuit and overload protection. A fixed slope compensa-
tion signal is added to the sensed current to maintain
stability for duty cycles greater than 50%. The peak cur-
rent mode loop appears as a voltage-programmed cur-
rent source in parallel with the output capacitor. The
output of the voltage error amplifier programs the cur-
rent mode loop for the necessary peak switch current to
force a constant output voltage for all load and line con-
ditions. Internal loop compensation terminates the
transconductance voltage error amplifier output. The
reference voltage is internally set to program the con-
verter output voltage greater than or equal to 0.6V.
Soft Start/Enable
Soft start limits the current surge seen at the input and
eliminates output voltage overshoot. When pulled low,
the enable input forces the AAT2784 into a low-power,
non-switching state. The total input current during shut-
down is less than 1μA.
Low Dropout Operation
For conditions where the input voltage drops to the out-
put voltage level, the converter duty cycle increases to
100%. As the converter approaches the 100% duty
cycle, the minimum off time initially forces the high side
in time to exceed the 1.8MHz clock cycle and reduce the
effective switching frequency. Once the input drops
below the level where the converter can regulate the
output, the high side P-channel MOSFET is enabled con-
tinuously for 100% duty cycle. At 100% duty cycle the
output voltage tracks the input voltage minus the I*R
drop of the high side P-channel MOSFET.
Current Limit and
Over-Temperature Protection
For overload conditions, the peak input current is limit-
ed. To minimize power dissipation and stresses under
current limit and short-circuit conditions, switching is
terminated after entering current limit for a series of
pulses. Switching is terminated for seven consecutive
clock cycles after a current limit has been sensed for a
series of four consecutive clock cycles. Thermal protec-
tion completely disables switching when internal dissipa-
tion 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.
12
SystemPower
TM
w w w . a n a l o g i c t e c h . c o m
Under-Voltage Lockout
Internal bias of all circuits is controlled via the V
Under-voltage lockout (UVLO) guarantees sufficient V
bias and proper operation of all internal circuitry prior to
activation.
Component Selection
Inductor Selection: Channels 1 and 2
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 adjustable and
low voltage fixed versions of channels 1 and 2 is 0.6A/μ.
This equates to a slope compensation that is 75% of the
inductor current down slope for a 1.8V output and 2.2μH
inductor.
In this case a standard 4.7μH value is selected. Table 1
displays the suggested inductor values for channels 1
and 2. The 4.7μH CDRH2D11 series inductor selected
from Sumida has a 170mΩ DCR and a 0.88A DC current
rating. At full load the inductor DC loss is 15mW which
corresponds to a 1.5% loss in efficiency for a 300mA,
3.3V output.
inductor has a 260mΩ worst case DCR and a 475mA DC
current rating. At full 300mA load, the inductor DC loss
is 23mW which gives less than 7% loss in efficiency for
a 300mA, 3.3V output.
Inductor Selection: Channel 3
The internal slope compensation for the adjustable and
low voltage fixed versions of channel 3 is 0.75A/μs. This
equates to a slope compensation that is 75% of the
inductor current down slope for a 1.8V output and 1.8μH
inductor.
3-Channel Step-Down DC/DC Converter
m =
L =
m =
0.75 ⋅ V
0.75 ⋅ V
0.75 ⋅ V
For 4.7μH GLF2518T4R7M series TDK
m
L
L
PRODUCT DATASHEET
O
O
O
=
=
=
0.75 ⋅ 1.8V
0.75 ⋅ 1.8V
0.75 ⋅ 3.3V
1.8µH
0.6
2.2µH
AAT2784
µs
A
= 0.75
= 0.6
= 4.1µH
2784.2007.11.1.1
µs
A
µs
A
IN
input.
IN