LM34922 Datasheet, PDF (11/17 Pages) Texas Instruments – 28V, 2A Constant On-Time Switching Regulator with Adjustable Current Limit

plitude. The approximate operating frequency is calculated as
follows:
(1)
The buck switch duty cycle is approximately equal to:
(2)
When the load current is less than one half the inductor’s rip-
ple current amplitude the circuit operates in discontinuous
conduction mode. The off-time is longer than in continuous
conduction mode while the inductor current is zero, causing
the switching frequency to reduce as the load current is re-
duced. Conversion efficiency is maintained at light loads
since the switching losses are reduced with the reduction in
load and frequency. The approximate discontinuous operat-
ing frequency can be calculated as follows:
(3)
where R_L = the load resistance, and L1 is the circuit’s inductor.
The output voltage is set by the two feedback resistors
(R_FB1, R_FB2 in the Block Diagram). The regulated output volt-
age is calculated as follows:
V_OUT = 2.51V x (R_FB1 + R_FB2) / R_FB1
(4)
Ripple voltage, which is required at the input of the regulation
comparator for proper output regulation, is generated inter-
nally 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 R_T resistor
and the input voltage (V_IN), calculated from:
(5)
The inverse relationship with V_IN results in a nearly constant
frequency as V_IN is varied. To set a specific continuous con-
duction mode switching frequency (F_S), the R_T resistor is
determined from the following:
(6)
The on-time must be chosen greater than 90ns for proper op-
eration. Equations 1, 5 and 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 volt-
age 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 max-
imum continuous current into the RT pin must be less than
2mA. For high frequency applications, the maximum switch-
ing 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, pre-
vents achievement of the proper duty cycle.
Current Limit
Current limit detection occurs during the off-time by monitor-
ing the voltage across the external current sense resistor
R_S. 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 in-
ductor. If the voltage across the sense resistor exceeds the
threshold (V_ILIM) 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 R_S reduces 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 de-
tected, the on-time is reduced by ≊40% if the voltage at the
FB pin is below its threshold when the voltage across R_S re-
duces to its threshold (V_OUT is low due to current limiting).
Figure 2 illustrates the inductor current waveform during nor-
mal 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 in-
ductor current increases until the lower peak of the inductor
ripple current exceeds the threshold. During the “Current Lim-
ited” portion of Figure 2, 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 thresh-
old plus half the ripple amplitude (I_OCL), and the output voltage
is below the normal regulated value. Normal operation re-
sumes when the load current is reduced (to I_O2), allowing
V_OUT 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 dur-
ing each off time. The peak current allowed through the buck
switch is 3.5A, and the maximum allowed average current is
2.0A.
11
www.ti.com