Datasheet

Table Of Contents

OFF(CL)

x 0.93) + 0.28V

4 x 10

-6

((V

/31) + 0.15)

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Load Current Derating

5 Load Current Derating

Although the maximum load current for this evaluation board is specified as 4.5A, the data sheet for the

Si7465 PFET specifies a maximum continuous current of 3.2A. Since the input current, which is the

average current though Q1, increases as the input voltage is decreased, the load current must be derated

at low input voltage, seeFigure 2.

Figure 2. Maximum Load Current Derating

6 Operating at Low Voltage

When the input voltage is less than 5 V the PFET (Q1) is on continuously (100% duty cycle), and the

output voltage is equal to the input voltage, minus voltage drops across the sense resistor, Q1 and the

inductor. As the input voltage is increased above 5 V, switching commences at the PGATE pin and the

SW node as the LM5085 regulates the output at 5 V. Since the LM5085 does not have a required

minimum off-time, the circuit transitions smoothly from 100% duty cycle to a regulated output.

7 Current Limit

The LM5085 peak current limit detection operates by sensing the voltage across either the R

DS(ON)

of Q1,

or a sense resistor (R5), during the on-time and comparing it to the voltage across R3 at the ADJ pin. The

current limit threshold is reached when the sensed voltage exceeds the voltage across R3. When current

limit is reached Q1 is immediately switched off. The current limit function is much more accurate and

stable over temperature when a sense resistor is used. The R

DS(ON)

of a MOSFET has a wide process

variation and a large temperature coefficient.

Current sensing is disabled for a blanking time of ≊100 ns at the beginning of each on-time to prevent

false triggering of the current limit comparator due to leading edge current spikes. After Q1 is turned off

due to current limit detection, Q1 is held off for a longer-than-normal off-time. The extended off-time is a

function of the input voltage and the voltage at the FB pin, as shown below in the graph “Current Limit Off-

time vs. V

and V

”. The current limit off-time can be calculated from Equation 2:

(2)

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 long forced off-time, provides effective

protection from output load faults over a wide range of operating conditions.

SNVA357B–October 2008–Revised April 2013 AN-1878 LM5085 Evaluation Board

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