### Applicaton Note

```V I S H A Y I N T E R T E C H N O L O G Y, I N C .
LOW VALUE POWER METAL STRIP® RESISTORS
DC/DC Converter
DC/DC Converter Utilizes Current Sensing
Resistors with Ultra Low Ω, Low TCR, Tight
Tolerance, High Current Capability, and Low
Thermal EMF
Due to the increase in the sales of Notebook PC,
the demand for the DC/DC Converter has been
showing rapid growth because of its high energy
conversion efficiency (95% vs. 50 to 70% for the
Conventional IC Regulator), and its precise
current-limiting capability.
However, to assure the performance of the
Multiple Output DC/DC Converter, the currentlimiting voltage must be detected precisely to
protect the expensive Notebook PC from an
overload, which in general, is due to shorts within
the capacitors used in these devices.
For high energy conversion efficiency, various
control ICs have been developed that utilize
resistive components. (See page 2 for a typical
design.)
To achieve the perfect cutoff mode of the DC
Load Current at the programmed current-limiting
voltage of the Control IC*, a very stable and
accurate sense resistor with the following
characteristics is required for precise voltage
comparison:
• Tight Tolerance
For maximizing the current supply within the
limit of acceptable current, the tolerance of the
sense resistor must be ± 1% or tighter.
• Low TCR
Since the maximum normal DC load current
is in excess of 3 amps, in general, the
maximum ∆R due to self-heating must be as
low as 1000ppm across the ambient
temperature range of 0°C to + 60°C. Thus, a
low TCR is required for current sensing.
w w w . v i s h a y. c o m
* Current level is programmed by the user by
choosing ohmic value of the current sense
resistor.
• Resistance Value
For maximizing energy conversion efficiency
of the DC/DC Converter, the value of the
current sense resistor is calculated according
to the worst-case-low current-limiting
threshold voltage of the Control IC and the
peak inductor current by the following
equation:
VL
Rs = I peak
Where: Rs = Sense Resistor (Ω)
I peak = Peak Inductor Current* = Maximum
VL = Worst-case-low current-limiting
threshold voltage
The key factor in determining the resolution
(current-limiting threshold voltage) of the Control
IC is its semiconductor noise. Typical resistance
values utilized in various control ICs are 20 to
25 milli-ohms.
The Vishay Dale® WSL-0805, WSL-1206, WSL2010, WSL-2512, WSL-0805-18, WSL-1206-18,
WSL-2010-18, WSL-2512-18, WSR2 and WSR3
can provide extremely low values down to 1 milliohms with one resistor. This saves mounting
costs and space, and improves the MTBF of the
DC/DC Converter. In comparison, with
conventional cermet chips, four to six chips are
needed to achieve very low ohmic values; and
with conventional thin film chips, at least two
chips are needed due to their low power
capability.
VSD-AP0001-0301
APPLICATION NOTE
• Very Low Ohmic Value
For maximizing energy conversion efficiency
of the DC/DC Converter the ohmic value, for
minimizing power consumption at the current
sense resistor, should be below 25 milli-ohms.
• Low Thermal EMF
For an accurate comparison between the
programmed current-limiting voltage (typically,
≤ 100mV) of the Control IC and the detected
voltage by the sense resistor, the error due to
thermal EMF of the sense resistor must be
minimized.
Application Note WSL and WSR for DC/DC Converters
Vishay Dale
• Terminal Construction
Down to 1 milli-ohms, two terminal construction is accepted
for the DC/DC Converter. However, when better accuracy
is required the four-terminal WSK-2512 is available. Fourterminal construction will reduce as mounted resistance
tolerance variations: • reduce lead resistance
• reduce TCR of copper terminal
• reduce TCR of solder joint.
• High Current Capability
The maximum DC load current of an up-to-date Notebook
PC with high-speed CPU, large memory (D-RAM) and the
CD-ROM is in excess of 3 amps. In this case, the sense
resistor must be able to withstand a high continuous current
of at least 1.15 times the DC load current (peak inductor
current) due to the effect of the inductor connected in series
for smoothing the circuit. Otherwise, the DC/DC Converter
will fail to function due to a failed sense resistor.
• High Temperature Capability
Utilization of DC/DC converters in industrial and automotive
applications may expose components to higher temperature
environments. The sense resistor must be capable of
operating in high temperature conditions with minimal
reduction (derating) of rated power.
The Vishay Dale® WSL-0805, WSL-1206, WSL-2010, WSL2512, WSL-0805-18, WSL-1206-18, WSL-2010-18, WSL2512-18, WSR2 and WSR3 will withstand high temperatures.
In comparison cermet chips have poor high temperature
capability. See Chart 1 for high temperature comparison.
• Vishay Dale® sense resistors provide characteristics
required and demanded by DC/DC Converter operations
that cannot be met with other conventional cermet and
thin film resistive chip components.
* For smoothing circuit.
Typical Control IC for Dual Output Step-down
DC/DC Converter
The two key functions of a Control IC are to supply dual stepdown DC potential energy and to perform accurate current
limiting which is programmed by the user.
A. A voltage drop across a sense resistor connected in series
between the N-channel Power MOSFET and the output
terminal is monitored in order to compare the worst-caselow current-limiting threshold voltage of the Control IC with
the current level programmed by the user via sense resistor.
B. The sense resistor also functions as a part of the smoothing
circuit to supply high linear DC current.
The sequence of the Dual Output Step-down Converter's
steady state operations is first the gate (Ga) turns on the Nchannel Power MOSFET (A) and sinks the DC current into the
load until the terminal voltage reaches the programmed positive
voltage of the Control IC, then, second, (Ga) cuts off (A), and
(Gb) turns on (B), and leaks the DC current to the load until
the terminal voltage reaches the programmed negative voltage,
then, third (Gb) cuts off (B) • • •.
INPUT
5.5V to 30V
Gb
Ga
• TCR
b
OUTPUT #2
5.0V
A
R1
R2
CONTROL IC
At steady state conditions, Notebook PCs generally require
input currents in excess of 3 amps. For accurate currentlimiting control by the Control IC, the DR of the sense resistor,
due to self-heating, must be low enough to ensure that the
accuracy of comparison between the worst-case-low
current-limiting threshold voltage of the Control IC and the
sense resistor voltage is ± 5%.
b B
SOFT-START CAPACITORS
To meet this standard current limit, the dynamic tolerance (no
load, static resistance tolerance + % ∆R caused by self-heating
+ apparent % ∆R caused by thermal EMF) allocated to the
sense resistor is 2%; and, to assure 2% tolerance of the sense
resistor across the temperature range of 0°C to + 100°C
(ambient temperature + self-heating), the resistor must have
a low TCR. The Vishay Dale® WSL-0805, WSL-1206, WSL2010, WSL-2512, WSL-0805-18, WSL-1206-18, WSL-201018, WSL-2512-18, WSR2 and WSR3 have the lowest TCR in
the industry. See Chart 2 for TCR comparison.
for reducing surge current at start up
R1 and R2: Current Surge Resistors
A and B: N-Channel Power MOSFETs
V Ga
V Gb
b
I
0
b
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OUTPUT#2
3.3V
Application Note WSL and WSR for DC/DC Converters
Vishay Dale
CHART 1
% Power
POWER METAL STRIP - HIGH TEMPERATURE CAPABILITY
Temperature °C
CHART 2
Resistance Temperature
Coefficient (ppm/°C)
POWER METAL STRIP - TCR PERFORMANCE
Resistance Value (mΩ)
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```