ISL28005_11 Datasheet, PDF (9/14 Pages) Intersil Corporation – Micropower, Rail-to-Rail Input Current Sense Amplifier

ISL28005
Test Circuits and Waveforms
V_CC
+
V_RS+
-
+
V_SENSE
-
RS+
RS-
GND
OUT
1MΩ R_L ^VOUT
+
V_RS+
-
V_R1
V_CC
R1
+
V_SENSE
-
R2
V_R2
RS+
RS-
GND
OUT
1MΩ R_L ^VOUT
FIGURE 13. I_S, V_OS, V_OA, CMRR, PSRR, GAIN ACCURACY
V_CC
V_RS+
V_RS-
RS+
RS-
OUT
GND
1MΩ R_L ^VOUT
PULSE
GENERATOR
FIGURE 15. SLEW RATE, t_s, SATURATION RECOVERY TIME
FIGURE 14. INPUT BIAS CURRENT, LEAKAGE CURRENT
SIGNAL
GENERATOR
V_CC
V_RS+
V_SENSE
RS+
RS-
OUT
GND
1MΩ R_L ^VOUT
FIGURE 16. GAIN vs FREQUENCY
V_RS+
V_CC
RS+
RS-
OUT
GND
1MΩ R_L ^VOUT
PULSE
GENERATOR
FIGURE 17. 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 18 shows the active
circuitry for high-side current sense applications where the sense
voltage is between 1.35V to 28V. Figure 19 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
FN6973.4
July 1, 2011