AAT2510 Datasheet, PDF (11/19 Pages) Advanced Analogic Technologies – Dual 400mA, 1MHz Step-Down DC-DC Converter

DATA SHEET
AAT2510
Dual 400mA, 1MHz Step-Down DC/DC Converter
A laboratory test set-up typically consists of two long
wires running from the bench power supply to the evalu-
ation 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 during load tran-
sients. Errors in the loop phase and gain measurements
can also result.
Since the inductance of a short printed circuit board
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
converter performance, a high ESR tantalum or alumi-
num electrolytic capacitor 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 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. As the loop responds, the induc-
tor current increases to match the load current demand.
This typically takes two to three switching cycles and can
be estimated by:
C_OUT
=
3 · ΔI_LOAD
V_DROOP · F_S
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 · 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
For applications requiring an adjustable output voltage,
the 0.6V version can be programmed externally. Resistors
R1 through R4 of Figure 2 program the output to regu-
late at a voltage higher than 0.6V. To limit the bias cur-
rent required for the external feedback resistor string,
the minimum suggested value for R2 and R4 is 59k.
Although a larger value will reduce the quiescent cur-
rent, 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 voltages with R2 and R4 set to either
59k for good noise immunity or 221k for reduced no
load input current.
R1
=
⎛ ^VOUT
⎝ V_REF
-1^⎞⎠
·
R2
=
⎛ ^1.5V
⎝ 0.6V
-
1 ^⎞⎠
·
59kΩ
=
88.5kΩ
The adjustable version of the AAT2510 in combination
with an external feedforward capacitor (C4 and C5 of
Figure 2) delivers enhanced transient response for
extreme pulsed load applications. The addition of the
feedforward capacitor typically requires a larger output
capacitor (C1 and C2) for stability.
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.
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