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LT3999EMSE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LT3999
7
3999fa
For more information www.linear.com/LT3999
OPERATION
Overview
The LT3999 is a monolithic isolated push-pull DC trans-
former driver. It includes functions such as duty cycle
control, soft-start and protection features.
Push-Pull Topology
In a push-pull topology, a pair of switches operating out
of phase generate a square wave voltage pulse on the
primary side of a center tapped transformer. The diodes
on the secondary side rectify the voltage
and generate
the output voltage. This voltage is simply V
IN
times the
transformer turns ratio.
Duty Cycle Control
The LT3999 duty cycle control provides, to a degree, line
regulation. The duty cycle is programmed by a resistor
on the RDC pin and the OVLO/DC voltage. By making the
OVLO/DC voltage a function of V
IN
the duty cycle will
adjust with varying V
IN
thereby keeping V
OUT
constant.
This feature is useful in cases where an LDO is used to post
regulate the output of the LT3999. By pseudo regulating
the output with the duty cycle control the power dissipa-
tion in the LDO is minimized.
Leaving the RDC pin floating or connecting it to the OVLO/
DC pin disables the duty cycle function and the LT3999
operates at close to 50%
duty cycle.
Current Limit and Soft-Start
The LT3999 ILIM/SS pin programs the cycle-by-cycle
switch current limit and the soft-start time. A resistor on
the ILIM/SS pin sets the current limit. A capacitor on the
pin in conjunction with the resistor sets the soft-start time.
When the programmed current limit is reached the switch
is immediately turned off and remains off
for the remainder
of the cycle. Leaving the ILIM/SS pin unconnected will
disable the programmable current limit and the LT3999
will default to its internal current limit.
The soft-start function ramps the maximum switch current
over the programmed soft-start time. The purpose of the
soft-start is to reduce inrush current from the input supply.
Other Features
The LT3999 protection features include overvoltage
lock-
out (OVLO), undervoltage lockout (UVLO) and thermal
shutdown.
The OVLO function is programmed with the OVLO/DC pin.
Switching is disabled during an OVLO event. An internal
overvoltage lockout on the V
IN
pin is also provided to
protect the LT3999.
The UVLO function is programmed with the UVLO pin.
Switching is disabled during a UVLO event. The UVLO
pin is also used to put the
LT3999 into a low quiescent
shutdown state.
At a junction temperature above the operating tempera-
ture range the thermal shutdown function turns off both
switches.
LT3999
8
3999fa
For more information www.linear.com/LT3999
APPLICATIONS INFORMATION
Switching Frequency
The LT3999 drives two output power switches out of
phase, thus the oscillator frequency is two times the actual
switching frequency of each power switch. The choice
of switching frequency is a trade-off between power ef-
ficiency and the size of capacitive and inductive storage
components.
Operating at low switching frequency reduces the switch-
ing losses (transient losses) and consequently improves the
power converter efficiency. However, the lower switching
frequency requires greater inductance for a given amount
of ripple current, resulting in a larger design footprint and
higher cost.
The LT3999 switching frequency is set in the range of 50kHz
to 1MHz. The value of R
T
for a given operating frequency
is chosen from Table 1 or from the following equation:
Table 1. Recommended 1% Standard Values
R
T
f
SW
316kΩ 50kHz
158kΩ 100kHz
76.8kΩ 200kHz
49.9kΩ 300kHz
36.5kΩ 400kHz
28kΩ 500kHz
22.6kΩ 600kHz
19.1kΩ 700kHz
16.2kΩ 800kHz
14kΩ 900kHz
12.1kΩ 1000kHz
R
T
kΩ
( )
=
1
2• f
SW
–70ns
•3.25•10
10
Oscillator Sync
In applications where a more precise frequency is desired
to accurately place high frequency harmonics, the LT3999
oscillator can be synchronized to an external clock. Set
the internal oscillator frequency 10% to 50% lower than
the external sync frequency. The switching frequency is
one-half the sync frequency.
Drive the SYNC pin with a 2V or greater square wave.
The rising edge of the sync square wave
will initiate clock
discharge. If unused, connect the SYNC pin to ground.
Duty Cycle
To run the LT3999 at full duty cycle leave the RDC pin
unconnected.
Variations in V
IN
are, to a first order, compensated with
the LT3999 duty cycle control function. The duty cycle
function is implemented with a resistor divider on V
IN
connected to the OVLO/DC pin and a resistor to
ground
on the RDC pin. Use the following formula to calculate
the RDC resistor or duty cycle:
Duty Cycle DC
( )
=
1.25•RDC
V
IN
•
R
B
R
A
+R
B
•R
T
•4
RDC=
V
IN
•
R
B
R
A
+R
B
•R
T
•DC•4
1.25
where R
A
and R
B
are the resistors from the V
IN
to OVLO/
DC resistor divider and R
T
is the frequency setting resis-
tor. See Figure 1. Setting the OVLO/DC pin to be 0.612V
at the nominal V
IN
voltage yields good line regulation over
a wide input range.
The duty cycle refers to the duty cycle of the individual
switch. Normally each switch operates at close
to 50%
duty cycle.
LT3999
9
3999fa
For more information www.linear.com/LT3999
APPLICATIONS INFORMATION
Soft-Start and Current Limit
The LT3999 soft-start ramps the peak switch current over
a time programmed by either a capacitor or a resistor and
capacitor on the ILIM/SS pin.
When programming the soft-start time with a capacitor only
the soft-start time is calculated with the following formula:
t
SS
(ms) = C
SS
• 80
where C
SS
is in µF.
The current limit defaults to the internally set value because
there is no resistor on the pin.
When programming the soft-start time with a resistor
and capacitor on the ILIM/SS pin the soft-start time is
calculated with the following formula:
τ = RC
where 3τ will be 95% of the maximum current.
The cycle-by-cycle current limit of the LT3999 is set with
a resistor on the ILIM/SS pin. Use the following formula
to calculate the value of the resistor:
R
ILIM
(kΩ) = I
LIM
• 86.4
OVLO/DC and UVLO
The UVLO pin has a precision voltage threshold with
hysteresis to enable the LT3999. The pin is typically con-
nected to V
IN
through a resistor divider; however, it can
be directly connected to V
IN
.
The OVLO/DC pin has a precision voltage threshold with
hysteresis to disable the LT3999 switching operation. The
pin is typically connected to V
IN
through a resistor divider.
The OVLO/DC pin can be directly connected to GND to
disable the function. It is possible to use two separate
resistor divider strings for OVLO/DC and UVLO pins or
combine them together and use one resistor divider string
to drive both pins. See Figure 1.
Resistors are chosen by first selecting R
B
. Then calculate
R
A
with the following formula:
R
A
=R
B
V
TH
1.25V
–1
where V
TH
is the V
IN
referred voltage at which the supply
is enabled (UVLO) or disabled (OVLO/DC).
Transformer Design
Table 3 lists recommended center tapped transformers
for a variety of input voltage, output voltage and power
combinations. These transformers will yield slightly high
output voltages so that they can accommodate an LDO
regulator on the output.
If your application is not listed, the LT C Applications group
is
available to assist in the choice and/or the design of the
transformer. In the design/selection of the transformer
the following characteristics are critical and should be
considered:
Table 3. Recommended Center Tapped Transformers
NOMINAL
INPUT
VOLTAGE (V)
NOMINAL
OUTPUT
VOLTAGE (V)
OUTPUT
POWER (W) PART NUMBER
5 5 5 Coilcraft PA6383
5 12 1 Coilcraft PA6381
5 12 3 Cooper Bussmann
CTX02-19064
12 12 10 Coilcraft PA6384
24 24 20 Cooper Bussmann
CTX02-19061
V
IN
UVLO
R
A2
R
A1
R
A
R
B
R
B
3999 F01
OVLO/DC
V
IN
UVLO
OR
OVLO/DC
Figure 1. Precision UVLO and OVLO Resistor Divider
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LT3999EMSE#PBF

Mfr. #:
Buy LT3999EMSE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control
Lifecycle:
New from this manufacturer.
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