AAT2503 Datasheet, PDF (15/22 Pages) Advanced Analogic Technologies – Adjustable 3-Channel Regulator

A laboratory test set-up typically consists of two long
wires running from the bench power supply to the eval-
uation board input voltage pins. The inductance of these
wires, along with the low-ESR ceramic input capacitor,
can create a high Q network that may affect converter
performance. This problem often becomes apparent in
the form of excessive ringing in the output voltage dur-
ing load transients. Errors in the loop phase and gain
measurements can also result.
Since the inductance of a short PCB trace feeding the
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. 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 4.7μF to
10μF 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-
DATA SHEET
AAT2503
Adjustable Three-Channel Regulator
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.
Adjustable Output Resistor Selection
The output voltage on the step-down converter is pro-
grammed with external resistors R2 and R6. To limit the
bias current required for the external feedback resistor
string while maintaining good noise immunity, the mini-
mum suggested value for R6 is 59k. Although a larger
value will further reduce quiescent current, it will also
increase the impedance of the feedback node, making it
more sensitive to external noise and interference. Table
2 summarizes the resistor values for various output volt-
ages with R6 set to either 59k for good noise immu-
nity or 221k for reduced no load input current.
With enhanced transient response for extreme pulsed
load application, an external feed-forward capacitor (C1
in Fig.3) can be added.
V_OUT (V)
0.9*
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
2.8
3.0
3.3
R6 = 59k
R2 (k)
0
6.65
13.3
19.6
26.1
32.4
39.2
59.0
61.9
71.5
105
124
137
158
R6 = 221k
R2 (k)
0
24.3
48.7
73.2
97.6
124
147
221
232
274
392
464
511
590
Table 2: Step-Down Converter Resistor Values for
Various Output Voltages.
* For the 0.9V output, R6 is open.
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