AAT2138 Datasheet, PDF (13/18 Pages) Skyworks Solutions Inc. – High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch

DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Dropout Voltage
Dropout voltage is determined by R_DS(ON) and the current
passing through it. AAT2138 load switch typical R_DS(ON) is
80mΩ for USB application. So, for a 750mA load switch
current limit setting, the load switch dropout voltage can
be calculated by:
V_{DROPOUT_SWITCH} = 750mA · 80mΩ = 60mV
Step-Down Converter
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.
The inductor should be set equal to half the output volt-
age numeric value in μH. This guarantees that there is
sufficient internal slope compensation.
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.
C_REGIN Selection
C_REGIN 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
is lower than the V_OUT plus the dropout voltage, the
AAT2138 enters dropout mode. C_REGIN minimum value
can be calculated by the following steps:
First, calculate the allowed maximum delta voltage on
C_REGIN to keep V_OUT in regulation:
ΔV_REGIN = 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_SYSIN - V_{DROPOUT_SWITCH} ^{· η}
Next, calculate the maximum current C_REGIN should pro-
vide:
I_CREGIN = I_BUCKIN - I_LIM
Finally, derive the C_SYSOUT at certain load on period T_ON
C = _{REGIN_MIN}
I_CREGIN · T_ON
∆V_REGIN
Example: A 2A, 217Hz, 12.5% load pulse is applied on
3.8V V_OUT in 5V V_IN and 750mA load switch current limit,
under the condition, V_{DROPOUT_SWITCH} is 0.06V, V_{DROPOUT_BUCK}
is 0.17V. Therefore, considering the step-down converter
at 2A, 3.8V, V_OUT is 90%.
ΔV_REGIN = 5 - 3.8 - 0.06 - 0.17 = 0.970V
I_BUCKIN =
3.8 · 2
5 - 0.06 ^· 0.9
= 1.71A
I_CREGIN = 1.71 - 0.75 = 0.96A
t_ON is 576μs for a 217Hz, 12.5% duty cycle load pulse.
C = _{REGIN_MIN}
0.96 · 576
= 570μF
0.970
V_OUT
500mV/div
V_REGIN
0
2V/div
I_SYSIN
0
1A/div
I_OUT
1A/div
0
800μs/div
Figure 2: AAT2138 Operation Waveform
2A, 577μs Load Pulse (Applied)
V_IN = 5V, V_OUT = 3.8V.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013
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