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LTC1735CGN-1#PBF

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LTC1735-1
APPLICATIO S I FOR ATIO
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MOSFET from turning on and storing charge during the
dead-time, which could cost as much as 1% in efficiency.
A 3A Schottky is generally a good size for 10A to 12A regu-
lators due to the relatively small average current. Larger
diodes result in additional transition losses due to their
larger junction capacitance. The diode may be omitted if the
efficiency loss can be tolerated.
C
IN
Selection
In continuous mode, the source current of the top
N-channel MOSFET is a square wave of duty cycle V
OUT
/
V
IN
. To prevent large voltage transients, a low ESR input
capacitor sized for the maximum RMS current must be
used. The maximum RMS capacitor current is given by:
II
V
V
V
V
RMS O MAX
OUT
IN
IN
OUT
≅
()
/
–1
12
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that capacitor manufacturer’s ripple
current ratings are often based on only 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to
meet size or height requirements in the design. Always
consult the manufacturer if there is any question.
C
OUT
Selection
The selection of C
OUT
is primarily determined by the
effective series resistance (ESR) to minimize voltage ripple.
The output ripple (∆V
OUT
) in continuous mode is deter-
mined by:
∆∆V I ESR
fC
OUT L
OUT
≈+
1
8
where f = operating frequency, C
OUT
= output capacitance,
and ∆I
L
= ripple current in the inductor. The output ripple
is highest at maximum input voltage since ∆I
L
increases
with input voltage. Typically, once the ESR requirement
for C
OUT
has been met, the RMS current rating generally
far exceeds the I
RIPPLE(P-P)
requirement. With ∆I
L
=
0.3I
OUT(MAX)
and allowing for 2/3 of the ripple due to ESR,
the output ripple will be less than 50mV at max V
IN
assuming:
C
OUT
required ESR < 2.2 R
SENSE
C
OUT
> 1/(8fR
SENSE
)
The first condition relates to the ripple current into the ESR
of the output capacitance while the second term guaran-
tees that the output voltage does not significantly dis-
charge during the operating frequency period due to ripple
current. The choice of using smaller output capacitance
increases the ripple voltage due to the discharging term
but can be compensated for by using capacitors of very
low ESR to maintain the ripple voltage at or below 50mV.
The I
TH
pin OPTI-LOOP compensation components can be
optimized to provide stable, high performance transient
response regardless of the output capacitors selected.
The selection of output capacitors for CPU or other appli-
cations with large load current transients is primarily de-
termined by the voltage tolerance specifications of the load.
The resistive component of the capacitor, ESR, multiplied
by the load current change plus any output voltage ripple
must be within the voltage tolerance of the load (CPU).
The required ESR due to a load current step is:
R
ESR
< ∆V/∆I
where ∆I is the change in current from full load to zero load
(or minimum load) and ∆V is the allowed voltage deviation
(not including any droop due to finite capacitance).
The amount of capacitance needed is determined by the
maximum energy stored in the inductor. The capacitance
must be sufficient to absorb the change in inductor current
when a high current to low current transition occurs. The
opposite load current transition is generally determined by
the control loop OPTI-LOOP components, so make sure
not to over compensate and slow down the response. The
minimum capacitance to assure the inductors’ energy is
adequately absorbed is:
C
LI
VV
OUT
OUT
>
∆
()
∆
()
2
2
where ∆I is the change in load current.
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LTC1735-1
APPLICATIO S I FOR ATIO
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Manufacturers such as Nichicon, United Chemicon and
Sanyo can be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
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. Special
polymer surface mount capacitors offer very low ESR but
have much lower capacitive density per unit volume than
other capacitor types. These capacitors offer a very cost-
effective 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 TPSV or the KEMET T510 series of surface mount
tantalums, available in case heights ranging from 2mm to
4mm. 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 Sanyo OS-CON, Nichicon PL
series and Sprague 595D series. Consult manufacturers
for other specific recommendations.
INTV
CC
Regulator
An internal P-channel low dropout regulator produces the
5.2V supply that powers the drivers and internal circuitry
within the LTC1735-1. The INTV
CC
pin can supply a
maximum RMS 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 required to supply the high transient cur-
rents required by the MOSFET gate drivers.
Higher input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC1735-1 to be
exceeded. The system supply current is normally domi-
nated by the gate charge current. Additional loading of
INTV
CC
also needs to be taken into account for the power
dissipation calculations. The total INTV
CC
current can be
supplied by either the 5.2V internal linear regulator or by
the EXTV
CC
input pin. When the voltage applied to the
EXTV
CC
pin is less than 4.7V, all of the INTV
CC
current is
supplied by the internal 5.2V 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 dependant on operating frequency as discussed in the
Efficiency Consideration section. The junction tempera-
ture can be estimated by using the equations given in Note
2 of the Electrical Characteristics. For example, the
LTC1735CS-1 is limited to less than 17mA from a 30V
supply when not using the EXTV
CC
pin as follows:
T
J
= 70°C + (17mA)(30V)(110°C/W) = 126°C
Use of the EXTV
CC
input pin reduces the junction tempera-
ture to:
T
J
= 70°C + (17mA)(5V)(110°C/W) = 79°C
To prevent maximum junction temperature from being
exceeded, the input supply current must be checked
operating in continuous mode at maximum V
IN
.
EXTV
CC
Connection
The LTC1735-1 contains an internal P-channel MOSFET
switch connected between the EXTV
CC
and INTV
CC
pins.
Whenever the EXTV
CC
pin is above 4.7V the internal 5.2V
regulator shuts off, the switch closes and INTV
CC
power is
supplied via EXTV
CC
until EXTV
CC
drops below 4.5V. This
allows the MOSFET gate drive and control power to be
derived from the output or other external source during
normal operation. When the output is out of regulation
(start-up, short circuit) power is supplied from the internal
regulator. Do not apply greater than 7V to the EXTV
CC
pin
and ensure that EXTV
CC
< V
IN
.
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LTC1735-1
APPLICATIO S I FOR ATIO
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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 5V regulators this
simply means connecting the EXTV
CC
pin directly to V
OUT
.
However, for dynamic (VID-like) programmed regulators
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 5.2V 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 5V to 7V output regulator and provides
the highest efficiency. For output voltages > 5V, EXTV
CC
is required to connect to V
OUT
so the SENSE pins
absolute maximum ratings are not exceeded.
3. EXTV
CC
Connected to an External Supply (This Option
is the Most Likely Used). If an external supply is
available in the 5V to 7V range, such as notebook main
5V system power, it may be used to power EXTV
CC
providing it is compatible with the MOSFET gate drive
requirements. This is the typical case as the 5V power
is almost always present and is derived by another high
efficiency regulator.
4. EXTV
CC
Connected to an Output-Derived Boost Net-
work. For low output voltage regulators, efficiency
gains can still be realized by connecting EXTV
CC
to an
output-derived voltage that has been boosted to greater
than 4.7V. This can be done with either the inductive
boost winding or capacitive charge pump circuits.
Refer to the LTC1735 data sheet for details. The charge
pump has the advantage of simple magnetics.
Output Voltage Programming
The output voltage is set by an external resistive divider
according to the following formula:
VV
R
R
OUT
=+
08 1
2
1
.
The resistive divider is connected to the output as shown
in Figure 3 allowing remote voltage sensing.
The output voltage can be digitally set to voltages between
any two levels with the addition of a resistor and small
signal N-channel MOSFET as shown in the circuit of
Figure 1. Dynamic output voltage selection can be accom-
plished with this technique. Output voltages of 1.30V and
1.55V are set by the resistors R1 to R3. With the gate of
the MOSFET low, (V
G
= 0), the output voltage is set by the
ratio of R1 to R2. When the MOSFET is on (V
G
= high), the
output voltage is the ratio of R1 to the parallel combina-
tion of R2 and R3. With the available power good output
(PGOOD), the circuit in Figure 1 creates a low cost Intel
Pentium III
mobile processor compliant supply.
The LTC1735-1 has remote sense capability. The top of the
internal resistive divider is connected to V
OSENSE
and is
referenced to the SGND pin. This allows a kelvin connec-
tion for remotely sensing the output voltage directly across
the load, eliminating any PC board trace resistance errors.
Topside MOSFET Driver Supply (C
B
, D
B
)
An external bootstrap capacitor C
B
connected to the BOOST
pin supplies the gate drive voltage for the topside
MOSFET. Capacitor C
B
in the Functional Diagram is charged
though external diode D
B
from INTV
CC
when the SW pin is
low. Note that the voltage across C
B
is about a diode drop
below INTV
CC
. When the topside MOSFET is to be turned
on, the driver places the C
B
voltage across the gate-source
of the 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
+ INTV
CC
. The value of
the boost capacitor C
B
needs to be 100 times greater than
the total input capacitance of the topside MOSFET. In most
applications 0.1µF to 0.33µF is adequate. The reverse
breakdown on D
B
must be greater than V
IN(MAX) .
Figure 3. Setting the LTC1735-1 Output Voltage
V
OSENSE
V
OUT
R2
1735-1 F03
LTC1735-1
R1
47pF
SGND
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LTC1735CGN-1#PBF

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Description:
Switching Voltage Regulators Hi Eff Sync Buck Sw Reg
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