Datasheet

Data Sheet ADP2384
Rev. 0 | 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. The rms current rating of the input
capacitor should be 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 ADP2384 is set by an external
resistive divider. The resistor values are calculated using
V
OUT
= 0.6 ×








+
BOT
TOP
R
R
1
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
< 30 kΩ.
Table 6 lists the recommended resistor divider values for
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 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 ADP2384 is typically 125 ns. The
minimum output voltage for a given input voltage and switching
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 ADP2384 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 cycle.
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 but
degrades efficiency, due to a larger inductor ripple current;
using a large inductor value leads to smaller ripple current
and better efficiency but 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 ADP2384 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.