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LT3995HMSE#PBF

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LT3995
10
3995f
For more information www.linear.com/LT3995
OPERATION
The LT3995 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 LT3995 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 LT3995 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 LT3995’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 LT3995 can provide up to 3A of output current. A cur-
rent limit foldback feature throttles back the current limit
during overload conditions to limit the power dissipation.
When SS is below 2V, the LT3995 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 LT3995 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 LT3995 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 LT3995 is shut down the PG pin is
actively pulled low.
LT3995
11
3995f
For more information www.linear.com/LT3995
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT3995 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 LT3995 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 LT3995 consumes 1.7μA, but when it turns
on all the circuitry to deliver a current pulse, the LT3995
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 LT3995 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 LT3995. The feedback resistors should
preferably be on the order of MΩ and the Schottky catch
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.
Figure 1. Switching Frequency in Burst Mode Operation
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 30mV of output ripple
with one 47µF ceramic output capacitor in a typical ap-
plication. 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 two 47µF output capacitors,
the output ripple is about 15mV, and with four 47µF output
capacitors the output ripple is about 7.5mV. The output
voltage ripple can continue to be decreased by increas-
ing the output capacitance, though care must be taken
to minimize the effects of output capacitor ESR and ESL.
At higher output loads (above 90mA for the front page
application) the LT3995 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 LT3995 will pulse skip at light loads. At very 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
LOAD CURRENT (mA)
0
50
SWITCHING FREQUENCY (kHz)
100
200
300
400
600
20
40 60 80
3995 F01
100 120
500
V
OUT
= 5V
f
SW
= 500kHz
L = 10µH
V
OUT
= 3.3V
f
SW
= 300kHz
L = 8.2µH
V
IN
= 12V
V
SW
20V/DIV
V
OUT
50mV/DIV
I
L
1A/DIV
5µs/DIVV
IN
= 48V
V
OUT
= 3.3V
I
LOAD
= 70mA
C
OUT
= 47µF
3995 F02
LT3995
12
3995f
For more information www.linear.com/LT3995
APPLICATIONS INFORMATION
clock is 11µA. 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 LT3995 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.
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
To estimate the required R
T
value, use the following
equation:
R
T
=
51.1
f
SW
( )
1.09
– 9.27
where f
SW
is the desired switching frequency in MHz and
R
T
is in kΩ.
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 (~0.24V 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 LT3995’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.
A good choice of switching frequency should allow ad-
equate input voltage range (see next two sections) and
keep the inductor and capacitor values small.
Maximum Input Voltage Range
The LT3995 can operate from input voltages of up to 60V.
Often the highest allowed V
IN
during normal operation
(V
IN(OP-MAX)
) is limited by the minimum duty cycle rather
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LT3995HMSE#PBF

Mfr. #:
Buy LT3995HMSE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 60V, 3A, 2MHz Step-Down Switching Regulator with 2.7 A Quiescent Current
Lifecycle:
New from this manufacturer.
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