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LTC3727LXEUH-1#PBF

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16
LTC3727LX-1
3727lx1fa
capacitor available from Sanyo has the lowest (ESR)(size)
product of any aluminum electrolytic at a somewhat
higher price. An additional ceramic capacitor in parallel
with OS-CON capacitors is recommended to reduce the
inductance effects.
In surface mount applications multiple capacitors may
need to be used in parallel to meet the ESR, RMS current
handling and load step requirements of the application.
Aluminum electrolytic, dry tantalum and special polymer
capacitors are available in surface mount packages. Spe-
cial polymer surface mount capacitors offer very low ESR
but have lower storage capacity per unit volume than other
capacitor types. These capacitors offer a very cost-effec-
tive output capacitor solution and are an ideal choice when
combined with a controller having high loop bandwidth.
Tantalum capacitors offer the highest capacitance density
and are often used as output capacitors for switching
regulators having controlled soft-start. Several excellent
surge-tested choices are the AVX TPS, AVX TPS Series III
or the KEMET T510 series of surface mount tantalums,
available in case heights ranging from 1.2mm to 4.1mm.
Aluminum electrolytic capacitors can be used in cost-
driven applications providing that consideration is given
to ripple current ratings, temperature and long term
reliability. A typical application will require several to
many aluminum electrolytic capacitors in parallel. A
combination of the above mentioned capacitors will
often result in maximizing performance and minimizing
overall cost. Other capacitor types include Nichicon PL
series, NEC Neocap, Cornell Dubilier ESRE and Sprague
595D series. Consult manufacturers for other specific
recommendations.
INTV
CC
Regulator
An internal P-channel low dropout regulator produces
7.5V at the INTV
CC
pin from the V
IN
supply pin. INTV
CC
powers the drivers and internal circuitry within the
LTC3727LX-1. The INTV
CC
pin regulator can supply a
peak current of 50mA and must be bypassed to ground
with a minimum of 4.7µF tantalum, 10µF special polymer,
or low ESR type electrolytic capacitor. A 1µF ceramic
capacitor placed directly adjacent to the INTV
CC
and
PGND IC pins is highly recommended. Good bypassing is
necessary to supply the high transient currents required
by the MOSFET gate drivers and to prevent interaction
between channels.
Higher input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3727LX-1 to
be exceeded. The system supply current is normally
dominated by the gate charge current. Additional external
loading of the INTV
CC
and 3.3V linear regulators also
needs to be taken into account for the power dissipation
calculations. The total INTV
CC
current can be supplied by
either the 7.5V internal linear regulator or by the EXTV
CC
input pin. When the voltage applied to the EXTV
CC
pin is
less than 7.3V, all of the INTV
CC
current is supplied by the
internal 7.5V linear regulator. Power dissipation for the IC
in this case is highest: (V
IN
)(I
INTVCC
), and overall efficiency
is lowered. The gate charge current is dependent on
operating frequency as discussed in the Efficiency Consid-
erations section. The junction temperature can be esti-
mated by using the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC3727LX-1
V
IN
current is limited to less than 24mA from a 24V supply
when not using the EXTV
CC
pin as follows:
T
J
= 70°C + (24mA)(24V)(95°C/W) = 125°C
Use of the EXTV
CC
input pin reduces the junction tempera-
ture to:
T
J
= 70°C + (24mA)(7.5V)(95°C/W) = 87°C
Dissipation should be calculated to also include any added
current drawn from the internal 3.3V linear regulator. To
prevent maximum junction temperature from being ex-
ceeded, the input supply current must be checked operat-
ing in continuous mode at maximum V
IN
.
EXTV
CC
Connection
The LTC3727LX-1 contains an internal P-channel MOS-
FET switch connected between the EXTV
CC
and INTV
CC
pins. When the voltage applied to EXTV
CC
rises above
7.3V, the internal regulator is turned off and the switch
closes, connecting the EXTV
CC
pin to the INTV
CC
pin
thereby supplying internal power. The switch remains
closed as long as the voltage applied to EXTV
CC
remains
above 7.0V. This allows the MOSFET driver and control
power to be derived from the output during normal opera-
APPLICATIO S I FOR ATIO
WUUU
17
LTC3727LX-1
3727lx1fa
tion (7.2V < V
OUT
< 8.5V) and from the internal regulator
when the output is out of regulation (start-up, short-
circuit). If more current is required through the EXTV
CC
switch than is specified, an external Schottky diode can be
added between the EXTV
CC
and INTV
CC
pins. Do not apply
greater than 8.5V to the EXTV
CC
pin and ensure that
EXTV
CC
<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 a
factor of (Duty Cycle)/(Efficiency). For 7.5V regulators this
supply means connecting the EXTV
CC
pin directly to V
OUT
.
However, for 3.3V and other lower voltage regulators,
additional 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 7.5V regulator
resulting in an efficiency penalty of up to 10% at high
input voltages.
2. EXTV
CC
Connected directly to V
OUT
. This is the normal
connection for a 7.5V regulator and provides the high-
est efficiency.
3. EXTV
CC
Connected to an External supply. If an external
supply is available in the 7.5V to 8.5V range, it may be
used to power EXTV
CC
providing it is compatible with
the MOSFET gate drive requirements.
4. EXTV
CC
Connected to an Output-Derived Boost Net-
work. For 3.3V and other low voltage regulators, effi-
ciency gains can still be realized by connecting EXTV
CC
to an output-derived voltage that has been boosted to
greater than 7.5V. This can be done with the inductive
boost winding as shown in Figure 6.
Topside MOSFET Driver Supply (C
B
, D
B
)
External bootstrap capacitors C
B
connected to the BOOST
pins supply the gate drive voltages for the topside MOS-
FETs. Capacitor C
B
in the functional diagram is charged
though external diode D
B
from INTV
CC
when the SW pin is
low. When one of the topside MOSFETs is to be turned 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 follows. With the topside
MOSFET on, the boost voltage is above the input supply:
V
BOOST
= V
IN
+ V
INTVCC
. The value of the boost capacitor
C
B
needs to be 100 times that of the total input capacitance
of the topside MOSFET(s). The reverse breakdown of the
external Schottky diode must be greater than V
IN(MAX)
.
When adjusting the gate drive level, the final arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the efficiency has
improved. If there is no change in input current, then there
is no change in efficiency.
Output Voltage
The LTC3727LX-1 output voltages are each set by an
external feedback resistive divider carefully placed across
the output capacitor. The resultant feedback signal is
compared with the internal precision 0.800V voltage
reference by the error amplifier. The output voltage is
given by the equation:
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
08 1
2
1
.
where R1 and R2 are defined in Figure 2.
APPLICATIO S I FOR ATIO
WUUU
Figure 6. Secondary Output Loop & EXTV
CC
Connection
EXTV
CC
FCB
SGND
V
IN
TG1
SW
BG1
PGND
LTC3727LX-1
R
SENSE
V
OUT
V
SEC
+
C
OUT
+
1µF
3727LX1 F06
N-CH
N-CH
R6
+
C
IN
V
IN
T1
1:N
OPTIONAL EXTV
CC
CONNECTION
7.5V < V
SEC
< 8.5V
R5
18
LTC3727LX-1
3727lx1fa
Figure 7. RUN/SS Pin Interfacing
3.3V OR 5V RUN/SS
RUN/SS
D1
C
SS
C
SS
3727LX1 F07
(7a) (7b)
SENSE
+
/SENSE
–
Pins
The common mode input range of the current comparator
sense pins is from 0V to 14V. Continuous linear operation
is guaranteed throughout this range allowing output volt-
age setting from 0.8V to 14V. A differential NPN input
stage is biased with internal resistors from an internal
2.4V source as shown in the Functional Diagram. This
requires that current either be sourced or sunk from the
SENSE pins depending on the output voltage. If the output
voltage is below 2.4V current will flow out of both SENSE
pins to the main output. The output can be easily preloaded
by the V
OUT
resistive divider to compensate for the current
comparator’s negative input bias current. The maximum
current flowing out of each pair of SENSE pins is:
I
SENSE
+
+ I
SENSE
–
= (2.4V – V
OUT
)/24k
Since V
OSENSE
is servoed to the 0.8V reference voltage, we
can choose R1 in Figure 2 to have a maximum value to
absorb this current.
Rk
V
VV
MAX
OUT
124
08
24
()
.
.–
=
⎛
⎝
⎜
⎞
⎠
⎟
for V
OUT
< 2.4V
Regulating an output voltage of 1.8V, the maximum value
of R1 should be 32K. Note that for an output voltage above
2.4V, R1 has no maximum value necessary to absorb the
sense currents; however, R1 is still bounded by the
V
OSENSE
feedback current.
Soft-Start/Run Function
The RUN/SS1 and RUN/SS2 pins are multipurpose pins
that provide a soft-start function and a means to shut
down the LTC3727LX-1. Soft-start reduces the input
power source’s surge currents by gradually increasing the
controller’s current limit (proportional to V
ITH
). This pin
can also be used for power supply sequencing.
An internal 1.2µA current source charges up the C
SS
capacitor
.
When the voltage on RUN/SS1 (RUN/SS2)
reaches 1.5V, the particular 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 45mV/
R
SENSE
to 135mV/R
SENSE
. The output current limit ramps
up slowly, taking an additional 1.25s/µF to reach full
current. The output current thus ramps up slowly, reduc-
ing the starting surge 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
.
.
./
t
VV
A
CsFC
IRAMP SS SS
=
−
µ
=µ
()
315
12
125
.
.
./
By pulling both RUN/SS pins below 1V, the LTC3727LX-1
is put into low current shutdown (I
Q
= 20µA). The RUN/SS
pins can be driven directly from logic as shown in Fig-
ure 7. Diode D1 in Figure 7 reduces the start delay but
allows C
SS
to ramp up slowly providing the soft-start
function. Each RUN/SS pin has an internal 6V zener clamp
(See Functional Diagram).
APPLICATIO S I FOR ATIO
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Fault Conditions: Current Limit and Current Foldback
The LTC3727LX-1 current comparator has a maximum
sense voltage of 135mV resulting in a maximum MOSFET
current of 135mV/R
SENSE
. The maximum value of current
limit generally occurs with the largest V
IN
at the highest
ambient temperature, conditions that cause the highest
power dissipation in the top MOSFET.
The LTC3727LX-1 includes current foldback to help fur-
ther limit load current when the output is shorted to
ground. The foldback circuit is active even when the
overload shutdown latch described above is overridden. If
the output falls below 70% of its nominal output level, then
the maximum sense voltage is progressively lowered from
135mV to 45mV. Under short-circuit conditions with very
low duty cycles, the LTC3727LX-1 will begin cycle skip-
ping in order to limit the short-circuit current. In this
situation the bottom MOSFET will be dissipating most of
the power but less than in normal operation. The short-
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LTC3727LXEUH-1#PBF

Mfr. #:
Buy LTC3727LXEUH-1#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Dual, 2-Phase Step-Down Controller in QFN
Lifecycle:
New from this manufacturer.
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