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SC310XSKTRT

High-Side Current-Sense Amplifier with Low Impedance Voltage Output

Semtech Corporation 

SC310

POWER MANAGEMENT

Application Information

Choosing Rsense

Common Mode Input

Full scale sense voltage for the amplifier is 150mV max.

The resistor (Rsense) is selected depending on the load of

the system. For light loads a bigger value of Rsense 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 sense voltage is limited at low values of common mode

input voltage (VCMR ). 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.

The voltage at the OUT pin of the amplifier is given as:

28V

Vout = (Gain) * (Iload) * (Rsense)

Similarly, for heavy loads, a smaller value of Rsense resis-

tor is used not to exceed the max “Sense Voltage” rating

of the IC. Also at high loads, a power rating of Rsense re-

sistor is used and must be acceptable (greater than I2R

power dissipation in the resistor).

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 Function

SC310C

SC310B

SC310A

5V

3.5V

2.7V

0

35mV

Not drawn to scale

70mV

Vsense

150mV

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.

Typical Values for Sense Resistors

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.

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 (VCC) 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 RSENSE as close as possible. Use

minimum width traces to reduce capacitive coupling.

Output Impedance

The output impedance of the SC310 is very low. Typical

value is 60Ω. 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.

 2005 Semtech Corp.

9

www.semtech.com

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