- 65 ns

4.15 x 10

-11

x

- 0.5 k:

4.15 x 10

-11

+ 65 ns

LM34930

SNVS571B – MAY 2008 – REVISED APRIL 2013

www.ti.com

Output voltage regulation requires a minimum of 25 mVp-p ripple voltage be supplied to the feedback pin (FB). In

the typical application circuit shown with the Block Diagram, ripple is generated by the inductor’s ripple current

passing through R3 in series with the output capacitor. The output ripple is passed to the FB pin by C6, avoiding

attenuation by resistors R1 and R2.

On-Time Timer

(5)

(6)

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

valid only when the regulator is not in current limit. When the LM34930 operates in current limit, the on-time is

reduced by

≊50%. 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 2 mA. For high frequency applications, the

maximum switching frequency is limited at the maximum input voltage by the minimum on-time one-shot period.

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

which may prevent achievement of the proper duty cycle.

Current Limit

Current limit detection occurs during the off-time by monitoring the recirculating diode current flowing out of the

ISEN pin. Referring to the Block Diagram, during the off-time the inductor current flows through the load, into the

GND pin, through the internal sense resistor, out of ISEN and through D1 to the inductor. If that current exceeds

the current limit threshold the current limit comparator delays the start of the next on-time period. The next on-

time starts when the current out of ISEN reduces to the threshold and the voltage at FB is below 2.52V. The

operating frequency is typically lower in the current limited condition due to longer-than-normal off-times.

reduce the inductor’s peak current, the next on-time after current limit detection is reduced by

≊50% if the voltage

is low due to current limiting).

Figure 18 illustrates the inductor current waveform during normal operation and in current limit. During the first

resistance is reduced, the inductor current increases until the lower peak of the inductor ripple current exceeds

the current limit threshold. During the “Current Limited” portion of Figure 18, each on-time is reduced by

≊50%,

resulting in lower ripple amplitude for the inductor’s current. During this time the LM34930 is in a constant current

output voltage is below the normal regulated value. Normal operation resumes when the load current is reduced

Operation”, even 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, and the ISEN pin, is 2A, and the maximum allowed average

current is 1.5A.

10

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