PRODUCT DATASHEET
AAT1189
TM
SwitchReg
High Voltage Step-Down Regulator
IQ is the step-down converter and LDO quiescent cur-
rents respectively. The term tSW is used to estimate the
full load step-down converter switching losses.
Thermal Protection
The AAT1189 has an internal thermal protection circuit
which will turn on when the device die temperature
exceeds 135°C. The internal thermal protection circuit
will actively turn off the high side regulator output device
to prevent the possibility of over temperature damage.
The Buck regulator output will remain in a shutdown
state until the internal die temperature falls back below
the 135°C trip point. The combination and interaction
between the short circuit and thermal protection sys-
tems allows the Buck regulator to withstand indefinite
short-circuit conditions without sustaining permanent
damage.
For asynchronous Step-Down converter, the power dis-
sipation is only in the internal high side MOSFET during
the on time. When the switch is off, the power dissipates
on the external Schottky diode. Total package losses for
AAT1189 reduce to the following equation:
PTOTAL = IOUT2 · RDS(ON)H · D + (tSW · FS · IOUT + IQ) · VIN
VOUT
VIN
where D =
is the duty cycle.
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.
Thermal Calculations
There are two types of losses associated with the
AAT1189 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 losses is given by:
Given the total losses, the maximum junction tempera-
ture can be derived from the θJA for the TDFN34-16
package, which is 50°C/W.
TJ(MAX) = PTOTAL · θJA + TAMB
IOUT2 · (RDS(ON)H · VOUT + RDS(ON)L · [VIN - VOUT])
PTOTAL
=
VIN
+ (tSW · FS · IOUT + IQ) · VIN
VOUT
5V/2.5A
5.0V/2.5A
LX
LX
L1
4.7µH
L1
4.7µH
C4
68nF
R1
6.34k
C4
68nF
R1
6.34k
RS
OS
RS
OS
R8
R6
R7
R6
Figure 2: Resistor Network to Adjust the
Current Limit Less than the Pre-Set
Over-Current Threshold (Add R6, R7).
Figure 3: Resistor Network to Adjust the
Current Limit Greater than the Pre-Set
Over-Current Level (Add R6, R8).
w w w . a n a l o g i c t e c h . c o m
1189.2008.06.1.0
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ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
