LM34910CSD National Semiconductor, LM34910CSD Datasheet - Page 7

• Manufacturer Part Number: LM34910CSD
• Description: High Voltage (50V, 1.25A) Step Down Switching Regulator
• Manufacturer: National Semiconductor

Softstart pin is released to allow the Softstart capacitor (C6)
to charge up.
The minimum input voltage is determined by the regulator’s
dropout voltage, the V
the frequency. When V
V
loaded, the minimum input voltage increases since the output
impedance at V
Regulation Comparator
The feedback voltage at FB is compared to the voltage at the
Softstart pin (2.5V). In normal operation (the output voltage is
regulated), an on-time period is initiated when the voltage at
FB falls below 2.5V. The buck switch stays on for the on-time,
causing the FB voltage to rise above 2.5V. After the on-time
period, the buck switch stays off until the FB voltage falls be-
low 2.5V. Bias current at the FB pin is nominally 100 nA.
Over-Voltage Comparator
The voltage at FB is compared to an internal 2.875V refer-
ence. If the voltage at FB rises above 2.875V the on-time
pulse is immediately terminated. This condition can occur if
the input voltage or the output load changes suddenly, or if
the inductor (L1) saturates. The buck switch remains off until
the voltage at FB falls below 2.5V.
ON-Time Timer, and Shutdown
The on-time for the LM34910C is determined by the R
sistor and the input voltage (V
See Figure 3. The inverse relationship with V
nearly constant frequency as V
lected for a minimum on-time (at maximum V
200 ns. This requirement limits the maximum frequency for
each application, depending on V
the following:
CC
UVLO activates to shut off the output. If V
CC
is
≊
CC
140Ω. See Figure 2.
CC
UVLO falling threshold (
falls below the falling threshold the
IN
IN
), and is calculated from:
is varied. R
IN
and V
OUT
ON
, calculated from
FIGURE 5. Self Biased Configuration
IN
CC
IN
) greater than
should be se-
≊
is externally
results in a
5.7V), and
ON
re-
(4)
7
To reduce power dissipation in the start-up regulator, an aux-
iliary voltage can be diode connected to the V
the auxiliary voltage to between 8V and 14V shuts off the in-
ternal regulator, reducing internal power dissipation. The sum
of the auxiliary voltage and the input voltage (V
not exceed 70V. Internally, a diode connects V
Figure 5.
The LM34910C can be remotely shut down by taking the
R
pin is internally grounded, the on-timer is disabled, and bias
currents are reduced. Releasing the R
mal operation to resume. The voltage at the R
between 1.5V and 3.0V, depending on V
sistor.
Current Limit
Current limit detection occurs during the off-time by monitor-
ing the recirculating current through the free-wheeling diode
(D1). Referring to Figure 1, when the buck switch is turned off
the inductor current flows through the load, into S
ON
/SD pin below 0.65V. See Figure 6. In this mode the SS
FIGURE 6. Shutdown Implementation
30023511
ON
/SD pin allows nor-
IN
and the R
CC
CC
CC
ON
GND
www.national.com
pin. Setting
30023513
to V
+ V
/SD pin is
, through
IN
IN
ON
) can-
. See
(5)
re-