LM34917A Datasheet, PDF (10/16 Pages) National Semiconductor (TI) – Ultra Small 33V, 1.25A Constant On-Time Buck Switching Regulator with Intelligent Current Limit

time period. The next on-time starts when the current out of
ISEN is below the threshold and the voltage at FB falls below
2.5V. The operating frequency is typically lower due to longer-
than-normal off-times.
The valley current limit threshold is a function of the input
voltage (V_IN) and the output voltage sensed at FB, as shown
in the graph “Valley Current Limit Threshold vs. V_FB and
V_IN”. This feature reduces the inductor current’s peak value
at high line and load. To further reduce the inductor’s peak
current, the next cycle’s on-time is reduced by approximately
50% if the voltage at FB is below its threshold when the in-
ductor current reduces to the current limit threshold (V_OUT is
low due to current limiting).
Figure 3 illustrates the inductor current waveform during nor-
mal operation and in current limit. During the first “Normal
Operation” the load current is I_OUT1, the average of the ripple
waveform. As the load resistance is reduced, the inductor
current increases until it exceeds the current limit threshold.
During the “Current Limited” portion of Figure 3, the current
limit threshold lowers since the high load current causes
V_OUT (and the voltage at FB) to reduce. The on-time is re-
duced by approximately 50%, resulting in lower ripple ampli-
tude for the inductor’s current. During this time the LM34917A
is in a constant current mode, with an average load current
equal to the current limit threshold + ΔI/2 (I_OUT2). Normal op-
eration resumes when the load current is reduced to I_OUT3,
allowing V_OUT, the current limit threshold, 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 cur-
rent exceeds the current limit threshold during part of each
cycle, the circuit is not in current limit since the current falls
below the threshold before the feedback voltage reduces to
its threshold to initiate the next on-time.
The peak current allowed through the buck switch, and the
ISEN pin, is 2A, and the maximum allowed average current
is 1.5A.
FIGURE 3. Inductor Current - Normal and Current Limit Operation
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N - Channel Buck Switch and Driver
The LM34917A integrates an N-Channel buck switch and as-
sociated floating high voltage gate driver. The gate driver
circuit works in conjunction with an external bootstrap capac-
itor and an internal high voltage diode. A 0.022 µF capacitor
(C4) connected between BST and SW provides the voltage
to the driver during the on-time. During each off-time, the SW
pin is at approximately -1V, and C4 is recharged for the next
on-time from V_CC through the internal diode. The minimum
off-time ensures a minimum time each cycle to recharge the
bootstrap capacitor.
Softstart
The softstart feature allows the converter to gradually reach
a steady state operating point, thereby reducing start-up
stresses and current surges. Upon turn-on, after V_CC reaches
the under-voltage threshold, an internal 11.6 µA current
source charges up the external capacitor at the SS pin to 2.5V
(t₂ in Figure 1). The ramping voltage at SS (and the non-in-
verting input of the regulation comparator) ramps up the
output voltage in a controlled manner.
An internal switch grounds the SS pin if V_CC is below the un-
der-voltage lockout threshold, if the RON/SD pin is grounded,
or if V_IN exceeds the overvoltage threshold.
Thermal Shutdown
The LM34917A should be operated so the junction tempera-
ture does not exceed 125°C. If the junction temperature in-
creases above that, an internal Thermal Shutdown circuit
activates (typically) at 175°C, taking the controller to a low
power reset state by disabling the buck switch. This feature
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