PRODUCT DATASHEET
AAT1130
TM
SwitchReg
2.5MHz 500mA Step-Down DC/DC Converter
tion mode. In this mode the on-time remains the same
as it is in continuous conduction mode, and therefore the
inductor ripple current remains the same in both modes.
But reduced load current requires more time for the out-
put capacitor to discharge to the regulation voltage
reducing the switching frequency. This has the added
benefit of reducing the switching transition losses
improving efficiency at light loads.
Example:
Given that VOUT = 1.2V, then COUT > 4.24ꢀF, therefore a
4.7ꢀF capacitor is suitable.
Due to the unique control method, the “inside” current
control loop does not have the inherent instability that
plagues most fixed frequency current-mode DC-DC reg-
ulators.
Stability
Soft-Start
The AAT1130 requires no additional compensation com-
ponents to guarantee stability. The only requirement for
stability is to choose the appropriate output capacitor.
When the AAT1130 is enabled, it enters soft-start mode.
In this mode, the output voltage slowly rises over 150ꢀs
allowing the output capacitor to charge without drawing
excessive input current. This feature prevents over-
stressing the battery or other input power source.
Current-mode control simplifies compensation by con-
trolling the inductor current to regulate the output volt-
age. This approximates a single pole response in the
loop gain even though a complex pole pair exists due to
the LC filter. Therefore the crossover frequency is
approximated as the DC loop gain multiplied by the
single pole. The AAT1130 DC loop gain is a function of
the 60mΩ current sense resistor and is determined by
the equation:
Current Limit
The AAT1130 includes a cycle-by-cycle current limit to
prevent damage to itself and external circuitry. The cur-
rent limit is a valley current limit using the n-channel
synchronous rectifier to measure the current. If the syn-
chronous rectifier current is above the valley current
limit, the AAT1130 holds the synchronous rectifier on
until the current is below the limit. This allows the
AAT1130 to control the current in current limit even with
a hard shorted output.
VOUT RLOAD
0.6V 60mΩ
ALOOP(DC)
=
·
And the dominant pole frequency is:
1
Anti-Ringing Switch
fP =
2π · RLOAD · COUT
The AAT1130 includes an anti-ringing switch that dissi-
pates any energy left in the inductor when the current is
approximately zero. The anti-ringing switch turns on
when both the p-channel switch and n-channel synchro-
nous rectifier are off and the inductor current is approx-
imately zero. The switch shorts the LX and VOUT nodes
together, effectively shorting the inductor. The low on-
resistance of the anti-ringing switch dissipates any
energy left in the inductor preventing ringing at light
loads. When either the switch or synchronous rectifier
are on, the anti-ringing switch remains off.
Therefore the crossover frequency is:
fC = ALOOP(DC) · fP
VOUT
=
2π · 0.6V · 60mΩ · COUT
The only requirement for stability is that the crossover
frequency be much less than the 2.5MHz switching fre-
quency. The crossover frequency can be as high as 1/2
of the switching frequency, or 1.25MHz. Therefore calcu-
late the output capacitor by the equation:
Over-Temperature
The AAT1130 includes thermal protection that automati-
cally turns off the regulator when the die temperature
exceeds a safe level. The thermal protection turns on at
a die temperature of 140°C and has a 15°C hysteresis.
VOUT
COUT
>
2π · 0.6V · 60mΩ · 1.25MHz
w w w . a n a l o g i c t e c h . c o m
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1130.2009.03.1.4
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
