LM25085AMYSLASHNOPB datasheet(12/31 Pages) TI1

12 / 31 page

'I =

(VOUT + VFD + VESR) x tOFF

'I =

(VIN - VOUT) x tON

tOFF(CL) =

8 x 10

-6 x ((V

IN/31) + 0.15)

(VFB x 0.93) + 0.56V

LM25085A

SNVS601B – JANUARY 2009 – REVISED MARCH 2013

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CURRENT LIMITING

The LM25085A current limiting operates by sensing the voltage across either the RDS(ON) of Q1, or a sense

resistor, during the on-time and comparing it to the voltage across the resistor RADJ (see Figure 26). The current

limit function is much more accurate and stable over temperature when a sense resistor is used. The RDS(ON) of a

MOSFET has a wide process variation and a large temperature coefficient.

If the voltage across RDS(ON) of Q1, or the sense resistor, is greater than the voltage across RADJ, the current limit

comparator switches to turn off Q1. Current sensing is disabled for a blanking time of

≊100 ns at the beginning of

the on-time to prevent false triggering of the current limit comparator due to leading edge current spikes.

Because of the blanking time and the turn-on and turn-off delays created by the PFET, the on-time at the PGATE

pin should not be set less than 150 ns. An on-time shorter than that may prevent the current limit detection circuit

from properly detecting an over-current condition. The duration of the subsequent forced off-time is a function of

the input voltage and the voltage at the FB pin, as shown in Figure 10. The longer-than-normal forced off-time

allows the inductor current to decrease to a low level before the next on-time. This cycle-by-cycle monitoring,

followed by a forced off-time, provides effective protection from output load faults over a wide range of operating

conditions.

The voltage across the RADJ resistor is set by an internal 40 µA current sink at the ADJ pin. When using Q1’s

RDS(ON) for sensing, the current at which the current limit comparator switches is calculated from:

ICL = 40 µA x RADJ/RDS(ON)

(6)

When using a sense resistor (RSEN) the thrshold of the current limit comparator is calculated from:

ICL = 40 µA x RADJ/RSEN

(7)

When using Equation 6 or Equation 7, the tolerances for the ADJ pin current sink and the offset of the current

limit comparator should be included to ensure the resulting minimum current limit is not less than the required

maximum switch current. Simultaneously increasing the values of RADJ and RSEN decreases the effects of the

current limit comparator offset, but at the expense of higher power dissipation. When using a sense resistor, the

RSEN resistor value should be chosen within the practical limitations of power dissipation and physical size. For

example, for a 10A current limit, setting RSEN = 0.005Ω results in a power dissipation as high as 0.5W. Current

sense connections to the RSEN resistor, or to Q1, must be Kelvin connections to ensure accuracy.

The CADJ capacitor filters noise from the ADJ pin, and helps prevent unintended switching of the current limit

comparator due to input voltage transients. The recommended value for CADJ is 1000 pF.

CURRENT LIMIT OFF-TIME

When the current through Q1 exceeds the current limit threshold, the LM25085A forces an off-time longer than

the normal off-time defined by Equation 1. See Figure 10, or calculate the current limit off-time from the following

equation:

(8)

where VIN is the input voltage, and VFB is the voltage at the FB pin at the time current limit was detected. This

feature is necessary to allow the inductor current to decrease sufficiently to offset the current increase which

occurred during the on-time. During the on-time, the inductor current increases an amount equal to:

(9)

During the off-time the inductor current decreases due to the reverse voltage applied across the inductor by the

output voltage, the freewheeling diode’s forward voltage (VFD), and the voltage drop due to the inductor’s series

resistance (VESR). The current decrease is equal to:

(10)

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