AAT1138 Datasheet, PDF (15/21 Pages) Skyworks Solutions Inc. – 2A Step-Down Converter with Adjustable Input Current Limit

DATA SHEET
AAT1138
2A Step-Down Converter with Adjustable Input Current Limit
To 2A step-down converter, the maximum dropout volt-
age is:
V_{Dropout_Buck} = 2A · 200mΩ = 0.4V
Operation in Heavy Load Pulse
When a heavy load pulse is applied to the output of
AAT1138 as typical application shows, the input current
is limited to the value of I_LIM determined by R_SET. At this
time, C_SYSOUT has the important role of providing enough
current and voltage to the step-down converter input to
bring it into regulation. The duty cycle of the heavy load
pulse should not exceed the maximum value which allows
sufficient time to charge C_SYSOUT from V_IN and balance the
capacitor charging and discharging to make the operation
normally. Figure 2 shows the operation waveform at 5V
V_IN and 3.8V V_OUT with a 2A load pulse applied when R_SET
= 11.5kΩ, C_IN = 1μF, C_SYSOUT = 22μF ceramic capacitor +
4x330μF tantalum capacitor, and C_OUTB = 22μF.
V_IN
(100mV/div)
5V
V_SYSOUT
(AC Coupled)
(1V/div)
V_OUT
3.8V
(200mV/div)
I_IN
(500mA/div)
0A
I_OUT
0A
(2A/div)
Time (1ms/div)
Figure 2: AAT1138 Operation Waveform when 2A
217Hz 12.5% Load Pulse is Applied.
Capacitor Selection
C_SYSOUT Selection
C_SYSOUT is not only the load switch output capacitor but
also the step-down converter input capacitor. It is
designed to provide the additional input current and
maintain the SYSOUT voltage for the step-down con-
verter when load switch limits the input current from
SYSIN. If the input voltage of the step-down converter
(V_PVIN and V_SVIN) is lower than the V_OUT plus the dropout
voltage, the AAT1138 enters dropout mode.
C_SYSOUT minimum value can be calculated by the following
steps:
First, calculate the allowed maximum delta voltage on
C_SYSOUT to keep V_out in regulation:
∆V_SYSOUT = V_IN - V_OUT - V_{Dropout_Switch} - V_{Dropout_Buck}
Second, calculate the required input current at SYSOUT
for the step-down converter:
I_BUCKIN =
V_OUT · I_OUT
(V_IN - V ) _{Dropout_Switch} · η
Next, calculate the maximum current C_SYSOUT should pro-
vide:
I_CSYSOUT = I_BUCKIN - I_LIM
Finally, derive the C_SYSOUT at certain load on period T_ON.
C_{SYSOUT_Min} =
I_CSYSOUT · T_ON
ΔV_SYSOUT
Example: A 2A 217Hz 12.5% load pulse is applied on
3.8V V_OUT in 5V V_IN and 500mA load switch current limit.
Under the condition, V_{Dropout_Switch} is 0.15V. V_{Dropout_Buck} is
0.4V. Therefore:
∆V_SYSOUT = 5 - 3.8 - 0.15 - 0.4 = 0.65V
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013
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