AAT2158_0810 Datasheet, PDF (13/17 Pages) Advanced Analogic Technologies – 1.5A Low Noise Step-Down Converter

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PRODUCT DATASHEET
AAT2158
1.5A Low Noise Step-Down Converter
V_IN
2.4V - 5.5V
R1
R2
0 100K
C1
10µF
3
2
1
EN
C2
open
U1
12 _VP
LX ¹⁵
11 _VP
LX ¹⁴
10
VP
9 _VCC
LX ¹³
FB ⁴
AAT2158
7 _EN
N/C ¹⁶
3 _{PGND N/C} 6
2 _{PGND N/C} 8
L1
1.0 -1.5µH
R3
100K
R4
100K
1 _{PGND SGND} 5
V_OUT
C8
1nF
(optional)
C3
22µF
1.2V/1.5A
C4
22µF
U1
C1
C3
C4
C8
R1-R4
L1
AAT2158 AnalogicTech, Hi-Voltage Buck, TDFN34-16
Cap, MLC, 10µF/6.3V,X7R, 0805
Cap, MLC, 22µF/6.3V, X7R,0805
Cap, MLC, 22µF/6.3V, X7R, 0805
Cap, MLC, 1nF/6.3V, 0402
Carbon film resistor, 0402
TDK, VLS3012T-1R5N1R7, 1.5µH, I_SAT = 1.9A, DCR = 68mΩ
Figure 2: AAT2158 Alternate Application Schematic.
The schematic in Figure 2 shows the configuration for
improved transient response in an application where the
output is stepped down to 1.2V. The adjustments consist
of adding an additional 22µF output capacitor C4,
increasing the value of the feed forward capacitor C8 to
1nF, and eliminating the RC filter network R1 and C2.
Thermal Calculations
There are three types of losses associated with the
AAT2158 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 losses is
given by:
P_TOTAL
=
I_O2
·
(R_DS(ON)H
·
V_O
+ R_DS(ON)L
V_IN
·
[V_IN
-
V_O])
+ (t_sw · F_S · 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 con-
verter switching losses.
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
P_TOTAL
=
I_O2
·
(R_DS(ON)H
·
V_O
+ R_DS(ON)L
V_IN
·
[V_IN
-
V_O])
+ (t_sw · F_S · I_O + I_Q) · 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 q_JA for the QFN33-16 pack-
age, which is 50°C/W.
T_J(MAX) = P_TOTAL · Θ_JA + T_AMB
2158.2008.10.1.5
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