LM2695SD datasheet(10/14 Pages) NSC

10 / 14 page

ON-Time Timer, and Shutdown

(Continued)

The LM2695 can be remotely shut down by taking the

RON/SD pin below 0.8V. See Figure 6. In this mode the SS

pin is internally grounded, the on-timer is disabled, and bias

currents are reduced. Releasing the RON/SD pin allows

normal operation to resume. The voltage at the RON/SD pin

is between 1.5V and 3.0V, depending on V

IN and the RON

resistor.

Current Limit

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

ing the recirculating current through the free-wheeling diode

(D1). Referring to Figure 3, when the buck switch is turned

off the inductor current flows through the load, into SGND,

through the sense resistor, out of ISEN and through D1. If

that current exceeds 1.25A the current limit comparator out-

put switches to delay the start of the next on-time period if

the voltage at FB is below 2.5V. The next on-time starts

when the current out of ISEN is below 1.25A and the voltage

at FB is below 2.5V. If the overload condition persists caus-

ing the inductor current to exceed 1.25A 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 7 illustrates the inductor current waveform. During

normal operation the load current is Io, the average of the

ripple waveform. When the load resistance decreases the

current ratchets up until the lower peak reaches 1.25A.

During the Current Limited portion of Figure 7, the current

ramps down to 1.25A 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:

∆I=(V

IN -VOUT)xtON /L1

During this time the LM2695 is in a constant current mode,

with an average load current (I

OCL) equal to 1.25A +

∆I/2.

The current limit threshold can be increased by connecting

an external resistor between SGND and ISEN. The external

resistor will typically be less than 1

Ω. The peak current out of

SW and ISEN must not exceed 2A. The average current out

of SW must be less than 1.5A.

N - Channel Buck Switch and

Driver

The LM2695 integrates an N-Channel buck switch and as-

sociated floating high voltage gate driver. The peak current

allowed through the buck switch is 2A, and the maximum

allowed average current is 1.5A. The gate driver circuit

works in conjunction with an external bootstrap capacitor

and an internal high voltage diode. A 0.022 µF capacitor (C4)

connected between BST and SW provides the voltage to the

driver during the on-time. During each off-time, the SW pin is

at approximately -1V, and C4 charges from V

CC through the

internal diode. The minimum off-time of 250 ns ensures a

minimum time each cycle to recharge the bootstrap capaci-

tor.

20170413

FIGURE 6. Shutdown Implementation

20170414

FIGURE 7. Inductor Current - Current Limit Operation

LM2695SD Datasheet(PDF) 10 Page - National Semiconductor (TI)

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