AAT2315_07 Datasheet, PDF (12/22 Pages) Advanced Analogic Technologies – Dual 600mA Step-Down Converter with Synchronization

Table 1 displays the suggested inductor values for
the AAT2513.
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
inductor's saturation characteristics. The inductor
should not show any appreciable saturation under
all 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 2.2uH CDRH2D11 series inductor selected
from Sumida has a 98mΩ DCR and a 1.27A DC
current rating. At full load the inductor DC loss is
35mW which corresponds to a 3.2% loss in effi-
ciency for a 600mA, 1.8V output.
Input Capacitor
A key feature of the AAT2513 is that the funda-
mental switching frequency ripple at the input can
be reduced by operating the two converters 180°
out of phase. This reduces the input ripple by
roughly half, reducing the required input capaci-
tance. An X5R ceramic input capacitor as small as
1µF is often sufficient. To estimate the required
input capacitor size, determine the acceptable
input ripple level (V_PP) and solve for C. The calcu-
lated value varies with input voltage and is a maxi-
mum 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
AAT2513
Dual 600mA Step-Down
Converter with Synchronization
This equation provides an estimate for the input
capacitor required for a single channel.
The equation below solves for the input capacitor
size for both channels. It makes the worst case
assumption that both converters are operating at
50% duty cycle with in phase synchronization.
1
C_IN =
⎛ V_PP
⎝ I_O1 + I_O2
- ESR^⎞⎠ · 4 · F_S
Because the AAT2513 channels will generally
operate at different duty cycles the actual ripple will
vary and be less than the ripple (V_PP) used to solve
for the input capacitor in the above equation.
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 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 converters combined.
I = _RMS(MAX)
I + I _{O1(MAX) O2(MAX)}
2
Configuration
0.6V adjustable
with external
resistive divider
12
Output Voltage
0.6V-2.0V
2.5V
3.3V
Inductor
2.2µH
3.3µH
4.7µH
Table 1: Inductor Values.
Slope Compensation
0.6A/µs
2513.2007.04.1.1