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ADP2325ACPZ-R7 数据表(PDF) 20 Page - Analog Devices

部件名 ADP2325ACPZ-R7
功能描述  Dual 5 A, 20 V Synchronous Step-Down
PDF  32 Pages
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制造商  AD [Analog Devices]
网页  http://www.analog.com
标志 AD - Analog Devices

ADP2325ACPZ-R7 数据表(HTML) 20 Page - Analog Devices

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ADP2325
Data Sheet
Rev. 0 | Page 20 of 32
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:
(
)
D
D
I
I
OUT
_rms
IN
C
−
×
×
=
1
OUTPUT VOLTAGE SETTING
The output voltage of the ADP2325 can be set by an external
resistor divider using the following equation:


+
×
=
BOT
TOP
OUT
R
R
V
1
6
.
0
To limit output voltage accuracy degradation due to FBx pin
bias current (0.1 µA maximum) to less than 0.5% (maximum),
ensure that RBOT is less than 30 kΩ. Table 8 provides the recom-
mended resistor divider for various output voltage options.
Table 8. Resistor Divider for Various Output Voltages
VOUT (V)
RTOP, ±1% (kΩ)
RBOT, ±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 limited by the minimum on time. The
minimum on time of the ADP2325 is typically 130 ns. The
minimum output voltage in CCM mode at a given input voltage
and frequency can be calculated using the following equation:
VOUT_MIN = VIN × tMIN_ON × fSW − (RDSON1 − RDSON2) × IOUT_MIN ×
tMIN_ON × fSW − (RDSON2 + RL) × IOUT_MIN
where:
VOUT_MIN is the minimum output voltage.
tMIN_ON is the minimum on time.
IOUT_MIN is the minimum output current.
fSW is the switching frequency.
RDSON1 is the high-side MOSFET on resistance.
RDSON2 is the low-side MOSFET on resistance.
RL is the series resistance of the output inductor.
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:
VOUT_MAX = VIN × (1 − tMIN_OFF × fSW) − (RDSON1 − RDSON2) ×
IOUT_MAX × (1 − tMIN_OFF × fSW) − (RDSON2 + RL) × IOUT_MAX
where:
VOUT_MAX is the maximum output voltage.
tMIN_OFF is the minimum off time.
IOUT_MAX is the maximum output current.
The maximum output voltage that is limited by the maximum
duty cycle at a given input voltage can be calculated using the
following equation:
VOUT_MAX = DMAX × VIN
where DMAX is the maximum duty cycle.
As the previous equations demonstrate, reducing the switching
frequency alleviates the minimum on time and minimum off time
limitation.
CURRENT-LIMIT SETTING
The ADP2325 has two selectable current-limit thresholds. Make
sure that the selected current-limit value is larger than the peak
current of the inductor, IPEAK.
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, ΔIL, is typically set to
one-third of the maximum load current. The inductor value can
be calculated by using the following equation:
(
)
SW
L
OUT
IN
f
I
D
V
V
L
×
∆
×
−
=
where:
VIN is the input voltage.
VOUT is the output voltage.
ΔIL is the inductor ripple current.
fSW is the switching frequency.
D is the duty cycle.
IN
OUT
V
V
D =



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