AAT2120 Datasheet, PDF (12/19 Pages) Advanced Analogic Technologies – 500mA Low Noise Step-Down Converter

transient response and low temperature operation appli-
cation, a 10μF (X5R, X7R) ceramic capacitor is recom-
mended to stabilize extreme pulsed load conditions.
The output voltage droop due to a load transient is domi-
nated 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 relationship of the output volt-
age droop during the three switching cycles to the output
capacitance can be estimated 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 4.7μF. 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.
The maximum output capacitor RMS ripple current is
given by:
I = _RMS(MAX)
1
2·
·
3
V_OUT · (V_IN(MAX) - V_OUT)
L · F_S · V_IN(MAX)
Dissipation due to the RMS current in the ceramic output
capacitor ESR is typically minimal, resulting in less than
a few degrees rise in hot-spot temperature.
Adjustable Output Resistor Selection
Resistors R1 and R2 of Figure 1 program the output to
regulate at a voltage higher than 0.6V. To limit the bias
current required for the external feedback resistor string
while maintaining good noise immunity, the suggested
value for R2 is 59kΩ. Decreased resistor values are nec-
essary to maintain noise immunity on the FB pin, result-
ing in increased quiescent current. Table 2 summarizes
the resistor values for various output voltages.
R1
=
⎛ ^VOUT
⎝ V_REF
-1^⎞⎠
·
R2
=
⎛ ^3.3V
⎝ 0.6V
-
1 ^⎞⎠
·
59kΩ
=
267kΩ
DATA SHEET
AAT2120
500mA Low-Noise, Step-Down Converter
With enhanced transient response for extreme pulsed
load application, an external feed-forward capacitor, (C3
in Figure 1), can be added.
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
3.3
R2 = 59kΩ
R1 (kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
R2 = 221kΩ
R1 (kΩ)
75
113
150
187
221
261
301
332
442
464
523
715
1000
Table 2: Adjustable Resistor Values for
Step-Down Converter.
Thermal Calculations
There are three types of losses associated with the
AAT2120 step-down converter: switching losses, conduc-
tion 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 switch-
ing devices. At full load, assuming continuous conduction
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_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 inves-
tigated over the complete input voltage range.
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