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LTC1649CS#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
7
LTC1649
APPLICATIONS INFORMATION
WUU
U
MOSFET Gate Drive
The LTC1649 is designed to operate from supplies as low
as 2.7V while using standard 5V logic-level N-channel
external MOSFETs. This poses somewhat of a challenge—
from as little as 2.7V, the LTC1649 must provide a 0V to
5V signal to the lower MOSFET, Q2, while the upper
MOSFET, Q1, requires a gate drive signal that swings from
0V to (V
IN
+ 5V). The LTC1649 addresses this situation
with two specialized circuits. An onboard charge pump
boosts the input voltage at V
IN
to a regulated 5V at CP
OUT
.
This 5V supply is used to power the PV
CC2
pin, which in
turn supplies 5V gate drive to Q2. This 5V supply is also
used to power the V
CC
pin, which allows the internal drive
circuitry to interface to the boosted driver supplies.
Gate drive for the top N-channel MOSFET, Q1, is supplied
by PV
CC1
. This supply must reach V
IN
+ 5V while Q1 is on.
Conveniently, the switching node at the source of Q1 rises
to V
IN
whenever Q1 is on. The LTC1649 uses this fact to
generate the required voltage at PV
CC1
with a simple
external charge pump as shown in Figure 2. This circuit
charges the flying capacitor C2 to the 5V level at CP
OUT
when the switching node is low. As the top MOSFET turns
on, the switching node begins to rise to V
IN
, and the PV
CC1
is pulled up to V
IN
+ 5V by C2. The 93% maximum duty
cycle (typical) means the switching node at the source of
Q1 will return to ground during at least 7% of each cycle,
ensuring that the charge pump will always provide ad-
equate gate drive to Q1.
Synchronous Operation
The LTC1649 uses a synchronous switching architecture,
with MOSFET Q2 taking the place of the diode in a classic
buck circuit (Figure 3). This improves efficiency by reduc-
ing the voltage drop and the resultant power dissipation
across Q2 to V
ON
= (I)(R
DS(ON)(Q2)
), usually much lower
than V
F
of the diode in the classical circuit. This more than
offsets the additional gate drive required by the second
MOSFET, allowing the LTC1649 to achieve efficiencies in
the mid-90% range for a wide range of load currents.
Another feature of the synchronous architecture is that
unlike a diode, Q2 can conduct current in either direction.
This allows the output of a typical LTC1649 circuit to sink
current as well as sourcing it while remaining in regula-
tion. The ability to sink current at the output allows the
LTC1649 to be used with reactive or other nonconventional
loads that may supply current to the regulator as well as
drawing current from it.
Figure 2. PV
CC1
Charge Pump
Figure 3b. Synchronous Buck Architecture
Figure 3a. Classical Buck Architecture
LTC1649
CP
OUT
PV
CC2
PV
CC1
Q1
L1
Q2
G1
G2
V
IN
C
OUT
V
OUT
1649 F02
+
10µF
+
1µF
D
CP
Q1
D1
V
IN
V
OUT
CONTROLLER
1649 F03a
Q1
V
IN
V
OUT
CONTROLLER
Q2
1649 F03b
8
LTC1649
EXTERNAL COMPONENT SELECTION
Power MOSFETs
Two N-channel power MOSFETs are required for most
LTC1649 circuits. These should be selected primarily by
on-resistance considerations; thermal dissipation is often
a secondary concern in high efficiency designs. The
LTC1649 is designed to be used with 5V logic-level MOS-
FETs; “standard” threshold MOSFETs with R
DS(ON)
speci-
fied at 10V only will not provide satisfactory performance.
MOSFET R
DS(ON)
should be chosen based on input and
output voltage, allowable power dissipation and maxi-
mum required output current. In a typical LTC1649 buck
converter circuit operating in continuous mode, the aver-
age inductor current is equal to the output load current.
This current is always flowing through either Q1 or Q2 with
the power dissipation split up according to the duty cycle:
DC (Q1) =
V
OUT
V
IN
DC (Q2) = 1 –
V
OUT
V
IN
=
(V
IN
– V
OUT
)
V
IN
The R
ON
required for a given conduction loss can now be
calculated by rearranging the relation P = I
2
R:
R
DS(ON)
(Q1) =
=
P
MAX
(Q1)
DC(Q1)(I
MAX
2
)
V
IN
(P
MAX
)(Q1)
V
OUT
(I
MAX
2
)
R
DS(ON)
(Q2) =
=
P
MAX
(Q2)
DC(Q2)(I
MAX
2
)
V
IN
(P
MAX
)(Q2)
(V
IN
– V
OUT
)(I
MAX
2
)
P
MAX
should be calculated based primarily on required
efficiency. A typical high efficiency circuit designed for
3.3V in, 2.5V at 10A out might require no more than 3%
efficiency loss at full load for each MOSFET. Assuming
roughly 90% efficiency at this current level, this gives a
P
MAX
value of (2.5V)(10A/0.9)(0.03) = 833mW per FET
and a required R
DSON
of:
R
DS(ON)
(Q1) = = 0.011Ω
(3.3V)(833mW)
(2.5V)(10A
2
)
R
DS(ON)
(Q2) = = 0.034Ω
(3.3V)(833mW)
(3.3V – 2.5V)(10A
2
)
Note that while the required R
DS(ON)
values suggest large
MOSFETs, the dissipation numbers are less than a watt per
device— large TO-220 packages and heat sinks are not
necessarily required in high efficiency applications. Siliconix
Si4410DY and International Rectifier IRF7801 are two
small, surface mount devices with R
ON
values of 0.03Ω or
below with 5V of gate drive; both work well in LTC1649
circuits. A higher P
MAX
value will generally decrease
MOSFET cost and circuit efficiency and increase MOSFET
heat sink requirements.
Inductor
The inductor is often the largest component in an LTC1649
design and should be chosen carefully. Inductor value and
type should be chosen based on output slew rate require-
ments and expected peak current. Inductor value is prima-
rily controlled by the required current slew rate. The
maximum rate of rise of the current in the inductor is set
by its value, the input-to-output voltage differential and the
maximum duty cycle of the LTC1649. In a typical 3.3V to
2.5V application, the maximum rise time will be:
93% =
(V
IN
– V
OUT
)
L
AMPS
SECOND
0.744A
µs
I
L
where L is the inductor value in µH. A 2µH inductor would
have a 0.37A/µs rise time in this application, resulting in a
14µs delay in responding to a 5A load current step. During
this 14µs, the difference between the inductor current and
the output current must be made up by the output capaci-
tor, causing a temporary droop at the output. To minimize
this effect, the inductor value should usually be in the 1µH
to 5µH range for most typical 3.3V to 2.xV LTC1649
circuits. Different combinations of input and output volt-
APPLICATIONS INFORMATION
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9
LTC1649
APPLICATIONS INFORMATION
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ages and expected loads may require different values.
Once the required value is known, the inductor core type
can be chosen based on peak current and efficiency
requirements. Peak current in the inductor will be equal to
the maximum output load current added to half the peak-
to- peak inductor ripple current. Ripple current is set by the
inductor value, the input and output voltage and the
operating frequency. If the efficiency is high and can be
considered approximately equal to 1, the ripple current is
approximately equal to:
∆I = DC
(V
IN
–
V
OUT
)
(f
OSC
)(L)
DC =
V
OUT
V
IN
f
OSC
= LTC1649 oscillator frequency = 200kHz
L = inductor value
Solving this equation with our typical 3.3V to 2.5V appli-
cation, we get:
= 1.5A
P–P
(0.8)(0.76)
(200kHz)(2µH)
Peak inductor current at 10A load:
= 10.8A10A +
1.5A
2
The inductor core must be adequate to withstand this peak
current without saturating, and the copper resistance in
the winding should be kept as low as possible to minimize
resistive power loss. Note that the current may rise above
this maximum level in circuits under current limit or under
fault conditions in unlimited circuits; the inductor should
be sized to withstand this additional current.
Input and Output Capacitors
A typical LTC1649 design puts significant demands on
both the input and output capacitors. Under normal steady
load operation, a buck converter like the LTC1649 draws
square waves of current from the input supply at the
switching frequency, with the peak value equal to the
output current and the minimum value near zero. Most of
this current must come from the input bypass capacitor,
since few raw supplies can provide the current slew rate to
feed such a load directly. The resulting RMS current flow
in the input capacitor will heat it up, causing premature
capacitor failure in extreme cases. Maximum RMS current
occurs with 50% PWM duty cycle, giving an RMS current
value equal to I
OUT
/2. A low ESR input capacitor with an
adequate ripple current rating must be used to ensure
reliable operation. Note that capacitor manufacturers’
ripple current ratings are often based on only 2000 hours
(3 months) lifetime; further derating of the input capacitor
ripple current beyond the manufacturer’s specification is
recommended to extend the useful life of the circuit.
The output capacitor in a buck converter sees much less
ripple current under steady-state conditions than the input
capacitor. Peak-to-peak current is equal to that in the
inductor, usually a fraction of the total load current. Output
capacitor duty places a premium not on power dissipation
but on low ESR. During an output load transient, the
output capacitor must supply all of the additional load
current demanded by the load until the LTC1649 can
adjust the inductor current to the new value. ESR in the
output capacitor results in a step in the output voltage
equal to the ESR value multiplied by the change in load
current. A 5A load step with a 0.05Ω ESR output capacitor
will result in a 250mV output voltage shift; this is a 10%
output voltage shift for a 2.5V supply! Because of the
strong relationship between output capacitor ESR and
output load transient response, the output capacitor is
usually chosen for ESR, not for capacitance value; a
capacitor with suitable ESR will usually have a larger
capacitance value than is needed to control steady-state
output ripple.
Electrolytic capacitors rated for use in switching power
supplies with specified ripple current ratings and ESR can
be used effectively in LTC1649 applications. OS-CON
electrolytic capacitors from Sanyo give excellent perfor-
mance and have a very high performance/size ratio for an
electrolytic capacitor. Surface mount applications can use
either electrolytic or dry tantalum capacitors. Tantalum
capacitors must be surge tested and specified for use in
switching power supplies; low cost, generic tantalums are
known to have very short lives followed by explosive
deaths in switching power supply applications. AVX TPS
series surface mount devices are popular tantalum capaci-
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LTC1649CS#PBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 3.3V In Hi Pwr Buck Sw Reg Cntr
Lifecycle:
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