PRODUCT DATASHEET
AAT1149
SwitchRegTM
3MHz FastTransient 400mA Step-Down Converter
Control Loop
Applications Information
The AAT1149 is a peak current mode step-down con-
verter. The current through the P-channel MOSFET (high
side) is sensed for current loop control, as well as short
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.
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.
Table 1 displays suggested inductor values for various
output voltages.
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. For the
adjustable output, the error amplifier reference is fixed
at 0.6V.
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.
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 AAT1149 into a low-power,
non-switching state. The total input current during shut-
down is less than 1μA.
The 1.8μH CDRH2D09 series inductor selected from
Sumida has a 131mW DCR and a 400mA saturation cur-
rent rating. At full load, the inductor DC loss is 21mW
which gives a 2.8% loss in efficiency for a 400mA, 1.8V
output.
Current Limit and
Over-Temperature Protection
Input Capacitor
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.
Select a 4.7μF to 10μF X7R or X5R ceramic capacitor for
the input. To estimate the required input capacitor size,
determine the acceptable input ripple level (VPP) and solve
for C. The calculated value varies with input voltage and
is a maximum when VIN is double the output voltage.
VO
VIN
⎛
VO ⎞
VIN ⎠
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.
· 1 -
⎝
CIN =
⎛ VPP
⎝ IO
⎞
⎠
- ESR ·FS
VO
VIN
⎛
VO ⎞
VIN ⎠
1
· 1 -
⎝
=
for VIN = 2 · VO
4
1
Under-Voltage Lockout
CIN(MIN)
=
⎛ VPP
⎝ IO
⎞
⎠
- ESR · 4 · FS
Internal bias of all circuits is controlled via the IN input.
Under-voltage lockout (UVLO) guarantees sufficient VIN
bias and proper operation of all internal circuitry prior to
activation.
Always examine the ceramic capacitor DC voltage coef-
ficient characteristics when selecting the proper value.
For example, the capacitance of a 10μF, 6.3V, X5R ceram-
ic capacitor with 5.0V DC applied is actually about 6μF.
w w w . a n a l o g i c t e c h . c o m
1149.2008.08.1.3
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