AAT1121 Datasheet, PDF (13/20 Pages) Advanced Analogic Technologies – 1.5MHz, 250mA Step-Down Converter

AAT1121
1.5MHz, 250mA Step-Down Converter
Thermal Calculations
There are three types of losses associated with
the AAT1121 step-down converter: switching loss-
es, conduction losses, and quiescent current loss-
es. Conduction losses are associated with the
R_DS(ON) characteristics of the power output switch-
ing devices. Switching losses are dominated by
the gate charge of the power output switching
devices. At full load, assuming continuous conduc-
tion mode (CCM), a simplified form of the losses is
given by:
P_TOTAL
=
I_O2
·
(R_DSON(H)
·
V_O
+
R_DSON(L)
V_IN
·
[V_IN
-
V_O])
+ (t_sw · F · I_O + I_Q) · V_IN
I_Q is the step-down converter quiescent current.
The term t_sw is used to estimate the full load step-
down converter switching losses.
For the condition where the step-down converter is
in dropout at 100% duty cycle, the total device dis-
sipation reduces to:
P_TOTAL = I_O2 · R_DSON(H) + I_Q · V_IN
Since R_DS(ON), quiescent current, and switching
losses all vary with input voltage, the total losses
should be investigated over the complete input
voltage range.
Given the total losses, the maximum junction tem-
perature can be derived from the θ_JA for the
TDFN22-8 package which is 50°C/W.
T_J(MAX) = P_TOTAL · Θ_JA + T_AMB
VIN
C1
4.7μF
GND
U1
1
VP
2 _VIN
8
PGND
L1
LX ^{7 LX}
3
GND
6
EN
4 _FB
N/C ⁵
AAT1121
C2
R1
4.7μF Adj.
+VOUT
C3
(optional)
100pF
Figure 1: AAT1121 Schematic.
R2
59kΩ
GND
1121.2006.04.1.0
13