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NCV47721 Datasheet(PDF) 12 Page - ON Semiconductor

No. de Pieza. NCV47721
Descripción  Adjustable LDO with Adjustable Current Limit
Descarga  16 Pages
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Fabricante  ONSEMI [ON Semiconductor]
Página de inicio  http://www.onsemi.com
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NCV47721 Datasheet(HTML) 12 Page - ON Semiconductor

 
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NCV47721
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12
is set to a temperature stable reference (VREF1) of 1.265 V.
The output voltage is calculated from the following formula.
Ignoring the bias current into the ADJ pin:
Vout_nom + VREF1 1 )
R
1
R
2
(eq. 2)
Use R2 < 50 k
W to avoid significant voltage output errors
due to ADJ bias current.
Designers should consider the tolerance of R1 and R2
during the design phase.
Setting the Output Current Limit
The output current limit can be set up to 300 mA by
external resistor RCSO (see Figure 1). Capacitor CCSO of
1
mF in parallel with RCSO is required for stability of current
limit control circuitry (see Figure 1).
V
CSO + Iout
R
CSO
1
100
(eq. 3)
I
LIM +
100
1
2.55
R
CSO
(eq. 4)
R
CSO +
100
1
2.55
I
LIM
(eq. 5)
where
RCSO − current limit setting resistor
VCSO ­ voltage at CSO pin proportional to Iout
ILIM − current limit value
Iout
− output current actual value
CSO pin provides information about output current actual
value. The CSO voltage is proportional to output current
according to Equation 3.
Once output current reaches its limit value (ILIM) set by
external resistor RCSO than voltage at CSO pin is typically
2.55 V. Calculations of ILIM or RCSO values can be done
using Equation 4 and Equation 5, respectively. Minimum
and maximum value of Output Current Limit can be
calculated according to Equations 6 and 7.
(eq. 6)
I
LIM_min + RATIOmin
V
CSO_min
R
CSO_max
(eq. 7)
I
LIM_max + RATIOmax
V
CSO_max
R
CSO_min
where
RATIOmin − minimum value of Output Current to
CSO Current Ratio from electrical
characteristics table and particular output
current range
RATIOmax − maximum value of Output Current to
CSO Current Ratio from electrical
characteristics table and particular output
current range
VCSO_min ­ minimum value of CSO Voltage Level at
Current Limit from electrical characteristics
table
VCSO_max ­ maximum value of CSO Voltage Level at
Current Limit from electrical characteristics
table
RCSO_min − minimum value of RCSO with respect its
accuracy
RCSO_max − maximum value of RCSO with respect its
accuracy
Designers should consider the tolerance of RCSO during
the design phase.
Diagnostic in OFF State
The NCV47721 contains also circuitry for OFF state
diagnostics for Short to Battery (STB) and Open Load (OL).
There are internal current source and Pull Down resistors
which provide additional cost savings for overall application
by excluding external components and their assembly cost
and saving PCB space and safe control IOs of a
Microcontroller Unit (MCU).
Simplified functional schematic and truth table is shown
in Figure 19 and related flowchart in Figure 20.
Figure 19. Simplified Functional Diagram of OFF
State Diagnostics (STB and OL)
Vout
+
− VREF_OFF
EF
PASS DEVICE is OFF in Diagnostics
Mode in OFF state
Vin
Current source enabled via EN and DE pins
Comparator active only in Diagnostic
state (DE = H).
IPU
EN – Enable (Logic Input)
DE – Diagnostics Enable (Logic Input)
EF – Error Flag Output (Open Collector Output)
EN
DE
RPD1
RPD2
Digital Diagnostics:
to MCU’s digital input
with pull−up resistor
to MCU’s DIO supply rail
EN DE IPU EF
Vout
Diagnostic Status/Action
L
L
OFF
HZ
Unknown
None (Diagnostics OFF)
L
H
OFF
L
Vout > Vout_OFF
Short to Battery (STB)
L
H
OFF
HZ Vout < Vout_OFF
Check for Open Load (OL)
HH
ON
L
Vout > Vout_OFF
Open Load (OL)
H
H
ON
HZ V
out < Vout_OFF
No Failure (V
out close to 0 V)


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