AAT1112 Datasheet, PDF (13/19 Pages) Advanced Analogic Technologies – 1.5A, 1.4MHz Step-Down Converter

input voltage is significantly lower than the power leads
from the bench power supply, most applications do not
exhibit this problem.
In applications where the input power source lead induc-
tance cannot be reduced to a level that does not affect
the converter performance, a high ESR tantalum or alu-
minum electrolytic should be placed in parallel with the
low ESR/ESL bypass ceramic capacitor. This dampens
the high Q network and stabilizes the system.
Output Capacitor
The output capacitor limits the output ripple and pro-
vides holdup during large load transitions. A 10μF to
22μF X5R or X7R ceramic capacitor typically provides
sufficient bulk capacitance to stabilize the output during
large load transitions and has the ESR and ESL charac-
teristics necessary for low output ripple.
The output voltage droop due to a load transient is
dominated by the capacitance of the ceramic output
capacitor. During a step increase in load current, the
ceramic output capacitor alone supplies the load current
until the loop responds. Within two or three switching
cycles, the loop responds and the inductor current
increases to match the load current demand. The rela-
tionship of the output voltage droop during the three
switching cycles to the output capacitance can be esti-
mated by:
C_OUT
=
3 · ΔI_LOAD
V_DROOP · F_S
Once the average inductor current increases to the DC
load level, the output voltage recovers. The above equa-
tion establishes a limit on the minimum value for the
output capacitor with respect to load transients.
The internal voltage loop compensation also limits the
minimum output capacitor value to 10μF. This is due to
its effect on the loop crossover frequency (bandwidth),
phase margin, and gain margin. Increased output capac-
itance will reduce the crossover frequency with greater
phase margin.
Adjustable Output Resistor Selection
The output voltage on the AAT1112 is programmed with
external resistors R1 and R2. To limit the bias current
required for the external feedback resistor string while
maintaining good noise immunity, the minimum sug-
DATA SHEET
AAT1112
1.5A, 1.4MHz Step-Down Converter
gested value for R2 is 59k. Although a larger value will
further reduce quiescent current, it will also increase the
impedance of the feedback node, making it more sensi-
tive to external noise and interference. Table 1 sum-
marizes the resistor values for various output voltages
with R2 set to either 59k for good noise immunity or
221k for reduced no load input current.
V_OUT (V)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.0
3.3
R2 = 59k
R1 (k)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
237
267
R2 = 221k
R1 (k)
75
113
150
187
221
261
301
332
442
464
523
715
887
1000
Table 1: AAT1112 Resistor Values for Various
Output Voltages.
Thermal Calculations
There are three types of losses associated with the
AAT1112 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.
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