LM5010 datasheet(12/19 Pages) NSC

12 / 19 page

Current Limit

Current limit detection occurs during the off-time by monitor-

ing the recirculating current through the free-wheeling diode

(D1). The detection threshold is 1.25A, ±0.25A. Referring to

Figure 1, when the buck switch is off the inductor current

flows through the load, into S

GND, through the sense resistor,

out of I

SEN and through D1. If that current exceeds the

threshold the current limit comparator output switches to

delay the start of the next on-time period. The next on-time

starts when the current out of I

SEN is below the threshold and

the voltage at FB is below 2.5V. If the overload condition

persists causing the inductor current to exceed the threshold

during each on-time, that is detected at the beginning of

each off-time. The operating frequency is lower due to

longer-than-normal off-times.

Figure 12 illustrates the inductor current waveform. During

normal operation the load current is I

O, the average of the

ripple waveform. When the load resistance decreases the

current ratchets up until the lower peak attempts to exceed

the threshold. During the Current Limited portion of Figure

12, the current ramps down to the threshold during each

off-time, initiating the next on-time (assuming the voltage at

FB is < 2.5V). During each on-time the current ramps up an

amount equal to:

(6)

During this time the LM5010 is in a constant current mode,

with an average load current (I

OCL) equal to the threshold +

∆I/2.

The “valley current limit” technique allows the load current to

exceed the current limit threshold as long as the lower peak

of the inductor current is less than the threshold.

The current limit threshold can be increased by connecting

an external resistor (R

CL) between SGND and ISEN. The

external resistor typically is less than 1

Ω, and its calculation

is explained in the Applications Information section.

The peak current out of SW and I

SEN must not exceed 3.5A.

The average current out of SW must be less than 3A, and

the average current out of I

SEN must be less than 2A.

N - Channel Buck Switch and

Driver

The LM5010 integrates an N-Channel buck switch and as-

sociated floating high voltage gate driver. The peak current

through the buck switch must not be allowed to exceed 3.5A,

and the average current must be less than 3A. The gate

driver circuit is powered by the external bootstrap capacitor

between BST and SW (C4). During each off-time, the SW pin

is at approximately -1V, and C4 is re-charged from V

through the internal high voltage diode. The minimum off-

time of 265 ns ensures a minimum time each cycle to

recharge the bootstrap capacitor. A 0.022 µF ceramic ca-

pacitor is recommended for C4.

Softstart

The softstart feature allows the converter to gradually reach

a steady state operating point, thereby reducing startup

stresses and current surges. Upon turn-on, after V

reaches the under-voltage threshold (t1 in Figure 8), an

internal 11.5 µA current source charges the external capaci-

tor at the Softstart pin to 2.5V (t2 in Figure 8). The ramping

voltage at SS (and at the non-inverting input of the regulation

comparator) ramps up the output voltage in a controlled

manner. This feature keeps the load current from going to

current limit during startup, thereby reducing inrush currents.

An internal switch grounds the Softstart pin if V

CC is below

the under-voltage lockout threshold, if a thermal shutdown

occurs, or if the circuit is shutdown using the R

ON/SD pin.

Thermal Shutdown

The LM5010 should be operated so the junction temperature

does not exceed 125˚C. If the junction temperature in-

creases above that, an internal Thermal Shutdown circuit

activates (typically) at 175˚C, taking the controller to a low

power reset state by disabling the buck switch and the

on-timer, and grounding the Softstart pin. This feature helps

prevent catastrophic failures from accidental device over-

20119920

FIGURE 12. Inductor Current - Current Limit Operation

LM5010 Datasheet(PDF) 12 Page - National Semiconductor (TI)

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