-11

- 500:

4.1 x 10

-11

+ 15 ns

LM34922

SNVS813A – JUNE 2012 – REVISED MARCH 2013

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Ripple voltage, which is required at the input of the regulation comparator for proper output regulation, is

generated internally in the LM34922. In the LM34922 the ERM (Emulated Ripple Mode) control circuit generates

the required internal ripple voltage from the ripple waveform at the CS pin.

On-Time Timer

(5)

(6)

The on-time must be chosen greater than 90ns for proper operation. Equation 1, Equation 5 and Equation 6 are

valid only during normal operation - i.e., the circuit is not in current limit. When the LM34922 operates in current

limit, the on-time is reduced by

≊40%. This feature reduces the peak inductor current which may be excessively

high if the load current and the input voltage are simultaneously high. This feature operates on a cycle-by-cycle

basis until the load current is reduced and the output voltage resumes its normal regulated value. The maximum

continuous current into the RT pin must be less than 2mA. For high frequency applications, the maximum

switching frequency is limited at the maximum input voltage by the minimum on-time one-shot period (90ns). At

minimum input voltage the maximum switching frequency is limited by the minimum off-time one-shot period,

which, if reached, prevents achievement of the proper duty cycle.

Current Limit

Current limit detection occurs during the off-time by monitoring the voltage across the external current sense

inductor, through the load, through the sense resistor, and through D1 to the inductor. If the voltage across the

next on-time period. The next on-time starts when the recirculating current decreases such that the voltage

lower due to longer-than-normal off-times. When current limit is detected, the on-time is reduced by

≊40% if the

to current limiting).

Figure 16 illustrates the inductor current waveform during normal operation and in current limit. During the first

reduced, the inductor current increases until the lower peak of the inductor ripple current exceeds the threshold.

During the “Current Limited” portion of Figure 16, each on-time is reduced by

≊40%, resulting in lower ripple

amplitude for the inductor’s current. During this time the LM34922 is in a constant current mode with an average

though the inductor’s peak current exceeds the current limit threshold during part of each cycle, the circuit is not

in current limit since the inductor current falls below the current limit threshold during each off time. The peak

current allowed through the buck switch is 3.5A, and the maximum allowed average current is 2.0A.

10

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