ISL28005 Datasheet, PDF (9/14 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
V_RS+
SIGNAL
GENERATOR
V_CC
V_SENSE
RS+
RS-
OUT
GND
1MΩ R_L ^VOUT
PULSE
GENERATOR
FIGURE 13. SLEW RATE, t_s, SATURATION RECOVERY
TIME
FIGURE 14. GAIN vs FREQUENCY
V_RS+
V_CC
RS+
RS-
OUT
GND
1MΩ R_L ^VOUT
PULSE
GENERATOR
FIGURE 15. 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 16 shows the
active circuitry for high-side current sense applications
where the sense voltage is between 1.35V to 28V.
Figure 17 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)
The input gm stage derives its ~2.86µA supply current
from the input source through the RS+ terminal as long
as the sensed voltage at the RS+ pin is >1.35V and the
gm_HI amplifier is selected. When the sense voltage at
R_S+ drops below the 1.35V threshold, the gm_LO
amplifier kicks in and the gm_LO output current reverses,
flowing out of the RS- pin.
9
FN6973.2
May 27, 2010