AAT4621_12 Datasheet, PDF (13/17 Pages) Skyworks Solutions Inc. – PC Card Current Limit Interface and Capacitor Charger

DATA SHEET
AAT4621
PC Card Current Limit Interface and Capacitor Charger
Power Loop
The AAT4621 power loop limits the load current in the
event that the device power dissipation becomes exces-
sive. The AAT4621 power loop regulates the die tempera-
ture to 110°C (or 130°C for the AAT4621-1), decreasing
the load current in increments of 1/32 of the current limit
set point when the die temperature reaches 110°C (or
130°C for the AAT4621-1). When the die temperature
exceeds 110°C (or 130°C for the AAT4621-1) it decreas-
es the load current by 1/32 of the current limit set point.
If die temperature is less than 110°C (or 130°C for the
AAT4621-1), it increases the load current in increments
of 1/32 of the current limit set point until the set current
limit point is reached or the die temperature exceeds
110°C (or 130°C for the AAT4621-1).
The delay time between die temperature measurements
varies depending on the load current limit set point. The
delay ranges from 0.5ms for a 75mA (or 150mA for the
AAT4621-1) current limit set point to 4ms for a 1.2A (or
2.4A for the AAT4621-1) current limit set point.
For the condition where there is no current limit set point
(shorting R_SETU or R_SETL to ground) the soft start incre-
ments the current limit is 1/32 steps of 1.2A (or 2.4A for
the AAT4621-1) until the current reaches 1.2A (or 2.4A
for the AAT4621-1), at which point the power MOSFET
turns on fully. At this point, the current is limited by the
R_DS(ON) of the power MOSFET and other series resistance.
The Power Loop and Over-Temperature Loop then takes
control to limit the current until the output voltage has
fully charged.
Short Circuit Protection
The series pass power MOSFET turns off completely after
the output has charged to within 18mV of the input volt-
age. This protects the device in the event where a short
circuit is applied to the output. After applying a short
circuit or heavy load to the output, which causes the
output to sink below the 18mV threshold plus some hys-
teresis, the current will immediately attempt to slew
from zero to the final programmed value. The device will
continue to output the current limit as long as the output
is VIN - 18mV or until the die temperature reaches
110°C. If temperature fold-back occurs, then the power
loop will become active.
Over-Temperature Protection
If the die temperature rate of rise is fast enough to
exceed the power loop regulated temperature, over-
temperature shutdown disables the device. The over-
temperature threshold is 145°C (or 150°C for the
AAT4621-1). After over-temperature shutdown, soft
start is initiated once the die temperature drops to
130°C (or 135°C for the AAT4621-1).
Under-Voltage Shutdown
Under-voltage lockout enables the device at 2.7V typical
with 120mV of hysteresis. The maximum UVLO level is
3.0V.
Digitally Programmed
Soft Start Current Limit
At turn on, the digital soft start increases the load cur-
rent in discrete levels equal to 1/32 of the final pro-
grammed current limit set point from zero to the final
programmed current limit level.
Once the AAT4621 has fully charged the supercap, the
soft start circuit is disabled. The soft start function will
only reactivate under the following conditions:
1. The power source or enable is cycled off and back on
2. The thermal shutdown is activated
3. Switching from ENIU to ENIL or vice versa
4. Following an over-current event.
Reverse Current Blocking
An internal comparator disables the internal pass tran-
sistor when the input voltage is less than the output
voltage blocking any reverse current from the output to
the input.
Evaluation Board
The AAT4621/AAT4621-1 evaluation board schematic is
shown in Figure 2. The AAT4621/AAT4621-1 PCB layout
is shown in Figures 3 and 4.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
201944A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 15, 2012
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