NCV47823
www.onsemi.com
11
APPLICATIONS INFORMATION
Circuit Description
The NCV47823 is an integrated dual High Side Switch
(HSS) with current limit up to 350 mA per channel able to
operate in Constant Current Source (CCS) mode depending
on the output current load. The operation mode can be
expressed by equations as follows:
(eq. 1)
HSSmode :
(V
in
* V
in*out1,2
)
R
load1,2
+ I
out1,2
t I
LIM1,2
or
(eq. 2)
CCSmode :
(V
in
* V
in*out1,2
)
R
load1,2
+ I
out1,2
+ I
LIM1,2
where I
LIM1,2
value is preset by R
CSO1,2
. In HSS mode of
operation (eq. 1) output current I
out1,2
may exceed I
LIM1,2
(reduced inrush current). Voltage on CSO pin is proportional
to output current. The operation mode with PWM function
of Enable inputs is provided by the circuit. The integrated
current sense features diagnosis and system protection
functionality. The HSS is protected by both current limit and
thermal shutdown. Thermal shutdown occurs above 150°C
to protect the IC during overloads and extreme ambient
temperatures.
Enable Inputs
An enable pin is used to turn the channel on or off. By
holding the pin down to a voltage less than 0.99 V, the output
of the channel will be turned off. When the voltage on the
enable pin is greater than 2.31 V, the output of the channel
will be enabled to power its output to the regulated output
voltage. The enable pins may be connected directly to the
input pin to give constant enable to the output channel. As
mentioned above, the circuit allows using both Enable
inputs to obtain PWM of output current.
Setting the Output Current Limit
The output current value can be set up to 350 mA by
external resistor R
CSO1,2
(see Figure 1).
V
CSO1,2
+ I
out1,2
ǒ
R
CSO1,2
1
RATIO
Ǔ
(eq. 3)
I
LIM1,2
+
RATIO
1
2.55
R
CSO1,2
(eq. 4)
R
CSO1,2
+
RATIO
1
2.55
I
LIM1,2
(eq. 5)
where
R
CSO1,2
− current limit setting resistor
V
CSO1,2
− voltage at CSO pin proportional to I
out1,2
I
LIM1,2
− current limit value
I
out1,2
− output current actual value
RATIO − typical value of Output Current to CSO
Current Ratio for particular output current
range
CSO pin provides information about output current actual
value. The CSO voltage is proportional to output current
according to (eq. 3).
Once output current reaches its limit value (I
LIM1,2
) set by
external resistor R
CSO
than voltage at CSO pin is typically
2.55 V. Calculations of I
LIM1,2
or R
CSO1,2
values can be
done using (eq. 6) and (eq. 7).
(eq. 6)
I
LIM1,2_min
+ RATIO
min
V
CSO1,2_min
R
CSO1,2_max
(eq. 7)
I
LIM1,2_max
+ RATIO
max
V
CSO1,2_max
R
CSO1,2_min
where:
RATIO
min
– minimum value of Output Current to
CSO Current Ratio from electrical
characteristics table and particular output
current range
RATIO
max
– maximum value of Output Current to
CSO Current Ratio from electrical
characteristics table and particular output
current range
V
CSO1,2_min
– minimum value of CSO Voltage Level
at Current Limit from electrical characteristics
table
V
CSO1,2_max
– maximum value of CSO Voltage Level
at Current Limit from electrical characteristics
table
R
CSO1,2_min
– minimum value of R
CSO1,2
with
respect its accuracy
R
CSO1,2_max
– maximum value of R
CSO1,2
with
respect its accuracy
Designers should consider the tolerance of R
CSO1,2
during the design phase.
Diagnostic in OFF state
The NCV47823 contains also circuitry for OFF state
diagnostics for Short to Battery (STB) and Open Load (OL).
There are internal current sources and Pull Down resistors
providing 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 13 and related flowchart in Figure
14.
The diagnostics in OFF state shall be performed for each
channel separately. For diagnostics of Channel 1 the input
CS pin has to be put logic low, for diagnostics of Channel 2
the input CS pin has to be put logic high. Corresponding EN
pin has to be used for control (EN1 for Channel 1 and EN2
for Channel 2).
