Datasheet

Rsw
CBOOTx
4R7
HDRVx
SWx
0.1 PF
LM5642, LM5642X
www.ti.com SNVS219K –JUNE 2003–REVISED APRIL 2013
Figure 28. HDRV Series Resistor
CURRENT SENSING AND LIMITING
As shown in Figure 29, the KSx and RSNSx pins are the inputs of the current sense amplifier. Current sensing is
accomplished either by sensing the Vds of the top FET or by sensing the voltage across a current sense resistor
connected from VIN to the drain of the top FET. The advantages of sensing current across the top FET are
reduced parts count, cost and power loss.
The R
DS-ON
of the top FET is not as stable over temperature and voltage as a sense resistor, hence great care
must be used in layout for V
DS
sensing circuits. At input voltages above 30V, the maximum recommended output
current is 5A per channel.
Keeping the differential current-sense voltage below 200mV ensures linear operation of the current sense
amplifier. Therefore, the R
DS-ON
of the top FET or the current sense resistor must be small enough so that the
current sense voltage does not exceed 200 mV when the top FET is on. There is a leading edge blanking circuit
that forces the top FET on for at least 166ns. Beyond this minimum on time, the output of the PWM comparator
is used to turn off the top FET. Additionally, a minimum voltage of at least 50 mV across Rsns is recommended
to ensure a high SNR at the current sense amplifier.
Assuming a maximum of 200 mV across Rsns, the current sense resistor can be calculated as follows:
where
• Imax is the maximum expected load current, including overload multiplier (ie: 120%)
• Irip is the inductor ripple current (see Equation 17) (3)
The above equation gives the maximum allowable value for Rsns. Conduction losses will increase with larger
Rsns, thus lowering efficiency.
The peak current limit is set by an external resistor connected between the ILIMx pin and the KSx pin. An
internal 10 µA current sink on the ILIMx pin produces a voltage across the resistor to set the current limit
threshold which is then compared to the current sense voltage. A 10 nF capacitor across this resistor is required
to filter unwanted noise that could improperly trip the current limit comparator.
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