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LM34910 Datenblatt(PDF) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Teilenummer LM34910
Bauteilbeschribung  High Voltage (40V, 1.25A) Step Down Switching Regulator
Download  11 Pages
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Hersteller  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

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Functional Description
The LM34910 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 40V 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
IN. This feature allows
the operating frequency to remain relatively constant with
load and input voltage variations. The hysteretic control re-
quires no loop compensation resulting in very fast load tran-
sient response. 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 LM34910 can be applied in numerous applications to
efficiently regulate down higher voltages. Additional features
include: Thermal shutdown, V
CC under-voltage lockout, gate
drive under-voltage lockout, and maximum duty cycle limiter.
Hysteretic Control Circuit
Overview
The LM34910 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 inter-
nal reference (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). Follow-
ing the 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. Typi-
cally, 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 LM34910 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,
never 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
conduction mode is one-half the inductor’s ripple current
amplitude. The operating frequency is approximately:
(1)
The buck switch duty cycle is equal to :
(2)
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 continu-
ous conduction mode, and varies with load current. Conver-
sion efficiency is maintained at light loads since the switch-
ing losses reduce with the reduction in load and frequency.
The approximate discontinuous operating frequency can be
calculated as follows:
(3)
where R
L = the load resistance.
The output voltage is set by two external resistors (R1, R2).
The regulated output voltage is calculated as follows:
V
OUT =2.5x(R1+R2)/R2
Output voltage regulation is based on ripple voltage at the
feedback input, requiring a minimum amount of ESR for the
output capacitor C2. The LM34910 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 re-
quired the output can be taken directly from a low ESR
output capacitor as shown in Figure 4. However, R3 slightly
degrades the load regulation.
20110910
FIGURE 4. Low Ripple Output Configuration
www.national.com
6


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