PRODUCT DATASHEET
AAT2688
TM
SystemPower
4.5A PMIC Solution for 12V Adapter Systems with 2-Output High Performance Step-Down Converters
down mode, an internal 1.5kΩ resistor is connected
between VOUT and GND. This is intended to discharge COUT
when the LDO regulator is disabled. The internal 1.5kΩ
has no adverse effect on device turn-on time.
highly recommended. A larger value of CIN with respect
to COUT will affect a slower CIN decay rate during shut-
down, thus preventing VOUT from exceeding VIN. In appli-
cations where there is a greater danger of VOUT exceeding
VIN for extended periods of time, it is recommended to
place a Schottky diode across VIN to VOUT (connecting the
cathode to VIN and anode to VOUT). The Schottky diode
forward voltage should be less than 0.45V.
Channel 2 Short-Circuit Protection
The AAT2688 LDO contains an internal short-circuit pro-
tection circuit that will trigger when the output load cur-
rent exceeds the internal threshold limit. Under short-
circuit conditions, the output of the LDO regulator will be
current limited until the short-circuit condition is removed
from the output or LDO regulator package power dissi-
pation exceeds the device thermal limit.
Thermal Calculations
There are three types of losses associated with the
AAT2688 step-down converter: switching losses, con-
duction losses, and quiescent current losses. Conduction
losses are associated with the RDS(ON) characteristics of
the power output switching devices. Switching losses are
dominated by the gate charge of the power output
switching devices. At full load, assuming continuous con-
duction mode (CCM), a simplified form of the synchro-
nous step-down converter and LDO losses is given by:
Channel 2 Thermal Protection
The AAT2688 LDO has an internal thermal protection
circuit which will turn on when the device die tempera-
ture exceeds 135°C. The internal thermal protection
circuit will actively turn off the LDO regulator output pass
device to prevent the possibility of over temperature
damage. The LDO regulator output will remain in a shut-
down state until the internal die temperature falls back
below the 135°C trip point. The combination and interac-
tion between the short circuit and thermal protection
systems allows the LDO regulator to withstand indefinite
short-circuit conditions without sustaining permanent
damage.
2
IOUT1 · (RDS(ON)H · VOUT1 + RDS(ON)L · [VIN1 - VOUT1])
PTOTAL
=
VIN1
+ (tSW · FS · IOUT1 + IQ1) · VIN1 + (VIN2 - VOUT2) · IOUT2
IQ1 and IQ2 are the step-down converter and LDO quies-
cent currents respectively. The term tSW is used to esti-
mate the full load step-down converter switching losses.
For a synchronous Step-Down converter, the power dis-
sipation occurs in the internal high side MOSFET during
the on time and the external low side MOSFET during the
off time. When the internal high side switch is off, the
power dissipates on the external low side switch. The
total package losses for AAT2688 reduce to the following
equation:
Channel 2 No-Load Stability
The AAT2688 is designed to maintain output voltage
regulation and stability under operational no load condi-
tions. This is an important characteristic for applications
where the output current may drop to zero.
PTOTAL = IOUT12 · RDS(ON)H · D + (tSW · FS · IOUT1 + IQ1) · VIN + (VIN2 - VOUT2) · IOUT2
Channel 2 Reverse Output-to-Input
Voltage Conditions and Protection
VOUT
VIN
Where: D =
is the duty cycle.
Under normal operating conditions, a parasitic diode
exists between the output and input of the LDO regula-
tor. The input voltage should always remain greater than
the output load voltage, maintaining a reverse bias on
the internal parasitic diode. Conditions where VOUT might
exceed VIN should be avoided since this would forward
bias the internal parasitic diode and allow excessive cur-
rent flow into the VOUT pin, possibly damaging the LDO
regulator. In applications where there is a possibility of
VOUT exceeding VIN for brief amounts of time during nor-
mal operation, the use of a larger value CIN capacitor is
Since RDS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be inves-
tigated over the complete input voltage range.
Given the total losses, the maximum junction tempera-
ture can be derived from the θJA for the TQFN45-24
package, which is 33°C/W.
TJ(MAX) = PTOTAL · θJA + TAMB
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2688.2008.06.1.0
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