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LTC3729LEUH-6#TRPBF

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16
LTC3729L-6
sn3729l6 3729l6fs
INTV
CC
Regulator
An internal P-channel low dropout regulator produces 5V
at the INTV
CC
pin from the V
IN
supply pin. The INTV
CC
regulator powers the drivers and internal circuitry of the
LTC3729L-6. The INTV
CC
pin regulator can supply up to
50mA peak and must be bypassed to power ground with
a minimum of 4.7µF tantalum or electrolytic capacitor. An
additional 1µF ceramic capacitor placed very close to the
IC is recommended due to the extremely high instanta-
neous currents required by the MOSFET gate drivers.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3729L-6 to
be exceeded. The supply current is dominated by the gate
charge supply current, in addition to the current drawn
from the differential amplifier output. The gate charge is
dependent on operating frequency as discussed in the
Efficiency Considerations section. The supply current can
either be supplied by the internal 5V regulator or via the
EXTV
CC
pin. When the voltage applied to the EXTV
CC
pin
is less than 4.7V, all of the INTV
CC
load current is supplied
by the internal 5V linear regulator. Power dissipation for
the IC is higher in this case by (I
IN
)(V
IN
– INTV
CC
) and
efficiency is lowered. The junction temperature can be
estimated by using the equations given in Note 1 of the
Electrical Characteristics. For example, the LTC3729L-6
V
IN
current is thermally limited to less than 54mA from a
30V supply:
T
J
= 70°C + (54mA)(30V)(34°C/W) = 125°C
Use of the EXTV
CC
pin reduces the junction temperature
to:
T
J
= 70°C + (54mA)(5V)(34°C/W) = 79.2°C
The input supply current should be measured while the
controller is operating in continuous mode at maximum
V
IN
and the power dissipation calculated in order to pre-
vent the maximum junction temperature from being ex-
ceeded.
EXTV
CC
Connection
The LTC3729L-6 contains an internal P-channel MOSFET
switch connected between the EXTV
CC
and INTV
CC
pins.
When the voltage applied to EXTV
CC
rises above
4.7V, the
internal regulator is turned off and the switch closes,
connecting the EXTV
CC
pin to the INTV
CC
pin thereby
supplying internal and MOSFET gate driving power. The
switch remains closed as long as the voltage applied to
EXTV
CC
remains above 4.5V. This allows the MOSFET
driver and control power to be derived from the output
during normal operation (4.7V < V
EXTVCC
< 7V) and from
the internal regulator when the output is out of regulation
(start-up, short-circuit). Do not apply greater than 7V to
the EXTV
CC
pin and ensure that EXTV
CC
< V
IN
+ 0.3V when
using the application circuits shown.
If an external voltage
source is applied to the EXTV
CC
pin when the V
IN
supply is
not present, a diode can be placed in series with the
LTC3729L-6’s V
IN
pin and a Schottky diode between the
EXTV
CC
and the V
IN
pin, to prevent current from backfeeding
V
IN
.
Significant efficiency gains can be realized by powering
INTV
CC
from the output, since the V
IN
current resulting
from the driver and control currents will be scaled by the
ratio: (Duty Factor)/(Efficiency). For 5V regulators this
means connecting the EXTV
CC
pin directly to V
OUT
. How-
ever, for 3.3V and other lower voltage regulators, addi-
tional circuitry is required to derive INTV
CC
power from the
output.
The following list summarizes the four possible connec-
tions for EXTV
CC:
1. EXTV
CC
left open (or grounded). This will cause INTV
CC
to be powered from the internal 5V regulator resulting in
a significant efficiency penalty at high input voltages.
2. EXTV
CC
connected directly to V
OUT
. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
3. EXTV
CC
connected to an external supply. If an external
supply is available in the 5V to 7V range, it may be used to
power EXTV
CC
providing it is compatible with the MOSFET
gate drive requirements. V
IN
must be greater than or equal
to the voltage applied to the EXTV
CC
pin.
4. EXTV
CC
connected to an output-derived boost network.
For 3.3V and other low voltage regulators, efficiency gains
can still be realized by connecting EXTV
CC
to an output-
derived voltage which has been boosted to greater than
4.7V but less than 7V. This can be done with either the
APPLICATIO S I FOR ATIO
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17
LTC3729L-6
sn3729l6 3729l6fs
inductive boost winding as shown in Figure 5a or the
capacitive charge pump shown in Figure 5b. The charge
pump has the advantage of simple magnetics.
Topside MOSFET Driver Supply (C
B
,D
B
) (Refer to
Functional Diagram)
External bootstrap capacitors C
B1
and C
B2
connected to
the BOOST1 and BOOST2 pins supply the gate drive
voltages for the topside MOSFETs. Capacitor C
B
in the
Functional Diagram is charged though diode D
B
from
INTV
CC
when the SW pin is low. When the topside MOSFET
turns on, the driver places the C
B
voltage across the gate-
source of the desired MOSFET. This enhances the MOSFET
and turns on the topside switch. The switch node voltage,
SW, rises to V
IN
and the BOOST pin rises to V
IN
+ V
INTVCC
.
The value of the boost capacitor C
B
needs to be 30 to 100
times that of the total input capacitance of the topside
MOSFET(s). The reverse breakdown of D
B
must be greater
than V
IN(MAX).
The final arbiter when defining the best gate drive ampli-
tude level will be the input supply current. If a change is
made that decreases input current, the efficiency has
improved. If the input current does not change then the
efficiency has not changed either.
Output Voltage
The LTC3729L-6 has a true remote voltage sense capablity.
The sensing connections should be returned from the load
back to the differential amplifier’s inputs through a com-
mon, tightly coupled pair of PC traces. The differential
amplifier rejects common mode signals capacitively or
inductively radiated into the feedback PC traces as well
as ground loop disturbances. The differential amplifier
output signal is divided down and compared with the
internal precision 0.6V voltage reference by the error
amplifier.
The output is an NPN emitter follower without any internal
pull-down current. A DC resistive load to ground is re-
quired in order to sink current. The output will swing from
0V to 10V. (V
IN
≥ V
DIFFOUT
␣ +␣ 2V.)
Soft-Start/Run Function
The RUN/SS pin provides three functions: 1) Run/Shut-
down, 2) soft-start and 3) a defeatable short-circuit latchoff
timer. Soft-start reduces the input power sources’ surge
currents by gradually increasing the controller’s current
limit I
TH(MAX)
. The latchoff timer prevents very short,
extreme load transients from tripping the overcurrent
latch. A small pull-up current (>5µA) supplied to the RUN/
SS pin will prevent the overcurrent latch from operating.
The following explanation describes how the functions
operate.
An internal 1.2µA current source charges up the C
SS
capacitor
.
When the voltage on RUN/SS reaches 1.5V, the
controller is permitted to start operating. As the voltage on
RUN/SS increases from 1.5V to 3.0V, the internal current
limit is increased from 25mV/R
SENSE
to 75mV/R
SENSE
.
The output current limit ramps up slowly, taking an
additional 1.4µs/µF to reach full current. The output
current thus ramps up slowly, reducing the starting surge
APPLICATIO S I FOR ATIO
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Figure 5a. Secondary Output Loop and EXTV
CC
Connection
Figure 5b. Capacitive Charge Pump for EXTV
CC
3729L-6 F05a
V
IN
TG1
N-CH
1N4148
N-CH
BG1
PGND
LTC3729L-6
SW1
EXTV
CC
OPTIONAL EXTV
CC
CONNECTION
5V < V
SEC
< 7V
T1
R
SENSE
V
SEC
6.8V
V
OUT
V
IN
+
C
IN
+
1µF
+
C
OUT
3729L-6 F05b
V
IN
TG1
N-CH
N-CH
BG1
PGND
LTC3729L-6
SW1
EXTV
CC
L1
BAT85
BAT85
BAT85 0.22µF
V
OUT
V
IN
+
C
IN
+
+
C
OUT
VN2222LL
R
SENSE
18
LTC3729L-6
sn3729l6 3729l6fs
current required from the input power supply. If RUN/SS
has been pulled all the way to ground there is a delay before
starting of approximately:
t
V
A
CsFC
DELAY SS SS
=
µ
=µ
()
15
12
125
.
.
./
The time for the output current to ramp up is then:
t
VV
A
CsFC
RAMP SS SS
=
−
µ
=µ
()
315
12
125
.
.
./
By pulling the RUN/SS pin below 0.8V the LTC3729L-6 is
put into low current shutdown (I
Q
< 40µA). RUN/SS can be
driven directly from logic as shown in Figure 6. Diode D1
in Figure 6 reduces the start delay but allows C
SS
to ramp
up slowly providing the soft-start function. The RUN/SS
pin has an internal 6V zener clamp (see Functional Dia-
gram).
Fault Conditions: Overcurrent Latchoff
The RUN/SS pin also provides the ability to latch off the
controllers when an overcurrent condition is detected. The
RUN/SS capacitor, C
SS
, is used initially to limit the inrush
current of both controllers. After the controllers have been
started and been given adequate time to charge up the
output capacitors and provide full load current, the RUN/
SS capacitor is used for a short-circuit timer. If the output
voltage falls to less than 70% of its nominal value after C
SS
reaches 4.1V, C
SS
begins discharging on the assumption
that the output is in an overcurrent condition. If the
condition lasts for a long enough period as determined by
the size of C
SS
, the controller will be shut down until the
RUN/SS pin voltage is recycled. If the overload occurs
during start-up, the time can be approximated by:
t
LO1
≈ (C
SS
• 0.6V)/(1.2µA) = 5 • 10
5
(C
SS
)
If the overload occurs after start-up, the voltage on C
SS
will
continue charging and will provide additional time before
latching off:
t
LO2
≈ (C
SS
• 3V)/(1.2µA) = 2.5 • 10
6
(C
SS
)
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor, R
SS
, to the RUN/SS pin as
shown in Figure 6. This resistance shortens the soft-start
period and prevents the discharge of the RUN/SS capaci-
tor during a severe overcurrent and/or short-circuit con-
dition. When deriving the 5µA current from V
IN
as in the
figure, current latchoff is always defeated. Diode-
connecting this pull-up resistor to INTV
CC
, as in
Figure␣ 6, eliminates any extra supply current during shut-
down while eliminating the INTV
CC
loading from prevent-
ing controller start-up.
Why should you defeat current latchoff? During the
prototyping stage of a design, there may be a problem with
noise pickup or poor layout causing the protection circuit
to latch off the controller. Defeating this feature allows
troubleshooting of the circuit and PC layout. The internal
short-circuit and foldback current limiting still remains
active, thereby protecting the power supply system from
failure. A decision can be made after the design is com-
plete whether to rely solely on foldback current limiting or
to enable the latchoff feature by removing the pull-up
resistor.
The value of the soft-start capacitor C
SS
may need to be
scaled with output voltage, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
C
SS
> (C
OUT
)(V
OUT
)(10
-4
)(R
SENSE
)
The minimum recommended soft-start capacitor of C
SS
=
0.1µF will be sufficient for most applications.
APPLICATIO S I FOR ATIO
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Figure 6. RUN/SS Pin Interfacing
3.3V OR 5V RUN/SS
V
IN
INTV
CC
RUN/SS
D1
D1*
C
SS
R
SS
*
C
SS
R
SS
*
3729L-6 F06
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
Phase-Locked Loop and Frequency Synchronization
The LTC3729L-6 has a phase-locked loop comprised of an
internal voltage controlled oscillator and phase detector.
This allows the top MOSFET turn-on to be locked to the
rising edge of an external source. The frequency range of
the voltage controlled oscillator is ±50% around the
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LTC3729LEUH-6#TRPBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators PolyPhase Controller QFN Package with 0.6V ref
Lifecycle:
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