AAT1138 Datasheet, PDF (14/21 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
Application Information
Setting the Load Switch Current Limit
The AAT1138's load switch current limit can be pro-
grammed by an external resistor R_SET from SET to GND.
In most applications, the variation in I_LIM must be taken
into account when determining R_SET. The I_LIM variation is
due to processing variations from part to part, as well as
variations in the voltages at SYSIN and SYSOUT, plus the
operating temperature. Together, these three factors add
up to a ±25% tolerance (see load switch I_LIM specification
in Electrical Characteristics section). Figure 1 illustrates
a cold device with a statistically higher current limit and
a hot device with a statistically lower current limit, both
with R_SET equal to 10.3kΩ. A 10.3kΩ R_SET resistor sets the
typical current limit to 0.665A. This figure shows that the
actual current limit will be at least 0.5A and no greater
than 0.83A.
1.2
1.0
0.83A
0.8
At -40°C
0.6
0.5A
0.4
At 85°C
0.2
0.0
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
V_SYSOUT (V)
Figure 1: Load Switch Current Limit
(R_SET = 10.3kΩ).
Though the relationship between typical I_LIM and R_SET is
not linear throughout the current limit setting range,
there is constant coefficient between them within a small
enough current limit range for the system designer to
select a suitable R_SET value. Table 1 shows the current
limit range using a standard 1% metal film resistor. To
determine R_SET, start with the maximum allowable input
current from SYSIN as minimum I_LIM and multiply by 1.33
to derive the typical I_LIM value. Next, refer to Table 1 to
find the constant coefficient I_LIM range which includes the
calculated I_LIM value and get the constant coefficient value
c. Then calculate the R_SET by the following formula:
R_SET = R_{SET_Max} - (I_LIM - I_{LIM_Range_Low} ) · c
R_{SET_Max} is the maximum standard R_SET resistance at the
certain constant coefficient current limit range.
I_LIM is the calculated typical current limit.
I_{LIM_Range_Low} is the low terminal of the current limit
range.
c is the coefficient of the current limit range.
Example: Select R_SET for 500mA current limit.
The typical current limit is I_LIM = 500 · 1.33 = 665mA.
The constant coefficient current range is 600mA to
700mA and c = 25000. Therefore:
R_SET = 13kΩ - (665mA - 600mA) · 25000 = 11.375kΩ
Select a standard 11.3kΩ resistor for R_SET. Considering
the ±25% tolerance, the current limit will be greater
than 500mA but less than 831mA.
I_LIM Typ.
(mA)
200
250
300
350
400
450
500
550
600
700
800
900
1000
1100
1200
1300
1400
Constant Coefficient c
of R_SET / I_LIM
186000
120000
56000
50000
36000
32000
30000
34000
25000
16300
13700
6900
7700
5500
5000
3500
1% Standard
R_SET (kΩ)
40.2
30.9
24.9
22.1
19.6
17.8
16.2
14.7
13.0
10.5
8.87
7.50
6.81
6.04
5.49
4.99
4.64
Table 1: Current Limit R_SET Values.
Dropout Voltage
Dropout voltage is determined by R_DS(ON) and the current
passing through it. AAT1138 load switch maximum
R_DS(ON) is 180mΩ for USB applicationa and the step-down
converter high side R_DS(ON) is maximum 200mΩ. So for a
500mA load switch current limit setting (the load switch
always limits below 831mA as described above), the load
switch maximum dropout voltage can be calculated by:
V_{Dropout_Switch} = 831mA · 180mΩ = 0.15V
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
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201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013