LM34917A Datasheet, PDF (9/16 Pages) National Semiconductor (TI) – Ultra Small 33V, 1.25A Constant On-Time Buck Switching Regulator with Intelligent Current Limit

zero, and the operating frequency remains relatively constant
with load and line variations. The minimum load current for
continuous conduction mode is one-half the inductor’s ripple
current amplitude. The approximate operating frequency is
calculated as follows:
(1)
The buck switch duty cycle is equal to:
(2)
In discontinuous conduction mode, where the inductor’s cur-
rent reaches zero during the off-time forcing a longer-than-
normal off-time, the operating frequency is lower than in
continuous conduction mode, and varies with load current.
Conversion efficiency is maintained at light loads since the
switching losses reduce with the reduction in load and fre-
quency. The approximate discontinuous operating 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 (R1,
R2 in the Block Diagram). The regulated output voltage is
calculated as follows:
V_OUT = 2.5 x (R1 + R2) / R2
Output voltage regulation is based on supplying ripple voltage
to the feedback input (FB pin) in phase with the SW pin. The
LM34917A requires a minimum of 25 mVp-p of ripple voltage
at the FB pin. The ripple is generated as a triangle wavefrom
at the junction of R3 and C8 as the SW pin switches high and
low, and fed to the FB pin by C7.
If the voltage at FB rises above 2.9V, due to a transient at
V_OUT or excessive inductor current which creates higher than
normal ripple at V_OUT, the internal over-voltage comparator
immediately shuts off the internal buck switch. The next on-
time starts when the voltage at FB falls below 2.5V and the
inductor current falls below the current limits threshold.
ON-Time Timer
The on-time for the LM34917A is determined by the R_ON re-
sistor and the input voltage (V_IN), calculated from:
(4)
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_SW), the R_ON resistor is
determined from the following:
(5)
Equations 1, 4 and 5 are valid only during normal operation -
i.e., the circuit is not in current limit. When the LM34917A
operates in current limit, the on-time is reduced by approxi-
mately 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. Equations
1, 4 and 5 have a ±25% tolerance.
Remote Shutdown
The LM34917A can be remotely shut down by taking the
RON/SD pin below 0.65V. See Figure 2. In this mode the SS
pin is internally grounded, the on-timer is disabled, and bias
currents are reduced. Releasing the RON/SD pin allows the
circuit to resume operation after the SS pin voltage is below
0.18V. The voltage at the RON/SD pin is normally between
1.4V and 3.5V, depending on V_IN and the R_ON resistor.
20216616
FIGURE 2. Remote Shutdown
Input Over-Voltage Shutdown
If the input voltage at VIN increases above 34.8V an internal
comparator disables the buck switch and the on-timer, and
grounds the soft-start pin. Normal operation resumes when
the V_IN voltage reduces below 34.8V, and when the soft-start
voltage (at the SS pin) has reduced below 0.18V.
Current Limit
Current limit detection occurs during the off-time by monitor-
ing the recirculating current flowing out of the ISEN pin.
Referring to the Block Diagram, during the off-time the induc-
tor current flows through the load, into SGND, 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 output delays the start of the next on-
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