ISL28006_14 Datasheet, PDF (19/26 Pages) Intersil Corporation – Micropower, Rail to Rail Input Current Sense Amplifier with Voltage Output

ISL28006
Applications Information
Functional Description
The ISL28006-20, ISL28006-50 and ISL28006-100 are single
supply, uni-directional current sense amplifiers with fixed gains
of 20V/V, 50V/V and 100V/V respectively. The ISL28006-ADJ is
single supply, uni-directional current sense amplifier with an
adjustable gain via external resistors (see Figure 80). The
ISL28006-ADJ is stable for gains of 20 and higher.
The ISL28006 is a 2-stage amplifier. Figure 76 shows the active
circuitry for high-side current sense applications where the sense
voltage is between 1.35V to 28V. Figure 77 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
variable gain (ADJ) has an additional FB pin and uses external
gain resistors to set the gain of the output. For the fixed gain
amps the only external component needed is a current sense
resistor (typically 0.001Ω to 0.01Ω, 1W to 2W).
The transfer function for the fixed gain parts is given in
Equation 1.
V_OUT = GAIN × (I_SR_S + V_OS)
(EQ. 1)
The transfer function for the adjustable gain part is given in
Equation 2.
V_OUT
=
⎛
⎜
⎝
1
+
R^-R----_GF^-_{⎠⎟⎞}
(
I_S
R_S
+
V_O
S
)
(EQ. 2)
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 69 and 70 for typical
input bias currents for High and Low side current sensing.
OPTIONAL
FILTER
CAPACITOR
I_S
+
R_S
-
R_S+
V_SENSE
R_S-
OPTIONAL
TRANSIENT
PROTECTION
LOAD
I = 2.86µA
V_SENSE
R₁
gm_HI
V_CC
HIGH-SIDE
SENSING
V_RS+ = 2V TO 28V
R₂
+
1.35V
I_MIRROR
-
R_f
R₃
gm_LO ‘V_SENSE
R₅
R_g
R₄
GND
V_CC = 2V to 28V
OUT
FIGURE 76. HIGH-SIDE CURRENT DETECTION
19
FN6548.6
November 22, 2013