Datasheet

Data Sheet ADP2386
Rev. A | Page 15 of 24
APPLICATIONS INFORMATION
INPUT CAPACITOR SELECTION
The input capacitor reduces the input voltage ripple caused by
the switch current on PVIN. Place the input capacitor as close
as possible to the PVIN pin. A ceramic capacitor in the 10 μF to
47 μF range is recommended. The loop that is composed of this
input capacitor, the high-side N-MOSFET, and the low-side N-
MOSFET must be kept as small as possible.
The voltage rating of the input capacitor must be greater than
the maximum input voltage. Ensure that the rms current rating
of the input capacitor is larger than the value calculated from
the following equation:
I
C
IN
_
RMS
= I
OUT
×
)1( DD −×
OUTPUT VOLTAGE SETTING
The output voltage of the ADP2386 is set by an external resistive
divider. The resistor values are calculated using
V
OUT
= 0.6 ×








+
BOT
TOP
R
R
1
To limit the output voltage accuracy degradation due to the FB
bias current (0.1 µA maximum) to less than 0.5% (maximum),
ensure that R
BOT
< 30 kΩ.
Table 6 lists the recommended resistor divider values for the
various output voltages.
Table 6. Resistor Divider Values for Various Output Voltages
V
OUT
(V) R
TOP
± 1% (kΩ) R
BOT
± 1% (kΩ)
1.0
1.2
1.5
1.8
2.5
3.3
5.0
10
10
15
20
47.5
10
22
15
10
10
10
15
2.21
3
VOLTAGE CONVERSION LIMITATIONS
The minimum output voltage for a given input voltage and
switching frequency is constrained by the minimum on time.
The minimum on time of the ADP2386 is typically 125 ns.
The minimum output voltage for a given input voltage and
switching frequency can be calculated using the following:
V
OUT_MIN
= V
IN
× t
MIN_ON
× f
SW
− (R
DSON_HS
− R
DSON_LS
) ×
I
OUT_MIN
× t
MIN_ON
× f
SW
− (R
DSON_LS
+ R
L
) × I
OUT_MIN
(1)
where:
V
OUT_MIN
is the minimum output voltage.
V
IN
is the input voltage.
t
MIN_ON
is the minimum on time.
f
SW
is the switching frequency.
R
DSON_HS
is the high-side MOSFET on resistance.
R
DSON_LS
is the low-side MOSFET on resistance.
I
OUT_MIN
is the minimum output current.
R
L
is the series resistance of the output inductor.
The maximum output voltage for a given input voltage and
switching frequency is constrained by the minimum off time
and the maximum duty cycle. The minimum off time is typically
200 ns, and the maximum duty cycle of the ADP2386 is
typically 90%.
The maximum output voltage, limited by the minimum off time
at a given input voltage and switching frequency, can be calculated
using the following equation:
V
OUT_MAX
= V
IN
× (1 − t
MIN_OFF
× f
SW
) − (R
DSON_HS
− R
DSON_LS
) ×
I
OUT_MAX
× (1 − t
MIN_OFF
× f
SW
) − (R
DSON_LS
+ R
L
) × I
OUT_MAX
(2)
where:
V
OUT_MAX
is the maximum output voltage.
V
IN
is the input voltage.
t
MIN_OFF
is the minimum off time.
f
SW
is the switching frequency.
R
DSON_HS
is the high-side MOSFET on resistance.
R
DSON_LS
is the low-side MOSFET on resistance.
I
OUT_MAX
is the maximum output current.
R
L
is the series resistance of the output inductor.
The maximum output voltage, limited by the maximum duty
cycle at a given input voltage, can be calculated using the
following equation:
V
OUT_MAX
= D
MAX
× V
IN
(3)
where D
MAX
is the maximum duty cycle; V
IN
is the input voltage.
As shown in Equation 1 to Equation 3, reducing the switching
frequency alleviates the minimum on time and minimum off
time limitation.
INDUCTOR SELECTION
The inductor value is determined by the operating frequency,
input voltage, output voltage, and inductor ripple current. Using
a small inductor value leads to a faster transient response; however,
it degrades efficiency, due to a larger inductor ripple current.
Using a large inductor value leads to smaller ripple current and
better efficiency, but it results in a slower transient response.
As a guideline, the inductor ripple current, ΔI
L
, is typically set
to one-third of the maximum load current. The inductor value
is calculated using the following equation:
L =
SWL
OUTIN
fI
DVV
×∆
×− )(
where:
V
IN
is the input voltage.
V
OUT
is the output voltage.
D is the duty cycle (D = V
OUT
/V
IN
).
ΔI
L
is the inductor current ripple.
f
SW
is the switching frequency.
The ADP2386 uses adaptive slope compensation in the current
loop to prevent subharmonic oscillations when the duty cycle
is larger than 50%. The internal slope compensation limits the
minimum inductor value.