ISL6617A_14 Datasheet, PDF (11/15 Pages) Intersil Corporation – PWM Doubler with Output Monitoring Feature

ISL6617A
Current Sensing
The ISL6617A senses current continuously for fast response. The
ISL6617A supports inductor DCR sensing, or resistive sensing
techniques. The associated channel current sense amplifier uses
the ISEN inputs to reproduce a signal proportional to the inductor
current, I_L. The sensed current, I_SEN, is proportional to the
inductor current. The sensed current is used for current balance
and load-line regulation.
The internal circuitry (shown in Figures 10 and 11) represents
one channel. This circuitry is repeated for each channel in the
doubler. The input bias current of the current sensing amplifier is
typically 60nA; less than 5kΩ input impedance is preferred to
minimize the offset error. In addition, the common mode input
voltage to the amplifier should be less than VCC-3V.
INDUCTOR DCR SENSING
An inductor’s winding is characteristic of a distributed resistance,
as measured by the DCR (Direct Current Resistance) parameter.
Consider the inductor DCR as a separate lumped quantity, as
shown in Figure 10.
V_IN
POWER
STAGE
PWMA/B
ILs
L
DCR
INDUCTOR
V_L
V_C(s)
R
C
V_OUT
C_OUT
ISL6617A
I_A/B
R_ISEN(A/B)
CURRENT
SENSE
+
CSEN(A/B)
-
CSRTN(A/B)
C_T
I_SEN
=
I_L
--^D-----^C----^R----
R_ISEN
FIGURE 10. DCR SENSING CONFIGURATION
The channel current I_L, flowing through the inductor, will also
pass through the DCR. Equation 2 shows the s-domain
equivalent voltage across the inductor V_L.
V_Ls = I_L  s  L + DCR
(EQ. 2)
A simple R-C network across the inductor extracts the DCR
voltage, as shown in Figure 10.
The voltage on the capacitor V_C, can be shown to be proportional
to the channel current I_L. See Equation 3.
V_Cs
=
 ^s

-----^L------
DCR
+
1_

DCR

I_L
----------------------------------------------------------------
s  RC + 1
(EQ. 3)
If the R-C network components are selected such that the RC
time constant matches the inductor time constant (RC = L/DCR),
the voltage across the capacitor V_C is equal to the voltage drop
across the DCR, i.e., proportional to the channel current.
With the internal low-offset current amplifier, the capacitor
voltage V_C is replicated across the sense resistor R_ISEN.
Therefore, the current out of ISEN+ pin, I_SEN, is proportional to
the inductor current.
Because of the internal filter at ISEN- pin, one capacitor, C_T, is
needed to match the time delay between the ISEN- and ISEN+
signals. Select the proper C_T to keep the time constant of R_ISEN
and C_T (R_ISEN x C_T) close to 27ns.
Equation 4 shows that the ratio of the channel current to the
sensed current, I_SEN, is driven by the value of the sense resistor
and the DCR of the inductor.
I_SEN
=
I_L

--^D-----^C----^R----
R_ISEN
(EQ. 4)
RESISTIVE SENSING
For more accurate current sensing, a dedicated resistor R_SENSE in
series with each output inductor can serve as the current sense
element (see Figure 11). This technique reduces overall converter
efficiency due to the additional power loss on the current sense
element R_SENSE.
IL
L
R_SENSE V_OUT
C_OUT
ISL6617A
I_A/B
R_ISEN(A/B)
CURRENT
SENSE
+
CSEN(A/B)
-
CSRT(A/B)
C_T
ISEN _{= IL -R---R-S----I-E-S---N-E----S-N---E---}
FIGURE 11. SENSE RESISTOR IN SERIES WITH INDUCTORS
The same capacitor C_T is needed to match the time delay between
ISEN- and ISEN+ signals. Select the proper C_T to keep the time
constant of R_ISEN and C_T (R_ISEN x C_T) close to 27ns.
Equation 5 shows the ratio of the channel current to the sensed
current I_SEN.
I_SEN
=
I_L

R_{-----S---E---N----S---E--}
R_ISEN
(EQ. 5)
Submit Document Feedback 11
FN7844.0
December 19, 2014