ADP1864 Analog Devices, ADP1864 Datasheet - Page 10

• Manufacturer Part Number: ADP1864
• Description: Constant Frequency Current-Mode Step-Down DC-to-DC Controller in TSOT
• Manufacturer: Analog Devices

ADP1864
APPLICATIONS INFORMATION
DUTY CYCLE
To determine the worst-case inductor ripple current, output
voltage ripple, and slope compensation factor, establish the
system maximum and minimum duty cycle. The duty cycle is
calculated by the equation
where V
A typical Schottky diode has a forward voltage drop of 0.5 V.
RIPPLE CURRENT
Choose the peak-to-peak inductor ripple current between 20%
and 40% of the maximum load current at the system’s highest
input voltage. A good starting point for a design is to pick the
peak-to-peak ripple current at 30% of the load current.
SENSE RESISTOR
Choose the sense resistor value to provide the desired current
limit. The internal current comparator measures the peak
current (sum of load current and positive inductor ripple
current) and compares it against the current limit threshold.
The current sense resistor value is calculated by the equation
where PCSV is the peak current sense voltage, typically 0.125 V.
To ensure the design provides the required output load current
over all system conditions, consider the variation in PCSV over
temperature (see the Specifications section) as well as increases
in ripple current due to inductor tolerance.
If the system is being operated with >40% duty cycle, incor-
porate the slope compensation factor into the calculation.
where SF is the slope factor correction ratio, taken from
Figure 13, at the system maximum duty cycle (minimum
input voltage).
ΔI
Duty
R
R
SENSE
SENSE
(PEAK)
D
is the diode forward drop.
Cycle
(
(
MIN
MIN
= 0.3 × I
)
)
=
=
(
DC
I
I
LOAD
LOAD
LOAD(MAX)
)
=
SF
(
(
V
MAX
MAX
V
OUT
PCSV
×
IN
)
)
PCSV
+
+
+
+
V
Δ
Δ
V
D
I
I
D
(
(
PEAK
PEAK
2
2
)
)
Rev. B | Page 10 of 16
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INDUCTOR VALUE
The inductor value choice is important because it dictates the
inductor ripple and, therefore, the voltage ripple at the output.
When operating the part at >40% duty cycle, keep the inductor
value low enough for the slope compensation to remain
effective.
The inductor ripple current is inversely related to the
inductor value.
where f is the oscillator frequency.
Smaller inductor values are usually less expensive, but increase
the ripple current and the output voltage ripple. Too large an
inductor value results in added expenses and can impede effective
load transient responses at >40% duty cycle because it reduces
the effect of slope compensation.
Start with the highest input voltage, and assuming the ripple
current is 30% of the maximum load current,
From this starting point, modify the inductance to obtain
the right balance of size, cost, and output voltage ripple, while
maintaining the inductor ripple current between 20% and 40%
of the maximum load current.
Δ
L
1.05
0.95
0.85
0.75
0.65
0.55
0.45
0.35
I
=
(
PEAK
0
0
3 .
)
×
(
V
0.1
=
I
Figure 13. Slope Factor (SF) vs. Duty Cycle
IN
LOAD
(
V
−
IN
0.2
V
L
(
MAX
−
OUT
×
V
0.3
f
)
OUT
)
×
f
)
0.4
DUTY CYCLE
×
×
⎛
⎜
⎜
⎝
⎛
⎜
⎜
⎝
V
0.5
V
V
V
OUT
OUT
IN
IN
+
0.6
+
+
+
V
V
V
V
D
D
D
0.7
D
⎞
⎟
⎟
⎠
⎞
⎟
⎟
⎠
0.8
0.9
1
.0
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