MSS1048-152NL AD [Analog Devices], MSS1048-152NL Datasheet - Page 20

• Manufacturer Part Number: MSS1048-152NL
• Description: Dual 5 A, 20 V Synchronous Step-Down
• Manufacturer: AD [Analog Devices]

APPLICATIONS INFORMATION
INPUT CAPACITOR SELECTION
The input decoupling capacitor attenuates high frequency noise
on the input and acts as an energy reservoir. This capacitor should
be a ceramic capacitor in the range of 10 µF to 47 µF and must
be placed close to the PVINx pin. The loop composed of 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. Ensure that the
rms current rating of the input capacitor is larger than that
expressed in following equation:
OUTPUT VOLTAGE SETTING
The output voltage of the
resistor divider using the following equation:
To limit output voltage accuracy degradation due to FBx pin
bias current (0.1 µA maximum) to less than 0.5% (maximum),
ensure that R
mended resistor divider for various output voltage options.
Table 8. Resistor Divider for Various Output Voltages
V
1.0
1.2
1.5
1.8
2.5
3.3
5.0
VOLTAGE CONVERSION LIMITATIONS
The minimum output voltage for a given input voltage and
switching frequency is limited by the minimum on time. The
minimum on time of the
minimum output voltage in CCM mode at a given input voltage
and frequency can be calculated using the following equation:
where:
V
t
I
f
R
R
R
ADP2325
SW
MIN_ON
OUT_MIN
OUT
DSON1
DSON2
L
OUT_MIN
is the series resistance of the output inductor.
is the switching frequency.
(V)
V
t
V
I
MIN_ON
C
OUT_MIN
is the high-side MOSFET on resistance.
is the low-side MOSFET on resistance.
OUT
is the minimum on time.
IN
is the minimum output current.
is the minimum output voltage.
_rms
=
× f
0
= V
=
BOT
6 .
SW
I
R
10
10
15
20
47.5
10
22
×
OUT
IN
is less than 30 kΩ. Table 8 provides the recom-
TOP
− (R




1
× t
, ±1% (kΩ)
+
×
MIN_ON
DSON2
R
R
TOP
BOT
D
ADP2325
×
ADP2325
+ R
× f




(
1
−
SW
L
) × I
D
− (R
)
OUT_MIN
is typically 130 ns. The
can be set by an external
DSON1
R
15
10
10
10
15
2.21
3
− R
BOT
DSON2
, ±1% (kΩ)
) × I
OUT_MIN
Rev. 0 | Page 20 of 32
×
The maximum output voltage for a given input voltage and
switching frequency is also limited by the minimum off time
and the maximum duty cycle. The minimum off time is typically
150 ns and the maximum duty is typically 90% in the ADP2325.
The maximum output voltage that is limited by the minimum off
time at a given input voltage and frequency can be calculated
using the following equation:
where:
V
t
I
The maximum output voltage that is limited by the maximum
duty cycle at a given input voltage can be calculated using the
following equation:
where D
As the previous equations demonstrate, reducing the switching
frequency alleviates the minimum on time and minimum off time
limitation.
CURRENT-LIMIT SETTING
The
sure that the selected current-limit value is larger than the peak
current of the inductor, I
INDUCTOR SELECTION
The inductor value is determined by the operating frequency,
input voltage, output voltage, and inductor ripple current. Using
a small inductor provides faster transient response but degrades
efficiency due to larger inductor ripple current, whereas a large
inductor value provides smaller ripple current and better effi-
ciency but results in a slower transient response. Thus, there is a
trade-off between the transient response and efficiency. As a
guideline, the inductor ripple current, ΔI
one-third of the maximum load current. The inductor value can
be calculated by using the following equation:
where:
V
V
ΔI
f
D is the duty cycle.
SW
MIN_OFF
OUT_MAX
OUT_MAX
IN
OUT
L
is the switching frequency.
is the inductor ripple current.
is the input voltage.
ADP2325
V
I
V
is the output voltage.
L
D =
OUT_MAX
OUT_MAX
OUT_MAX
=
is the minimum off time.
is the maximum output current.
MAX
is the maximum output voltage.
(
V
V
V
OUT
IN
IN
is the maximum duty cycle.
∆
× (1 − t
= V
= D
−
I
has two selectable current-limit thresholds. Make
L
V
×
OUT
IN
MAX
f
SW
× (1 − t
MIN_OFF
)
× V
×
D
PEAK
IN
MIN_OFF
× f
.
SW
) − (R
× f
SW
DSON2
) − (R
L
, is typically set to
+ R
DSON1
L
Data Sheet
) × I
− R
OUT_MAX
DSON2
) ×