RT = VOUT - (VIN x FS x 15 ns)
FS x 4.1 x 10-11 - 500:
tON = (VIN)
4.1 x 10-11 x (RT + 500:) + 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
The on-time for the LM34922 is determined by the RTresistor and the input voltage (VIN), calculated from:
(5)
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 RTresistor is determined from the following:
(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
resistor RS. Referring to the Block Diagram, during the off-time the recirculating current flows through the
inductor, through the load, through the sense resistor, and through D1 to the inductor. If the voltage across the
sense resistor exceeds the threshold (VILIM) the current limit comparator output switches to delay the start of the
next on-time period. The next on-time starts when the recirculating current decreases such that the voltage
across RSreduces to the threshold and the voltage at FB is below 2.51V. The operating frequency is typically
lower due to longer-than-normal off-times. When current limit is detected, the on-time is reduced by ≊40% if the
voltage at the FB pin is below its threshold when the voltage across RSreduces to its threshold (VOUT is low due
to current limiting).
Figure 16 illustrates the inductor current waveform during normal operation and in current limit. During the first
“Normal Operation” the load current is I01, the average of the inductor current waveform. As the load resistance is
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
load current equal to the current limit threshold plus half the ripple amplitude (IOCL), and the output voltage is
below the normal regulated value. Normal operation resumes when the load current is reduced (to IO2), allowing
VOUT and the on-time to return to their normal values. Note that in the second period of “Normal 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 is 3.5A, and the maximum allowed average current is 2.0A.
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