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LTC3851EGN#PBF

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LTC3851
16
3851fb
APPLICATIONS INFORMATION
mode, the TK/SS voltage is substantially higher than
0.8V at steady-state and effectively turns off D1. D2 and
D3 will therefore conduct the same current and offer
tight matching between V
FB
and the internal precision
0.8V reference. In the ratiometric mode, however, TK/SS
equals 0.8V at steady-state. D1 will divert part of the bias
current to make V
FB
slightly lower than 0.8V.
Although this error is minimized by the exponential I-V
characteristic of the diode, it does impose a fi nite amount
of output voltage deviation. Furthermore, when the master
supply’s output experiences dynamic excursion (under
load transient, for example), the slave channel output will
be affected as well. For better output regulation, use the
coincident tracking mode instead of ratiometric.
INTV
CC
Regulator
The LTC3851 features a PMOS low dropout linear regula tor
(LDO) that supplies power to INTV
CC
from the V
IN
supply.
INTV
CC
powers the gate drivers and much of the LTC3851 ’s
internal circuitry. The LDO regulates the voltage at the
INTV
CC
pin to 5V.
The LDO can supply a peak current of 50mA and must
be bypassed to ground with a minimum of 2.2F ceramic
capacitor or low ESR electrolytic capacitor. No matter
what type of bulk capaci tor is used, an additional 0.1F
ceramic capacitor placed directly adjacent to the INTV
CC
and GND pins is highly recommended. Good bypassing
is needed to supply the high transient 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 LTC3851 to be
exceeded. The INTV
CC
current, which is dominated by the
gate charge current, is supplied by the 5V LDO.
Power dissipation for the IC in this case is highest and
is approximately equal to V
IN
• I
INTVCC
. The gate charge
current is dependent on operating frequency as discussed
in the Effi ciency Considerations section. The junction
temperature can be estimated by using the equa tions given
in Note 3 of the Electrical Characteristics. For example,
the LTC3851 INTV
CC
current is limited to less than 14mA
from a 36V supply in the GN package:
T
J
= 70°C + (14mA)(36V)(110°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (MODE/PLLIN
= INTV
CC
) at maximum V
IN
.
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. 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
R3
V
OUT
R4
(4a) Coincident Tracking Setup
TO
V
FB
PIN
R3
V
MASTER
R4
TO
TK/SS
PIN
R1 R3
V
OUT
R4R2
3851 F04
(4b) Ratiometric Tracking Setup
TO
V
FB
PIN
TO
TK/SS
PIN
V
MASTER
–
+
II
D1
TK/SS
0.8V
V
FB
D2
D3
3851 F05
EA
Figure 4. Setup for Coincident and Ratiometric Tracking
Figure 5. Equivalent Input Circuit of Error Amplifi er
LTC3851
17
3851fb
APPLICATIONS INFORMATION
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 capa citance of the topside MOSFET.
The reverse break down of the external Schottky diode
must be greater than V
IN(MAX)
.
Undervoltage Lockout
The LTC3851 has two functions that help protect the
controller in case of undervoltage conditions. A precision
UVLO comparator constantly monitors the INTV
CC
voltage
to ensure that an adequate gate-drive voltage is present.
It locks out the switching action when INTV
CC
is below
3.2V. To prevent oscillation when there is a disturbance
on the INTV
CC
, the UVLO comparator has 400mV of preci-
sion hysteresis.
Another way to detect an undervoltage condition is to
monitor the V
IN
supply. Because the RUN pin has a precision
turn-on reference of 1.25V, one can use a resistor divider
to V
IN
to turn on the IC when V
IN
is high enough.
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:
I
RMS
≅I
O(MAX)
V
OUT
V
IN
V
IN
V
OUT
–1
⎛
⎝
⎜
⎞
⎠
⎟
1/ 2
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
O(MAX)
/2. This simple worst-case condition is
com monly used for design because even signifi cant
deviations do not offer much relief. Note that capacitor
manufacturers’ 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
determined by:
∆V
OUT
≈ ∆I
L
ESR+
1
8fC
OUT
⎛
⎝
⎜
⎞
⎠
⎟
where f = operating frequency, C
OUT
= output capaci-
tance 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 2/3 of the ripple to be
due to ESR, the output ripple will be less than 50mV at
maximum V
IN
if the I
LIM
pin is confi gured to fl oat and:
C
OUT
Required ESR < 2.2R
SENSE
C
OUT
>
1
8fR
SENSE
The fi rst condition relates to the ripple current into the ESR
of the output capacitance while the second term guaran tees
that the output capacitance does not signifi cantly discharge
during the operating frequency period due to ripple current.
The choice of using smaller output capaci tance 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 compo nents can be optimized
to provide stable, high perfor mance transient response
regardless of the output capaci tors selected.
The selection of output capacitors for applications with
large load current transients is primarily determined by the
voltage tolerance specifi cations 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.
LTC3851
18
3851fb
APPLICATIONS INFORMATION
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 fi nite capacitance).
The amount of capacitance needed is determined by the
maximum energy stored in the inductor. The capacitance
must be suffi cient 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
OUT
>
L ∆I
()
2
2 ∆V
()
V
OUT
where
∆
I is the change in load current.
Manufacturers such as Nichicon, United Chemi-Con and
Sanyo can be considered for high performance through-
hole capacitors. The OS-CON semiconductor electrolyte
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, ESR, RMS current han dling
and load step specifi cations may require multiple capacitors
in parallel. Aluminum electrolytic, dry tantalum and
special polymer capacitors are available in surface mount
packages. Special polymer surface mount capaci tors 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 capaci tors 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 rang ing from 1.5mm
to 4.1mm. Aluminum electrolytic capaci tors can be used
in cost-driven applications, provided that consideration
is given to ripple current ratings, tempera ture and long-
term reliability. A typical application will require several
to many aluminum electrolytic capacitors in parallel. A
combination of the above mentioned capaci tors will often
result in maximizing performance and minimizing overall
cost. Other capacitor types include Nichicon PL series, NEC
Neocap, Panasonic SP and Sprague 595D series. Consult
manufacturers for other specifi c recommendations.
Like all components, capacitors are not ideal. Each
ca pacitor has its own benefi ts and limitations. Combina-
tions of different capacitor types have proven to be a very
cost effective solution. Remember also to include high
frequency decoupling capacitors. They should be placed
as close as possible to the power pins of the load. Any
inductance present in the circuit board traces negates
their usefulness.
Setting Output Voltage
The LTC3851 output voltage is set by an external feedback
resistive divider carefully placed across the output,
as shown in Figure 6. The regulated output volt age is
determined by:
V
OUT
= 0.8V 1+
R
B
R
A
⎛
⎝
⎜
⎞
⎠
⎟
To improve the transient response, a feed-forward ca-
pacitor, C
FF
, may be used. Great care should be taken to
route the V
FB
line away from noise sources, such as the
inductor or the SW line.
LTC3851
V
FB
V
OUT
R
B
C
FF
R
A
3851 F06
Figure 6. Settling Output Voltage
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LTC3851EGN#PBF

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Buy LTC3851EGN#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Sync Buck Sw Reg Cntr
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