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LTC1876EG#PBF

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LTC1876
16
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then, sub-logic level threshold MOSFETs (V
GS(TH)
< 3V)
should be used. Pay close attention to the BV
DSS
specifi-
cation for the MOSFETs as well; most of the logic level
MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance R
DS(ON)
, reverse transfer capacitance C
RSS
,
input voltage and maximum output current. When the
LTC1876 is operating in continuous mode the duty cycles
for the top and bottom MOSFETs are given by:
Main SwitchDuty Cycle
V
V
OUT
IN
=
Synchronous SwitchDuty Cycle
VV
V
IN OUT
IN
=
–
The MOSFET power dissipations at maximum output
current are given by:
P
V
V
IR
kV I C f
MAIN
OUT
IN
MAX DS ON
IN MAX RSS
=
()
+
()
+
()( )( )()
2
2
1 δ
()
P
VV
V
IR
SYNC
IN OUT
IN
MAX DS ON
=
()
+
()
–
()
2
1 δ
where δ is the temperature dependency of R
DS(ON)
and k
is a constant inversely related to the gate drive current.
Both MOSFETs have I
2
R losses while the topside N-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
IN
< 20V the
high current efficiency generally improves with larger
MOSFETs, while for V
IN
> 20V the transition losses rapidly
increase to the point that the use of a higher R
DS(ON)
device
with lower C
RSS
actually provides higher efficiency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during a
short-circuit when the synchronous switch is on close to
100% of the period.
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized R
DS(ON)
vs temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs. C
RSS
is usually specified in the MOS-
FET characteristics. The constant k = 1.7 can be used to
estimate the contributions of the two terms in the main
switch dissipation equation.
The Schottky diode D1 shown in Figure 1 conducts during
the dead-time between the conduction of the two power
MOSFETs. This prevents the body diode of the bottom
MOSFET from turning on, storing charge during the dead-
time and requiring a reverse recovery period that could
cost as much as 3% in efficiency at high V
IN
. A 1A to 3A
Schottky is generally a good compromise for both regions
of operation due to the relatively small average current.
Larger diodes result in additional transition losses due to
their larger junction capacitance.
C
IN
Selection
The selection of C
IN
is simplified by the multiphase archi-
tecture and its impact on the worst-case RMS current
drawn through the input network (battery/fuse/capacitor).
It can be shown that the worst case RMS current occurs
when only one controller is operating. The controller with
the highest (V
OUT
)(I
OUT
) product needs to be used in the
formula below to determine the maximum RMS current
requirement. Increasing the output current, drawn from
the other out-of-phase controller, will actually decrease
the RMS ripple current from this maximum value (see
Figure 4). The out-of-phase technique typically reduces
the input capacitor’s RMS ripple current by a factor of 30%
to 70% when compared to a single phase power supply
solution.
The type of input capacitor, value and ESR rating have
efficiency effects that need to be considered in the selec-
tion process. The capacitance value chosen should be
sufficient to store adequate charge to keep high peak
battery currents down. 20µF to 40µF is usually sufficient
for a 25W output supply operating at 200kHz. The ESR of
the capacitor is important for capacitor power dissipation
as well as overall battery efficiency. All of the power (RMS
ripple current • ESR) not only heats up the capacitor but
wastes power from the battery.
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LTC1876
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Medium voltage (20V to 35V) ceramic, tantalum, OS-CON
and switcher-rated electrolytic capacitors can be used as
input capacitors, but each has drawbacks: ceramic voltage
coefficients are very high and may have audible piezoelec-
tric effects; tantalums need to be surge-rated; OS-CONs
suffer from higher inductance, larger case size and limited
surface-mount applicability; electrolytics’ higher ESR and
dryout possibility require several to be used. Multiphase
systems allow the lowest amount of capacitance overall.
As little as one 22µF or two to three 10µF ceramic capaci-
tors are an ideal choice in a 20W to 35W power supply due
to their extremely low ESR. Even though the capacitance
at 20V is substantially below their rating at zero-bias, very
low ESR loss makes ceramics an ideal candidate for
highest efficiency battery operated systems. Also con-
sider parallel ceramic and high quality electrolytic capaci-
tors as an effective means of achieving ESR and bulk
capacitance goals.
In continuous mode, the source current of the top N-chan-
nel 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 of one channel must
be used. The maximum RMS capacitor current is given by:
C quired I I
VVV
V
IN RMS MAX
OUT IN OUT
IN
Re
/
≈
−
()
[]
12
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst case condition is com-
monly 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.
The benefit of the LTC1876 multiphase controllers can be
calculated by using the equation above for the higher
power controller and then calculating the loss that would
have resulted if both controller channels switch on at the
same time. The total RMS power lost is lower when both
controllers are operating due to the reduced overlap of
current pulses required through the input capacitor’s ESR.
This is why the input capacitor’s requirement calculated
above for the worst-case controller is adequate for the
dual controller design. Remember that protection fuse
resistance, battery resistance and PC board trace resis-
tance losses are also reduced due to the reduced peak
currents in a multiphase system.
The overall benefit of a
multiphase design will only be fully realized when the
source impedance of the power supply/battery is included
in the efficiency testing.
The drains of the two top MOSFETS
should be placed within 1cm of each other and share a
common C
IN
(s). Separating the drains and C
IN
may pro-
duce undesirable voltage and current resonances at V
IN
.
For the boost regulator, the ripple requirement for the
input capacitor is less stringent. If the supply to the
regulator is obtained from one of the LTC1876 step-down
outputs, a 1µF to 4.7µF ceramic capacitor is sufficient.
However, if the step-down output is within close proximity
(< 1cm) to the boost supply input, there is no need for the
capacitor.
C
OUT
Selection
The selection of C
OUT
is driven by the required effective
series resistance (ESR). Typically once the ESR require-
ment is satisfied the capacitance is adequate for filtering.
For the step-down regulators, the output ripple (∆V
OUT
) is
determined by:
∆∆V I ESR
fC
OUT L
OUT
≈+
1
8
Where f = operating frequency, C
OUT
= output capacitance,
and ∆
L
= ripple current in the inductor. The output ripple is
highest at maximum input voltage since ∆I
L
increases
with input voltage. With ∆I
L
= 0.4I
OUT(MAX)
the output
ripple will typically be less than 50mV at max V
IN
assum-
ing:
C
OUT
Recommended ESR < 2 R
SENSE
and 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 capacitance does not significantly
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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
capacitors selected.
For the boost regulator, the output ripple (∆V
OUT
) is
determined by:
∆V I ESR
I
fC
OUT PK
OUT
OUT
≈+
15.
Since the boost regulator is operating at high frequency,
the second term will be small even with a small value of
C
OUT
. Hence, all efforts can be concentrated on finding a
low ESR capacitor. A ceramic capacitor can be used for the
output capacitor.
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. 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 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 applica-
tions providing that consideration is given to ripple current
ratings, temperature and long term reliability. A typical
application will require several to many aluminum electro-
lytic capacitors in parallel. A combination of the above
mentioned capacitors will often result in maximizing per-
formance and minimizing overall cost. Other capacitor
types include Nichicon PL series, NEC Neocap, Pansonic
SP and Sprague 595D series. For high value of ceramic
capacitors, Taiyo Yuden has a series of them. Select the
X5R or X7R series as these retain the capacitance over
wide voltage and temperature range. Consult manufactur-
ers for other specific recommendations.
INTV
CC
Regulator
An internal P-channel low dropout regulator produces 5V
at the INTV
CC
pin from the V
IN
supply pin. INTV
CC
powers
the drivers and internal circuitry within the LTC1876 step-
down controllers. 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 neces-
sary 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 LTC1876 to be
exceeded. The system supply current is normally domi-
nated 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 5V 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 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
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LTC1876EG#PBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 2-phase,Dual Step-dn + Boost Reg
Lifecycle:
New from this manufacturer.
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