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LM2695SDX Arkusz danych(PDF) 8 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Numer części LM2695SDX
Szczegółowy opis  High Voltage (30V, 1.25A) Step Down Switching Regulator
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Producent  NSC [National Semiconductor (TI)]
Strona internetowa  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM2695SDX Arkusz danych(HTML) 8 Page - National Semiconductor (TI)

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Functional Description
The LM2695 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 33V N-Channel buck
switch, is easy to implement, and is available in the thermally
enhanced LLP-10 and TSSOP-14EP packages. The regula-
tor’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 hyster-
etic control requires no loop compensation resulting in very
fast load transient response. The valley current limit detec-
tion 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 3.
The LM2695 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 LM2695 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 250
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 250 ns. Once
regulation is established, the off-times are longer.
When in regulation, the LM2695 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 LM2695 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 3).
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.
20170410
FIGURE 4. Low Ripple Output Configuration
www.national.com
8


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