AAT2504
Adjustable 3-Channel Regulator
equations and is a maximum when VO is twice VIN.
This is why the input voltage ripple and the input
capacitor RMS current ripple are a maximum at
50% duty cycle.
VO
VIN
⎛
· 1
⎝
VO
VIN
⎞
⎠
-
CIN =
⎛
⎝
VPP
IO
⎞
⎠
- ESR
·
FS
The input capacitor provides a low impedance loop
for the edges of pulsed current drawn by the
AAT2504. Low ESR/ESL X7R and X5R ceramic
capacitors are ideal for this function. To minimize
stray inductance, the capacitor should be placed as
closely as possible to the IC. This keeps the high
frequency content of the input current localized,
minimizing EMI and input voltage ripple.
VO
VIN
⎛
VO
VIN
⎞
⎠
1
4
· 1
-
=
for VIN = 2 · VO
⎝
1
CIN(MIN)
=
⎛
⎝
VPP
IO
⎞
⎠
- ESR
·
4
·
FS
A laboratory test set-up typically consists of two
long wires running from the bench power supply to
the evaluation board input voltage pins. The induc-
tance of these wires, along with the low-ESR
ceramic input capacitor, can create a high Q net-
work that may affect converter performance. This
problem often becomes apparent in the form of
excessive ringing in the output voltage during load
transients. Errors in the loop phase and gain meas-
urements can also result.
Always examine the ceramic capacitor DC voltage
coefficient characteristics when selecting the prop-
er value. For example, the capacitance of a 10µF,
6.3V, X5R ceramic capacitor with 5.0V DC applied
is actually about 6µF.
The maximum input capacitor RMS current is:
VO
VIN
⎛
· 1
⎝
VO
VIN
⎞
⎠
IRMS = IO
·
-
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.
The input capacitor RMS ripple current varies with
the input and output voltage and will always be less
than or equal to half of the total DC load current.
VO
VIN
⎛
· 1
⎝
VO
VIN
⎞
⎠
1
2
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.
-
=
D
· (1 - D) = 0.52 =
for VIN = 2 x VO:
IO
=
IRMS(MAX)
2
VO
VIN
⎛
⎝
VO
VIN
⎞
⎠
·
1 -
The term
appears in both the input
voltage ripple and input capacitor RMS current
Configuration
Output Voltage
Inductor
1.5µH
1V, 1.2V
0.9V Adjustable With
External Feedback
1.5V, 1.8V
2.5V, 3.3V
2.2µH
3.3µH
Table 1: Inductor Values.
2504.2007.06.1.0
13
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
