Direct Current Resistance (DCR) Using ISL28023 Digital

PRECISION ANALOG COOKBOOK
SENSOR SOLUTIONS - Current Sense
Direct Current Resistance (DCR) Using Digital Power
Monitor (DPM)
ISL28023
ISL28023 Key Features
The ISL28023 is a bidirectional high-side and low-side digital
current sense and voltage monitor with a serial interface. The
device monitors power supply current, voltage and provides
the digital results along with calculated power. The ISL28023
provides tight accuracy of 0.05% for both voltage and current
monitoring. The auxiliary input provides an additional power
monitor function.
• Integrated Analog Front End with Digital Output
The VCC power can either be externally supplied or internally
regulated, which allows the ISL28023 to handle a
common-mode input voltage range from 0V to 60V. The wide
range permits the device to handle telecom, automotive and
industrial applications with minimal external circuitry.
An 8-bit voltage DAC enables a DC/DC converter output
voltage margining. Fault indication includes Bus Voltage
window and overcurrent fast fault logic indication. A
temperature sensing option includes both internal
temperature sensor and a bias/monitor circuit for external
diode sensing.
The ISL28023 serial interface is PMBus compatible and
operates down to 1.2V voltage. It draws an average current of
just 800µA and is available in the space saving 24 Ld QFN
4mmx4mm package. The part operates across the full
industrial temperature range from -40°C to +125°C.
GND
DAC_OUT
PRIMARY CH
I2CVCC
INTERNAL
POWER
3.3V
LS
LS
SMBCLK
• Voltage, Current, Power Monitoring and Current Direction
-
Current Sense: High-side, Low-side, Bi-directional
Wide Common-mode Input Voltage Range: 0V to 60V
Digital Output
Internal 3.3V regulator
Internal temperature sense
8-bit voltage output DAC
Auxiliary channel available
• Precision /Accuracy
- 16-bit ADC
- Voltage/Current Measurement Error: <0.3%
- Internal Temperature Sensor Accuracy: 1°C
Lossless Current Sensing (DCR)
A DCR sense circuit is an alternative to a sense resistor. The
DCR circuit utilizes the parasitic resistance of an inductor to
measure the current to the load. A DCR circuit remotely
measures the current through an inductor. The lack of
components in series with the regulator to the load makes the
circuit lossless.
REG
REG_IN
{
VCC
DAC (8-BIT)
REG_OUT
- Analog Switch/MUX, ADC, Voltage Reference
- Digital Processing/Serial Communication Circuitry
(I2C/SMBus/PMBus)
Temp_V
VBUS_S
VBUS
I2C
SM BUS
PM BUS
TEMP
SENSE
DCR Circuit
Rsen
SMBDAT
REF
A0
A1
VINP
ADC
16-BIT
CM = 0 to 60V
VINM
FIR &
DIGITAL
LOGIC
16
A2
REG MAP
Buck
Regulator
Csen
DIV
VBUS_S
EXT_CLK
FB
CM = 0 to VCC
OV_TEMP_SET
VBUS_S
Temp_V
OC_SET
DIGITAL FILTER
0, 2, 4, 8µS
VIN_P
AUXP
AUXM
UV
DAC
Rdcr
UV_SET
VIN_M
AUX CH
OC
DAC
VINM
Lo
CLOCK
ADC
16-Bit
LOAD
SW Mux
{
AUXV
Rsen + Rdcr
Phase
OSC
OV/
TEMP
DAC
VINP
SMBALERT2
SMBALERT1
FIGURE 2. A SIMPLIFIED CIRCUIT EXAMPLE OF A DCR
ONLY FOR PRI CHL
FIGURE 1. ISL28023 Block Diagram
A properly matched DCR circuit has an equivalent circuit seen
by the ADC equals to Rdcr in Figure 2. Before deriving the
transfer function between the inductor current and voltage
seen by the ISL28023, let’s review the definition of an inductor
and capacitor in the Laplacian domain.
Xc( f )
1
j ( f )  C
XL( f )
j ( f )  L
(EQ. 1)
Xc is the impedance of a capacitor related to the frequency
and XL is the impedance of an inductor related to frequency. ω
equals to 2f. f is the chop frequency dictated by the regulator.
May 15, 2014
ST-109.0
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Copyright Intersil Americas LLC 2014. All Rights Reserved
Intersil (and design) is a trademark owned by Intersil Corporation or one of its subsidiaries.
All other trademarks mentioned are the property of their respective owners.
Direct Current Resistance (DCR) Using Digital Power Monitor (DPM)
Using Ohms law, the voltage across the DCR circuit in terms of
the current flowing through the inductor is define in Equation 2.
R dcr  j ( f)  L i L
V dcr( f )
(EQ. 2)
In Equation 2, Rdcr is the parasitic resistance of the inductor. The
voltage drop across the inductor (Lo) and the resistor (Rdcr)
circuit is the same as the voltage drop across the resistor (Rsen)
and the capacitor (Csen) circuit. Equation 3 defines the voltage
across the capacitor (Vcsen) in terms of the inductor current (IL).
 1   j(f)L  


R dcr  


R dcr
i
1  j(f)C senR sen L


 j(f)L  R dcr
V c(f)
1  j(f)C senR sen
(EQ. 3)
The relationship between the inductor load current (IL) and the
voltage across capacitor simplifies if the following component
selection holds true;
L
(EQ. 4)
C sen R sen
R dcr
If Equation 4 hold true, the numerator and denominator of the
fraction in Equation 3 cancels reducing the voltage across the
capacitor to the equation represented in Equation 5.
Vc
R dcr i L
Most inductor datasheets will specify the average value of the
Rdcr for the inductor. Rdcr values are usually sub 1mΩ with a
tolerance averaging 8%. Common chip capacitor tolerances
average to 10%.
Inductors are constructed out of metal. Metal has a high
temperature coefficient. The temperature drift of the inductor
value could cause the DCR circuit to be un-tuned. An un-tuned
circuit results in inaccurate current measurements along with a
chop signal bleeding into the measurement. To counter the
temperature variance, a temperature sensor may be incorporated
into the design to track the change in component values.
A DCR circuit is good for gross current measurements. As
discussed, inductors and capacitors have high tolerances and are
temperature dependent which will result in less than accurate
current measurements.
In Figure 2, there is a resistor in series with the ISL28023
negative shunt terminal, VINM, with the value of Rsen + Rdcr. The
resistor’s purpose is to counter the effects of the bias current
from creating a voltage offset at the input of the ADC.
Reference Documents
Intersil ISL28023 Data Sheet, FN8389
Reference Website
ISL28023 Product Page
(EQ. 5)
These are example simplified circuitry for DCR sensing application.
R1
1uF
VCC,FS,SS
Sync,Comp
Gnd
Phase
En
Vreg_in
I2C
SMBUS
SCL
Aux_N
AuxV
FB
SCL
8-Bit
DAC
Temp
Sense
GND
Vmcu
LOAD
VOUT =
0.6 +[ (0.6 – DAC O UT)
* R2/R1]
Vin
MCU
SDA
SMBALERT2
I2CVCC
DAC OUT
To SMBAlert1
GPIO/Int
SDA
PMBus
REG
MAP
R1
R_pullUp
VOUT =
0.8 +[ (0.8 – DAC OUT)
* R2/R1]
Vmcu
LOAD
To SMBAlert1
Ext_Temp
Place Diode
Near DCR
Com ponents
PG
A2
ADC
16-Bit
Aux_P
Boot
SMBALERT2
Temp
Sense
Rdcr
MCU
GPIO/Int
R_pullUp
8-Bit
DAC
Lo
0.1uF
ISL85415
I2CVCC
DAC OUT
A1
R_pullUp
AuxV
A0
(Rsen + Rdcr)
VINM
SDA
PMBus
REG
MAP
Csen
GND
3.3V
Vreg
VINP
R2
I2C
SMBUS
Aux_P
R_pullUp
En
SCL
DCR Circuit
Rsen
Vin
A2
ADC
16-Bit
Aux_N
FB
R2
PG
VINM
GND
Vin
Vout
A0
A1
(Rsen + Rdcr)
VCC
ISL28023
VBUS
SW MUX
Ext_Temp
Place Diode
Near DCR
Com ponents
LX2
GND
3.3V
Vreg
VINP
SW MUX
DCR Circuit
Csen
LX1
ISL9110
Adj
Buck/
Boost
SMBALERT1
VCC
ISL28023
VBUS
Rsen
Gnd,
PGnd
Rdcr
PVin
10uF
SMBALERT1
Lo
Mode
1uF
From 1.8V to 5.5V
Vin
Vreg_Out
Vreg_in
Vin
Vreg_Out
Vin = 4.5V 36V
SCL
SDA
GPIO
FIGURE 3. APPLICATION DIAGRAM with ISL9110
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FIGURE 4. APPLICATION DIAGRAM with ISL85415
ST-109.0
May 15, 2014

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