AAT1130 Datasheet, PDF (11/18 Pages) Advanced Analogic Technologies – 2.5MHz 500mA Step-Down DC/DC Converter

DATA SHEET
AAT1130
2.5MHz 400mA Step-Down DC/DC Converter
cent current and automatic transition to variable switch-
ing frequency mode makes it ideal for small battery
operated applications.
Light Load Operation
The AAT1130 monitors the synchronous rectifier current
and when the current drops to zero, it turns off the syn-
chronous rectifier to emulate an actual rectifier. This
allows the regulator to operate in discontinuous conduc-
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.
Stability
The AAT1130 requires no additional compensation com-
ponents to guarantee stability. The only requirement for
stability is to choose the appropriate output capacitor.
Current-mode control simplifies compensation by control-
ling the inductor current to regulate the output voltage.
This approximates a single pole response in the loop gain
even though a complex pole pair exists due to the LC fil-
ter. 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:
A_LOOP(DC)
=
V_OUT
0.6V
·
R_LOAD
60mΩ
And the dominant pole frequency is:
1
f_P = _{2π · RLOAD · COUT}
Therefore the crossover frequency is:
f_C = A_LOOP(DC) · f_P
=
2π
V_OUT
· 0.6V · 60mΩ · C_OUT
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:
C_OUT
>
2π
·
0.6V
·
V_OUT
60mΩ
·
1.25MHz
Example:
Given that V_OUT = 1.5V, then C_OUT > 5.3μF, therefore a
4.7μF capacitor is the closest standard value that can be
used for 1.2V to 1.5V output.
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.
Soft-Start
When the AAT1130 is enabled, it enters soft-start mode.
In this mode, the output voltage slowly rises over 150μs
allowing the output capacitor to charge without drawing
excessive input current. This feature prevents over-
stressing the battery or other input power source.
Valley Current Limit
The AAT1130 includes a cycle-by-cycle, valley current
limit to prevent damage to itself or the external compo-
nents. The valley current limit uses the low-side-N-
Channel synchronous rectifier to monitor the inductor
current. If the measured current exceeds the valley cur-
rent limit, the AAT1130 keeps the low-side synchronous
rectifier on until the current drops below the current
limit. Along with the predictive on-time control scheme,
the valley current limit protection allows the converter to
control and limit the inductor current, even with output
overload or short-circuit fault condition.
Since the AAT1130 uses a predictive on-time architec-
ture (constant on-time with input feed-forward), the
actual output current capability is a function of the
inductor ripple current (ΔI_L) and the current limit com-
parator delay (see Figure 1):
I_{OUT_MAX}
=
I_{LIM_VALLEY}
+
∆I_L
2
-
V_OUT
L
· t_DELAY
I = I + _{OUT_MAX LIM_VALLEY}
V_IN - V_OUT
2L
·
V_OUT
V_IN
· t_SW -
V_OUT
L
· t_DELAY
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201977B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013
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