AAT4621
PC Card Current Limit Interface and Capacitor ChargerSmartSwitchTM
PRODUCT DATASHEET
4621.2009.08.1.4 11
www.analogictech.com
For the condition where there is no current limit set point
(shorting RSETU or RSETL to ground) the soft start incre-
ments the current limit is 1/32 steps of 1.2A until the
current reaches 1.2A, at which point the power MOSFET
turns on fully. At this point, the current is limited by the
RDS(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.
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. After over-temperature
shutdown, soft start is initiated once the die tempera-
ture drops to 130°C.
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 cir-
cuit or heavy load to the output, which causes the output
to sink below the 18mV threshold plus some hysteresis,
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 foldback occurs, then the power loop will
become active.
Under-Voltage Shutdown
Under-voltage lockout enables the device at 2.7V typical
with 100mV 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 ENU to ENL or vice versa or
4. Following an over-current event, e.g. a hard output
short circuit.
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 evaluation board schematic is shown in
Figure 2. The AAT4621 PCB layout is shown in Figures 3
and 4.
Layout Guidance
When laying out the PC board, the following layout
guideline should be followed to ensure proper operation
of the AAT4621:
1. The exposed pad EP must be reliably soldered to
GND. The exposed thermal pad should be connected
to the board ground plane and Pin 12. The ground
plane should include a large exposed copper pad
under the package with VIAs to all board layers for
thermal dissipation.
2. The power traces, including GND traces and the VIN
trace should be kept short, direct and wide to allow
large current flow. Use several via pads when rout-
ing between layers.
3. The input capacitors (C1) should be connected as
close as possible to VCC (Pins 8, 9, 10, 11) and GND
(Pin 12) to get good power filtering.
4. The output capacitor (C2) should be connected as
close as possible to OUT (Pins 5, 6, 7) and GND (Pin
12) to get good power filtering as the device will
recharge the Supercap when the output voltage level
sensed on the OUT pin is reduced by approximately
18-20mV below its input voltage (VCC) level.