STOP

RUN

RON/SD

Input

Voltage

LM34919B

VIN

RON

RON =

VOUT x (VIN - 1.5V)

FS x 0.565 x 10

-10

x VIN

- 1.4 k:

LM34919B

LM34919B-Q1

SNVS623B – MAY 2010 – REVISED JULY 2013

www.ti.com

(6)

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

regulation and the minimum off-time. The minimum off-time limits the maximum duty cycle achievable with a low

voltage at VIN. At high values of VIN, the minimum on-time is limited to ≊ 90 ns.

The LM34919B can be remotely shut down by taking the RON/SD pin low (see Figure 27). 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 1.4V and 5.0V, depending on VIN

and the RON resistor.

Figure 27. 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 the 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 0.64A 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 ISEN is below 0.64A and the voltage at FB is below 2.5V. If the overload condition

persists causing the inductor current to exceed 0.64A 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 28 shows 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

0.64A. During the Current Limited portion of Figure 28, the current ramps down to 0.64A 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

(7)

During this time the LM34919B is in a constant current mode, with an average load current (IOCL) equal to 0.64A

+

ΔI/2.

Generally, in applications where the switching frequency is higher than

≊300 kHz and uses a small value

inductor, the higher dl/dt of the inductor's ripple current results in an effectively lower valley current limit threshold

due to the response time of the current limit detection circuit. However, since the small value inductor results in a

relatively high ripple current amplitude (

ΔI in Figure 28), the load current (IOCL) at current limit is typically in

excess of 640 mA.

14

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