ADP3820AR-41 View Datasheet(PDF) - Analog Devices
Part Name
Description
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ADP3820AR-41 Datasheet PDF : 8 Pages
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ADP3820
The current sense resistor for this application:
RS = VS/IO = 0.05/0.5 = 100 mΩ
FET Selection
The type and size of the pass transistor are determined by the
threshold voltage, input-output voltage differential and load
current. The selected PMOS must satisfy the physical and ther-
mal design requirements. To ensure that the maximum VGS
provided by the controller will turn on the FET at worst case
conditions, (i.e., temperature and manufacturing tolerances) the
maximum available VGS must be determined. Maximum VGS is
calculated as follows:
where
IOUTMAX
RS
VBE
VGS = VIN – VBE – IOUTMAX × RS
= Maximum Output Current
= Current Sense Resistor
~ 0.7 V (Room Temperature)
~ 0.5 V (Hot)
~ 0.9 V (Cold)
For example:
VIN = 5 V, and IOUTMAX = 1 A,
VGS = 5 V – 0.7 V – 1 A × 50 mΩ = 4.25 V
If VGS < 5 V, logic level FET should be considered.
If VGS > 5 V, either logic level or standard MOSFET can be
used.
The difference between VIN and VO (VDS) must exceed the
voltage drop due to the sense resistor plus the ON-resistance
of the FET at the maximum charge current. The selected
MOSFET must satisfy these criteria; otherwise, a different pass
device should be used.
VDS = VIN – VO = 5 V – 4.2 V = 0.8 V
The maximum RDS(ON) required at the available gate drive (VDR)
and Drain-to-Source voltage (VDS) is:
RDS(ON) = VDS/IOUTMAX
From the Drain-to Source current vs. Drain-to-Source voltage
vs. gate drive graph off the MOSFET data sheet, it can be de-
termined if the above calculated RDS(ON) is higher than the graph
indicates. However, the value read from the MOSFET data
sheet graph must be adjusted based on the junction temperature
of the MOSFET. This adjustment factor can be obtained from
the normalized RDS(ON) vs. junction temperature graph in the
MOSFET data sheet.
External Capacitors
The ADP3820 is stable with or without a battery load, and
virtually any good quality output filter capacitors can be used
(anyCAP™), independent of the capacitor’s minimum ESR
(Effective Series Resistance) value. The actual value of the
capacitor and its associated ESR depends on the gm and capaci-
tance of the external PMOS device. A 10 µF tantalum or alumi-
num electrolytic capacitor at the output is sufficient to ensure
stability for up to a 10 A output current.
Shutdown Mode
Applying a TTL high signal to the SD pin or tying it to the
input pin will enable the output. Pulling this pin low or tying
it to ground will disable the output. In shutdown mode, the
controller’s quiescent current is reduced to less than 1 µA.
Gate-to-Source Clamp
A 6 V gate-to-source voltage clamp is provided by the ADP3820 to
protect most MOSFET gates in the event the VIN > VGS allowed
and the output is suddenly shorted to ground. This allows use of
the new, low RDS(ON) MOSFETs.
Short Circuit Protection
The power FET is protected during short circuit conditions
with a foldback type of current limiting that significantly re-
duces the current. See Figure 13 for foldback current limit
information.
Current Sense Resistor
Current limit is achieved by setting an appropriate current sense
resistor (RS) across the current limit threshold voltage. Current
limit sense resistor, RS, is calculated as shown above. Proper
derating is advised to select the power dissipation rating of the
resistor.
The simplest and cheapest sense resistor for high current appli-
cations, (i.e., Figure 1) is a PCB trace. However, the tempera-
ture dependence of the copper trace and the thickness tolerances of
the trace must be considered in the design. The resistivity of
copper has a positive temperature coefficient of +0.39%/°C.
Copper’s Tempco, in conjunction with the proportional-to-
absolute temperature (± 0.3%) current limit voltage, can provide
an accurate current limit. Table I provides the typical resistance
values for PCB copper traces. Alternately, an appropriate sense
resistor, such as surface mount sense resistors, available from
KRL, can be used.
Table I. Printed Circuit Copper Resistance
Conductor
Thickness
1/2oz/ft2
(18 µm)
1oz/ft2
(35 µm)
2oz/ft2
(70 µm)
3oz/ft2
(106 µm)
Conductor
Width/Inch
0.025
0.050
0.100
0.200
0.500
0.025
0.050
0.100
0.200
0.500
0.025
0.050
0.100
0.200
0.500
0.025
0.050
0.100
0.200
0.500
Resistance
m⍀/In
39.3
19.7
9.83
4.91
1.97
19.7
9.83
4.91
2.46
0.98
9.83
4.91
2.46
1.23
0.49
6.5
3.25
1.63
0.81
0.325
anyCAP is a trademark of Analog Devices, Inc.
REV. A
–7–
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