OMO ElectronicOMO Electronic
Expand menu
Hello, Sign inMy Account
0 Cart

LT3975EMSE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LT3975
10
3975f
OPERATION
The LT3975 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by RT,
sets an RS flip-flop, turning on the internal power switch.
An amplifier and comparator monitor the current flowing
between the V
IN
and SW pins, turning the switch off when
this current reaches a level determined by the voltage at
V
C
(see Block Diagram). An error amplifier measures the
output voltage through an external resistor divider tied
to the FB pin and servos the V
C
node. If the error ampli-
fier’s output increases, more current is delivered to the
output; if it decreases, less current is delivered. An active
clamp on the V
C
pin provides current limit. The V
C
pin is
also clamped by the voltage on the SS pin; soft-start is
implemented by generating a voltage ramp at the SS pin
using an external capacitor.
An internal regulator provides power to the control circuitry.
The bias regulator normally draws power from the V
IN
pin, but if the OUT pin is connected to an external volt-
age higher than 3.2V, bias power will be drawn from the
external source (typically the regulated output voltage).
This improves efficiency.
If the EN pin is low, the LT3975 is shut down and draws
700nA from the input. When the EN pin falls below 1.02V,
the switching regulator will shut down, and when the EN
pin rises above 1.08V, the switching regulator will become
active. This accurate threshold allows programmable
undervoltage lockout.
The switch driver operates from either V
IN
or from the
BOOST pin. An external capacitor is used to generate a
voltage at the BOOST pin that is higher than the input
supply. This allows the driver to fully saturate the internal
bipolar NPN power switch for efficient operation.
To further optimize efficiency, the LT3975 automatically
switches to Burst Mode operation in light load situations.
Between bursts, all circuitry associated with controlling
the output switch is shut down reducing the input supply
current to 1.7μA. In a typical application, 2.7μA will be
consumed from the supply when regulating with no load.
The oscillator reduces the LT3975’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the output current during start-
up and overload.
The LT3975 can provide up to 2.5A of output current.
A current limit foldback feature throttles back the cur-
rent limit during overload conditions to limit the power
dissipation. When SS is below 2V, the LT3975 overrides
the current limit foldback circuit to avoid interfering with
start-up. Thermal shutdown further protects the part from
excessive power dissipation, especially in elevated ambient
temperature environments.
If the input voltage decreases towards the programmed
output voltage, the LT3975 will start to skip switch-off
times and decrease the switching frequency to maintain
output regulation. As the input voltage decreases below
the programmed output voltage, the output voltage will be
regulated 500mV below the input voltage. This enforced
minimum dropout voltage limits the duty cycle and keeps
the boost capacitor charged during dropout conditions.
Since sufficient boost voltage is maintained, the internal
switch can fully saturate yielding low dropout performance.
The LT3975 contains a power good comparator which
trips when the FB pin is at 91.6% of its regulated value.
The PG output is an open-drain transistor that is off when
the output is in regulation, allowing an external resistor
to pull the PG pin high. Power good is valid when V
IN
is
above 2V. When the LT3975 is shut down the PG pin is
actively pulled low.
LT3975
11
3975f
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT3975 operates
in low ripple Burst Mode operation, which keeps the out-
put capacitor charged to the desired output voltage while
minimizing the input quiescent current. In Burst Mode
operation the LT3975 delivers single pulses of current to
the output capacitor followed by sleep periods where the
output power is supplied by the output capacitor. When in
sleep mode the LT3975 consumes 1.7μA, but when it turns
on all the circuitry to deliver a current pulse, the LT3975
consumes several mA of input current in addition to the
switch current. Therefore, the total quiescent current will
be greater than 1.7μA when regulating.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 1) and the percentage
of time the LT3975 is in sleep mode increases, resulting
in much higher light load efficiency. By maximizing the
time between pulses, the converter quiescent current
gets closer to the 1.7μA ideal. Therefore, to optimize the
quiescent current performance at light loads, the current
in the feedback resistor divider and the reverse current
in the catch diode must be minimized, as these appear
to the output as load currents. Use the largest possible
feedback resistors and a low leakage Schottky catch diode
in applications utilizing the ultralow quiescent current
performance of the LT3975. The feedback resistors should
preferably be on the order of MΩ and the Schottky catch
Figure 1. Switching Frequency in Burst Mode Operation
diode should have less than a few µA of typical reverse
leakage at room temperature. These two considerations
are reiterated in the FB Resistor Network and Catch Diode
Selection sections.
It is important to note that another way to decrease the
pulse frequency is to increase the magnitude of each
single current pulse. However, this increases the output
voltage ripple because each cycle delivers more power to
the output capacitor. The magnitude of the current pulses
was selected to ensure less than 15mV of output ripple in
a typical application. See Figure 2.
Figure 2. Burst Mode Operation
While in Burst Mode operation, the burst frequency and
the charge delivered with each pulse will not change with
output capacitance. Therefore, the output voltage ripple will
be inversely proportional to the output capacitance. In a
typical application with a 22µF output capacitor, the output
ripple is about 10mV, and with a 47µF output capacitor
the output ripple is about 5mV. The output voltage ripple
can continue to be decreased by increasing the output
capacitance, though care must be taken to minimize the
effects of output capacitor ESR and ESL.
At higher output loads (above 150mA for the front page
application) the LT3975 will be running at the frequency
programmed by the R
T
resistor, and will be operating in
standard PWM mode. The transition between PWM and
low ripple Burst Mode operation is seamless, and will not
disturb the output voltage.
To ensure proper Burst Mode operation, the SYNC pin
must be grounded. When synchronized with an external
clock, the LT3975 will pulse skip at light loads. At very
LOAD CURRENT (mA)
0
SWITCHING FREQUENCY (kHz)
500
600
700
160
3975 F01
400
300
0
40
80
120
20
60
100
140
200
100
900
800
V
OUT
= 5V
f
SW
= 800kHz
V
OUT
= 3.3V
f
SW
= 600kHz
V
SW
5V/DIV
V
OUT
10mV/DIV
I
L
0.5A/DIV
5µs/DIVV
IN
= 12V
V
OUT
= 3.3V
I
LOAD
= 20mA
C
OUT
= 47µF
3975 F02
LT3975
12
3975f
APPLICATIONS INFORMATION
light loads, the part will go to sleep between groups of
pulses, so the quiescent current of the part will still be low,
but not as low as in Burst Mode operation. The quiescent
current in a typical application when synchronized with an
external clock is 11µA at no load. Holding the SYNC pin
DC high yields no advantages in terms of output ripple or
minimum load to full frequency, so is not recommended.
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
R1=R2
V
OUT
1.197V
–1
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
The total resistance of the FB resistor divider should be
selected to be as large as possible to enhance low current
performance. The resistor divider generates a small load
on the output, which should be minimized to optimize the
low supply current at light loads.
When using large FB resistors, a 10pF phase lead capacitor
should be connected from V
OUT
to FB.
Setting the Switching Frequency
The LT3975 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary R
T
value for a desired switching
frequency is in Table 1.
To estimate the necessary R
T
value for a desired switching
frequency, use the equation:
R
T
=
51.1
f
SW
( )
1.09
– 9.27
where R
T
is in kΩ and f
SW
is in MHz.
Table 1. Switching Frequency vs R
T
Value
SWITCHING FREQUENCY (MHz) R
T
VALUE (kΩ)
0.2 294
0.3 182
0.4 130
0.6 78.7
0.8 54.9
1.0 41.2
1.2 32.4
1.4 26.1
1.6 21.5
1.8 17.8
2.0 14.7
2.2 12.4
Operating Frequency Trade-Offs
Selection of the operating frequency is a trade-off between
efficiency, component size, minimum dropout voltage, and
maximum input voltage. The advantage of high frequency
operation is that smaller inductor and capacitor values
may be used. The disadvantages are lower efficiency, and
lower maximum input voltage. The highest acceptable
switching frequency (f
SW(MAX)
) for a given application
can be calculated as follows:
f
SW(MAX)
=
V
OUT
+ V
D
t
ON(MIN)
V
IN
– V
SW
+ V
D
( )
where V
IN
is the typical input voltage, V
OUT
is the output
voltage, V
D
is the catch diode drop (~0.5V), and V
SW
is
the internal switch drop (~0.22V at max load). This equa-
tion shows that slower switching frequency is necessary
to safely accommodate high V
IN
/V
OUT
ratio. This is due
to the limitation on the LT3975’s minimum on-time. The
minimum on-time is a strong function of temperature.
Use the typical minimum on-time curve to design for an
application’s maximum temperature, while adding about
30% for part-to-part variation. The minimum duty cycle that
can be achieved taking minimum on time into account is:
DC
MIN
= f
SW
• t
ON(MIN)
where f
SW
is the switching frequency, the t
ON(MIN)
is the
minimum switch on-time.
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24

LT3975EMSE#PBF

Mfr. #:
Buy LT3975EMSE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 42V, 2.5A, 2mHz Step-Down Switching Regulator with 3.4uA Quiescent Current
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet