AAT1184 Datasheet, PDF (13/18 Pages) Advanced Analogic Technologies – High Voltage Step-Down Regulator

Thermal Protection
The AAT1184 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.
Thermal Calculations
There are two types of losses associated with the
AAT1184 step-down converter: switching losses, con-
duction losses, and quiescent current losses. Conduction
losses are associated with the R_DS(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:
P_TOTAL
=
I_OUT2 · (R_DS(ON)H · V_OUT + R_DS(ON)L · [V_IN - V_OUT])
V_IN
+ (t_SW · F_S · I_OUT + I_Q) · V_IN
DATA SHEET
AAT1184
High Voltage Step-Down Regulator
I_Q is the step-down converter current. The term t_SW is
used to estimate the full load step-down converter
switching losses.
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
AAT1184 reduce to the following equation:
P_TOTAL = I_OUT2 · R_DS(ON)H · D + (t_SW · F_S · I_OUT + I_Q) · V_IN
where D = ^VOUT is the duty cycle.
V_IN
Since R_DS(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 θ_JA for the TSOPJW-12
package, which is 140°C/W.
T_J(MAX) = P_TOTAL · θ_JA + T_AMB
V_OUT
5V/1.2A
LX
L1
4.7µH
R1
2.32k
C4
68nF
RS
R3
OS
R6
V _OUT
5.0V/1.2A
LX
L1
4.7µH
R1
2.32k
C4
68nF
RS
R8
R3
OS
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).
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