AAT2510 Datasheet, PDF (10/19 Pages) Advanced Analogic Technologies – Dual 400mA, 1MHz Step-Down DC-DC Converter

DATA SHEET
AAT2510
Dual 400mA, 1MHz Step-Down DC/DC Converter
Manufacturer’s specifications list both the inductor DC
current rating, which is a thermal limitation, and the
peak current rating, which is determined by the satura-
tion characteristics. The inductor should not show any
appreciable saturation under normal load conditions.
Some inductors may meet the peak and average current
ratings yet result in excessive losses due to a high DCR.
Always consider the losses associated with the DCR and
its effect on the total converter efficiency when selecting
an inductor.
The 4.7μH CDRH3D16 series inductor selected from
Sumida has a 105m DCR and a 900mA DC current rat-
ing. At full load, the inductor DC loss is 17mW which gives
a 2.8% loss in efficiency for a 400mA 1.5V output.
Input Capacitor
Select a 4.7μF to 10μF X7R or X5R ceramic capacitor for
the input. To estimate the required input capacitor size,
determine the acceptable input ripple level (V_PP) and solve
for C. The calculated value varies with input voltage and
is a maximum when V_IN is double the output voltage.
V_O
V_IN
⋅
⎛
⎝
1
-
V_O ⎞
V_IN ⎠
C_IN =
⎛ V_PP
⎝ I_O
- ESR^⎞⎠ ⋅ F_S
This equation provides an estimate for the input capaci-
tor required for a single channel.
Configuration
0.6V Adjustable
With External
Resistive Divider
Fixed Output
Output
Voltage
0.6V to
2.0V
2.5V
0.6V to
2.0V
2.5V to
3.3V
Inductor
4.7μH
10μH
4.7μH
4.7μH
Slope
Compensation
0.24A/μsec
0.24A/μsec
0.24A/μsec
0.48A/μsec
Table 1: Inductor Values.
The equation below solves for input capacitor size for
both channels. It makes the worst-case assumptions
that both converters are operating at 50% duty cycle
and are synchronized.
1
C_IN =
⎛ V_PP
⎝ I_O1 + I_O2
- ESR^⎞⎠ • 4 • F_S
Because the AAT2510 channels will generally operate at
different duty cycles and are not synchronized, the actu-
al ripple will vary and be less than the ripple (V_PP) used
to solve for the input capacitor in the equation above.
Always examine the ceramic capacitor DC voltage coef-
ficient characteristics when selecting the proper value.
For example, the capacitance of a 10μF 6.3V X5R ceram-
ic capacitor with 5V DC applied is actually about 6μF.
The maximum input capacitor RMS current is:
⎛⎝ ⎞⎠ ⎛⎝ ⎞⎠ _{IRMS = IO1 ·}
V_O1
V_IN
·
⎛⎝_{1 -}
V_O1 ⎞
V_IN ⎠
+ I_O2 ·
V_{O2 ·}
V_IN
⎛⎝_{1 -}
V_O2 ⎞
V_IN ⎠
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 of both convert-
ers combined.
I = _RMS(MAX)
I + I _{O1(MAX) O2(MAX)}
2
This equation also makes the worst-case assumption
that both converters are operating at 50% duty cycle
and are synchronized. Since the converters are not syn-
chronized and are not both operating at 50% duty cycle,
the actual RMS current will always be less than this.
Losses associated with the input ceramic capacitor are
typically minimal.
The term
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
appears in both the input voltage
ripple and input capacitor RMS current equations. It is a
maximum when V_O is twice V_IN. This is why the input
voltage ripple and the input capacitor RMS current ripple
are a maximum at 50% duty cycle.
The input capacitor provides a low impedance loop for the
edges of pulsed current drawn by the AAT2510. Low ESR/
ESL X7R and X5R ceramic capacitors are ideal for this
function. To minimize the 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.
The proper placement of the input capacitor (C3 and C8)
can be seen in the evaluation board layout in Figure 4.
Since decoupling must be as close to the input pins as
possible, it is necessary to use two decoupling capaci-
tors. C3 provides the bulk capacitance required for both
converters, while C8 is a high frequency bypass capaci-
tor for the second channel (see C3 and C8 placement in
Figure 4).
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
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202020B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 19, 2013