
9
2005 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC310
PCB trace can also be used as the sense resistor. This
has the drawback of significant error at the output over
temperature. The trade-off, despite the error in the out-
put voltage, provides for a cost-effective solution.
SHDN F
SHDN Function
unction
The SHDN pin should not be kept floating and must be
tied to either ground or be held high. Connecting the pin
to ground is needed for normal operation. The voltage
level at the SHDN pin is independent of the functioning
of the IC.
TTTTTypical V
ypical V
ypical Values f
alues f
alues for Sense R
or Sense R
or Sense Resist
esist
esistor
or
orsssss
Table 1. lists the common resistor values for typical op-
eration of the SC310. SC310 is capable of sensing wide
ranges of currents providing proper sense resistors are
used.
Output Impedance
The output impedance of the SC310 is very low. Typical
value is 60
. Connecting the OUT pin to a high-imped-
ance input stage is recommended. Resistive loading on
the OUT pin will not affect the gain of the SC310.
Common Mode Input
The sense voltage is limited at low values of common mode
input voltage (V
CMR ).
The figure below specifies the de-
sired sense voltage for low common mode input volt-
age. In the figure below we see that SC310A can be op-
erated over full-scale sense voltage at Vcs+ = 2.7V,
whereas the voltage is limited for SC310B and SC310C.
Layout Guidelines
1. Place the SC310 between the pins and external cir-
cuitry to reduce the trace length.
2. Place the decoupling capacitor at the input (V
CC) as
close as possible to the pin.
3. An R-C filter at the input will help achieve the highest
noise rejection.
4. Run the traces from R
SENSE as close as possible.
Use
minimum width traces to reduce capacitive coupling.
Choosing R
sense
Full scale sense voltage for the amplifier is 150mV max.
The resistor (R
sense) is selected depending on the load of
the system. For light loads a bigger value of R
sense is se-
lected to minimize the error in the proportional output
voltage. Higher resistor value improves the strength of
the signal at the current-sense amplifier input and hence
gives an accurate output.
The voltage at the OUT pin of the amplifier is given as:
V
out = (Gain) * (Iload) * (Rsense)
Similarly, for heavy loads, a smaller value of R
sense resis-
tor is used not to exceed the max “Sense Voltage” rating
of the IC. Also at high loads, a power rating of R
sense re-
sistor is used and must be acceptable (greater than I2R
power dissipation in the resistor).
Application Information
Vsense
0
70mV
150mV
2.7V
3.5V
28V
V
CS
+
35mV
5V
SC310C
SC310B
SC310A
Not drawn to scale