LM34910CSD National Semiconductor, LM34910CSD Datasheet - Page 6

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

www.national.com
Functional Description
The LM34910C Step Down Switching Regulator features all
the functions needed to implement a low cost, efficient buck
bias power converter capable of supplying 1.25A to the load.
This high voltage regulator contains a 55V N-Channel buck
switch, is easy to implement, and is available in the thermally
enhanced LLP-10 package. The regulator’s operation is
based on a hysteretic control scheme, and uses an on-time
control which varies inversely with V
operating frequency to remain relatively constant with load
and input voltage variations. The hysteretic control requires
no loop compensation resulting in very fast load transient re-
sponse. The valley current limit detection circuit, internally set
at 1.25A, holds the buck switch off until the high current level
subsides. The functional block diagram is shown in Figure 1.
The LM34910C can be applied in numerous applications to
efficiently regulate down higher voltages. Additional features
include: Thermal shutdown, V
drive under-voltage lockout, and maximum duty cycle limiter.
Hysteretic Control Circuit Overview
The LM34910C buck DC-DC regulator employs a control
scheme based on a comparator and a one-shot on-timer, with
the output voltage feedback (FB) compared to an internal ref-
erence (2.5V). If the FB voltage is below the reference the
buck switch is turned on for a time period determined by the
input voltage and a programming resistor (R
on-time the switch remains off for a minimum of 280 ns, and
until the FB voltage falls below the reference. The buck switch
then turns on for another on-time period. Typically, during
start-up, or when the load current increases suddenly, the off-
times are at the minimum of 280 ns. Once regulation is
established, the off-times are longer.
When in regulation, the LM34910C operates in continuous
conduction mode at heavy load currents and discontinuous
conduction mode at light load currents. In continuous con-
duction mode current always flows through the inductor, nev-
er reaching zero during the off-time. In this mode the
operating frequency remains relatively constant with load and
line variations. The minimum load current for continuous con-
duction mode is one-half the inductor’s ripple current ampli-
tude. The operating frequency is approximately:
Start-up Regulator, V
The start-up regulator is integral to the LM34910C. The input
pin (V
with transient capability to 55V. The V
IN
) can be connected directly to line voltage up to 50V,
CC
under-voltage lockout, gate
IN
CC
. This feature allows the
CC
FIGURE 4. Low Ripple Output Configuration
output regulates at
ON
). Following the
6
The buck switch duty cycle is equal to :
In discontinuous conduction mode current through the induc-
tor ramps up from zero to a peak during the on-time, then
ramps back to zero before the end of the off-time. The next
on-time period starts when the voltage at FB falls below the
reference - until then the inductor current remains zero, and
the load current is supplied by the output capacitor (C2). In
this mode 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 loss-
es reduce with the reduction in load and frequency. The
approximate discontinuous operating frequency can be cal-
culated as follows:
where R
The output voltage is set by two external resistors (R1, R2).
The regulated output voltage is calculated as follows:
Output voltage regulation is based on ripple voltage at the
feedback input, requiring a minimum amount of ESR for the
output capacitor C2. The LM34910C requires a minimum of
25 mV of ripple voltage at the FB pin. In cases where the
capacitor’s ESR is insufficient additional series resistance
may be required (R3 in Figure 1).
For applications where lower output voltage ripple is required
the output can be taken directly from a low ESR output ca-
pacitor as shown in Figure 4. However, R3 slightly degrades
the load regulation.
7.0V, and is current limited to 9 mA. Upon power up, the reg-
ulator sources current into the external capacitor at V
When the voltage on the V
lockout threshold of 5.8V, the buck switch is enabled and the
L
= the load resistance.
V
OUT
= 2.5 x (R1 + R2) / R2
CC
30023510
pin reaches the under-voltage
CC
(C3).
(1)
(2)
(3)