Datasheet

Data Sheet ADP2381
Rev. 0 | Page 15 of 28
APPLICATIONS INFORMATION
INPUT CAPACITOR SELECTION
The input decoupling capacitor is used to attenuate high
frequency noise on the input. This capacitor should be a
ceramic capacitor in the range of 10 µF to 47 µF. It should be
placed close to the PVIN pin. The loop composed by this input
capacitor, high-side NFET, and low-side NFET must be kept as
small as possible.
The voltage rating of the input capacitor must be greater than
the maximum input voltage. The rms current rating of the input
capacitor should be larger than the following equation:
)1(
_
DDII
OUT
RMSC
IN
−××=
OUTPUT VOLTAGE SETTING
The output voltage of ADP2381 can be set by an external
resistive divider using the following equation:








+×=
BOT
TOP
OUT
R
R
V 16.0
To limit output voltage accuracy degradation due to FB bias
current (0.1 µA maximum) to less than 0.5% (maximum),
ensure that R
BOT
is less than 30 kΩ.
Table 6 gives the recommended resistor divider values for
various output voltage options.
Table 6. Resistor Divider for Different Output Voltages
V
OUT
(V) R
TOP
, ±1% (kΩ) R
BOT
, ±1% (kΩ)
1.0 10 15
1.2 10 10
1.5 15 10
1.8 20 10
2.5
47.5
15
3.3 10 2.21
5.0 22 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 ADP2381 is typically 120 ns. The
minimum output voltage at a given input voltage and frequency
can be calculated using the following equation:
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.
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 ADP2381
is typically 90%.
The maximum output voltage limited by the minimum off time
at a given input voltage and 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.
t
MIN_OFF
is the minimum off time.
I
OUT_MAX
is the maximum output current.
The maximum output voltage, limited by the maximum duty
cycle at a given input voltage, can be calculated by using the
following equation:
V
OUT_MAX
= D
MAX
× V
IN
(3)
where D
MAX
is the maximum duty.
As Equation 1 to Equation 3 show, 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 leads to a faster transient response, but it
degrades efficiency due to larger inductor ripple current,
whereas 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 1/3 of the maximum load current. The inductor can be
calculated using the following equation:
( )
D
fI
VV
L
SW
L
OUT
IN
×
×∆
−
=
where:
V
IN
is the input voltage.
V
OUT
is the output voltage.
ΔI
L
is the inductor current ripple.
f
SW
is the switching frequency.
D is the duty cycle.
IN
OUT
V
V
D =
The ADP2381 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.