AAT4620_12 Datasheet, PDF (11/14 Pages) Skyworks Solutions Inc. – PCMCIA Current Limit Interface and Capacitor Charger

DATA SHEET
AAT4620
PCMCIA Current Limit Interface and Capacitor Charger
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.10 0.30 0.50 0.70 0.90 1.10 1.30 1.50
R_SETU and R_SETL (MΩ)
Figure 1: Calculated Current Limit R_SET
Characteristic.
System Ready Hysteresis
(RHYS) and System Ready (RDY)
In internal comparator senses the output voltage and
delivers a ready signal to the external micro controller
when the output voltage reaches 98% of the final value
(V_CC). The comparator hysteresis is programmed by a
resistor from the RHYS pin to ground. The RHYS voltage
determines the hysteresis voltage and is equal to the
RHYS source current (1μA) times the RHYS resistance.
V_HYST = R_HYS · 1μA
= 200kΩ · 1μA
= 0.2V
The system ready pin (RDY) is an open drain output that
switches low when the output voltage reaches 98% of
the input voltage. It requires an external pull up resistor
tied to the input voltage with a typical value of 100k.
Power Loop
The AAT4620 power loop limits the load current in the
event that the device power dissipation becomes exces-
sive. The AAT4620 power loop regulates the die tempera-
ture to 100°C, decreasing the load current in increments
of 1/32 of the current limit set point when the die tem-
perature reaches 100°C. When the die temperature
exceeds 100°C it decreases the load current by 1/32 of
the current limit set point. If die temperature is less than
100°C, 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 100°C.
The delay time between die temperature measurements
varies depending on the load current limit set point. The
delay ranges from 1ms for a 75mA current limit set point
to 8ms for a 1.5A 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 until the
current reaches 1.2A, 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.
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 temperature
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 of a short
circuit applied to the output. Applying a short circuit or
heavy load to the output initiates the digital soft start
where the current ramps from zero to the final pro-
grammed value. The power loop will be activated once
the digital soft start begins.
Under-Voltage Shutdown
Under-voltage lockout enables the device at 2.7V typical
with 100mV of hysteresis. The maximum UVLO level is
3.0V.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
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201943A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 15, 2012