ISL28005_14 Datasheet, PDF (10/15 Pages) Intersil Corporation – Micropower, Rail-to-Rail Input Current Sense Amplifier with Voltage Output

ISL28005
Test Circuits and Waveforms (Continued)
V_CC
V_RS+
V_RS-
RS+
RS-
OUT
GND
1MΩ R_L ^VOUT
PULSE
GENERATOR
FIGURE 23. SLEW RATE, t_s, SATURATION RECOVERY TIME
V_RS+
SIGNAL
GENERATOR
V_CC
V_SENSE
RS+
RS-
OUT
GND
1MΩ R_L ^VOUT
FIGURE 24. GAIN vs FREQUENCY
V_CC
V_RS+
RS+
RS-
OUT
GND
1MΩ R_L ^VOUT
PULSE
GENERATOR
FIGURE 25. SLEW RATE
Applications Information
Functional Description
The ISL28005-20, ISL28005-50 and ISL28005-100 are single
supply, uni-directional current sense amplifiers with fixed gains
of 20V/V, 50V/V and 100V/V respectively.
The ISL28005 is a 2-stage amplifier. Figure 26 shows the active
circuitry for high-side current sense applications where the sense
voltage is between 1.35V to 28V. Figure 27 shows the active
circuitry for ground sense applications where the sense voltage is
between 0V to 1.35V.
The first stage is a bi-level trans-conductance amp and level
translator. The gm stage converts the low voltage drop (V_SENSE)
sensed across an external milli-ohm sense resistor, to a current
(@ gm = 21.3µA/V). The trans-conductance amplifier forces a
current through R₁ resulting to a voltage drop across R₁ that is
equal to the sense voltage (V_SENSE). The current through R₁ is
mirrored across R₅ creating a ground-referenced voltage at the
input of the second amplifier equal to V_SENSE.
The second stage is responsible for the overall gain and
frequency response performance of the device. The fixed gains
(20, 50, 100) are set with internal resistors R_f and R_g. The only
external component needed is a current sense resistor (typically
0.001Ω to 0.01Ω, 1W to 2W).
The transfer function is given in Equation 1.
V_OUT = GAIN × (I_SR_S + V_OS)
(EQ. 1)
Where I_SR_S is the product of the load current and the sense
resistor and is equal to V_SENSE.
When the sensed input voltage is >1.35V, the gm_HI amplifier
path is selected and the input gm stage derives its ~2.86µA
supply current from the input source through the RS+ terminal.
When the sense voltage at R_S+ drops below the 1.35V threshold,
the gm_LO amplifier is enabled for Low Side current sensing. The
gm_LO input bias current reverses, flowing out of the RS- pin.
Since the gm_LO amplifier is sensing voltage around ground, it
cannot source current to R5. A current mirror referenced off Vcc
supplies the current to the second stage for generating a ground
referenced output voltage. See Figures 19 and 20 for typical
input bias currents for High and Low side current sensing.
10
FN6973.5
October 24, 2013