LM2695 datasheet(9/21 Pages) TI

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STOP

RUN

RON/SD

Input

Voltage

LM2695

VIN

RON

tON(min) =

VOUT x 288 ns

(VIN(min) - VOUT)

RON =

VOUT

FS x 1.3 x 10

-10

LM2695

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SNVS413A – JANUARY 2006 – REVISED APRIL 2013

See Figure 4. The inverse relationship with VIN results in a nearly constant frequency as VIN is varied. To set a

specific continuous conduction mode switching frequency (FS), the RON resistor is determined from the following:

(6)

In high frequency applicatons the minimum value for tON is limited by the maximum duty cycle required for

regulation and the minimum off-time of (250 ns, ±15%). The minimum off-time limits the maximum duty cycle

achievable with a low voltage at VIN. The minimum allowed on-time to regulate the desired VOUT at the minimum

VIN is determined from the following:

(7)

The LM2695 can be remotely shut down by taking the RON/SD pin below 0.8V. See Figure 7. 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 VIN

and the RON resistor.

Figure 7. Shutdown Implementation

Current Limit

Current limit detection occurs during the off-time by monitoring the recirculating current through the free-wheeling

diode (D1). Referring to Typical Application Circuit and Block Diagram, 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 output 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 causing 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 8 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 8, 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:

ΔI = (VIN - VOUT) x tON / L1

(8)

During this time the LM2695 is in a constant current mode, with an average load current (IOCL) equal to 1.25A +

ΔI/2.

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