LM27402_1 Datasheet, PDF (1/4 Pages) National Semiconductor (TI) – Current Limit Application Circuits Application Circuits

LM27402 Current Limit
Application Circuits
National Semiconductor
Application Note 2060
Daniel Tennant
June 10, 2010
Introduction
Sensing the inductor DCR voltage can be an accurate and
lossless technique of obtaining current information in DC/DC
regulators. The absence of a series current sense resistor
permits high current and high efficiency designs at a lower
cost and higher density. Inductor DCR sensing also allows
continuous monitoring of the inductor current as opposed to
MOSFET R_DS(ON) sensing which usually samples the voltage
information across a FET during a small interval of time. Un-
like MOSFETs, inductors can be purchased with low toler-
ance DCR specifications thus increasing the overall current
sense accuracy.
The LM27402 is equipped with a low offset current sense
comparator to handle inductor DCR current sense applica-
tions. A +10 µA current in series with a resistor provides a
voltage offset compared to the DCR voltage to set the current
limit level. This application note will walk through the process
of choosing the DCR current sense circuit components to op-
timize the current limit performance of the LM27402 DC/DC
buck regulator controller.
DCR Current Sense Topology
Figure 1 shows a typical topology for sensing the DCR voltage
in a buck regulator.
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FIGURE 1. DCR Current Sense Topology
Components R_S and C_S create an RC filter. The time constant
of the RC filter should match the time constant related to the
inductor and its DCR to accurately reproduce the DCR volt-
age across C_S. Given the following circuit:
The derivation for matching time constants is as follows:
If the time constants do not match, the voltage V_S will either
lead or lag V_DCR. Particularly:
If τ_RC > τ_L, V_S will lag V_DCR by a factor of τ_RC/τ_L at any point in
time << τ_RC.
If τ_RC < τ_L, V_S will lead V_DCR by factor of τ_RC/τ_L at any point in
time << τ_RC.
Time constant mismatching can be useful in some situations
where tight or loose limiting of current limit must be applied.
For example, if a buck regulator responds readily to fast dI/dt
load transients, the inductor current must supply the output
current in addition to the output capacitor current. This can
cause the inductor current to temporarily overshoot thereby
exceeding the current limit setpoint. The V_S signal can filter
this event if τ_RC is designed moderately larger than τ_L in an-
ticipation of inductor overshoot. However, time constant mis-
matching must be used carefully. If τ_RC is much greater than
τ_L , the inductor current may reach dangerous levels before
the current limit condition is detected.
LM27402 DCR Current Sense
Design
The LM27402 is designed to detect the voltage across the
DCR of an inductor through the use of a sensitive current limit
comparator. Five current limit events within 32 switching cy-
cles must occur to mitigate the effects of noise and transitory
over current events before hiccup mode activates. This allows
the current limit level to be set close to the peak inductor ripple
current during maximum output current where the voltage
across the comparator may only be several milivolts. Figure
3 shows the typical circuit used to set the current limit level:
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FIGURE 2. DCR Sense Equivalent Circuit
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