AAT1121IES-0.6-T1 Advanced Analog Technology, Inc., AAT1121IES-0.6-T1 Datasheet - Page 12

• Manufacturer Part Number: AAT1121IES-0.6-T1
• Description: 1.5MHz, 250mA Step-Down Converter
• Manufacturer: Advanced Analog Technology, Inc.

In applications where the input power source lead
inductance cannot be reduced to a level that does
not affect the converter performance, a high ESR
tantalum or aluminum 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
provides holdup during large load transitions. A
4.7µF to 10µF X5R or X7R ceramic capacitor typi-
cally provides sufficient bulk capacitance to stabi-
lize the output during large load transitions and has
the ESR and ESL characteristics necessary for low
output ripple. For enhanced transient response
and low temperature operation application, a 10µF
(X5R, X7R) ceramic capacitor is recommended to
stabilize extreme pulsed load conditions.
The output voltage droop due to a load transient is
dominated by the capacitance of the ceramic out-
put capacitor. During a step increase in load cur-
rent, 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
Once the average inductor current increases to the
DC load level, the output voltage recovers. The
above equation 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 capacitance will reduce the
crossover frequency with greater phase margin.
12
12
capacitance
C
OUT
=
V
can
3 · ΔI
DROOP
LOAD
be
· F
S
estimated
by:
1.5MHz, 250mA Step-Down Converter
The maximum output capacitor RMS ripple current
is given by:
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 resis-
tor string while maintaining good noise immunity, the
suggested value for R2 is 59kΩ. Decreased resistor
values are necessary to maintain noise immunity on
the FB pin, resulting in increased quiescent current.
Table 2 summarizes the resistor values for various
output voltages.
With enhanced transient response for extreme
pulsed load application, an external feed-forward
capacitor, (C3 in Figure 1), can be added.
R1 =
V
OUT
Table 2: Adjustable Resistor Values For
1.85
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
2.0
2.5
3.3
I
⎛
⎝
RMS(MAX)
(V)
V
V
OUT
REF
-1 · R2 =
Step-Down Converter.
=
⎞
⎠
2
R2 = 59kΩ
·
1
R1 (kΩ)
3
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
124
137
187
267
118
·
⎛
⎝
V
3.3V
0.6V
OUT
L · F · V
· (V
- 1 · 59kΩ = 267kΩ
IN(MAX)
⎞
⎠
AAT1121
R2 = 221kΩ
IN(MAX)
R1 (kΩ)
- V
1121.2007.03.1.2
1121.2007.03.1.2
1000
150
187
221
261
301
332
442
464
523
715
113
75
OUT
)