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LTC3809EMSE#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LTC3809
13
3809fc
APPLICATIONS INFORMATION
short-circuit current sense threshold ΔV
SC
is set approxi-
mately 90mV when IPRG is fl oating (60mV when IPRG is
t i e d l o w ; 15 0 m V w h e n I P R G i s t i e d h i g h). T h e o n - r e s i s t a n c e
of N-channel MOSFET is determined by:
R
V
I
DS ON MAX
SC
SC PEAK
()
()
=
Δ
The short-circuit current limit (I
SC(PEAK)
) should be larger
than the I
OUT(MAX)
with some margin to avoid interfering
with the peak current sensing loop. On the other hand,
in order to prevent the MOSFETs from excessive heating
and the inductor from saturation, I
SC(PEAK)
should be
smaller than the minimum value of their current ratings.
A reasonable range is:
I
OUT(MAX)
< I
SC(PEAK)
< I
RATING(MIN)
Therefore, the on-resistance of N-channel MOSFET should
be chosen within the following range:
Δ
<<
ΔV
I
R
V
I
SC
RATING MIN
DS ON
SC
OUT MAX()
()
()
where ΔV
SC
is 90mV, 60mV or 150mV with IPRG being
fl oated, tied to GND or V
IN
respectively.
The power dissipated in the MOSFET strongly depends
on its respective duty cycles and load current. When the
LTC3809 is operating in continuous mode, the duty cycles
for the MOSFETs are:
Top P-Channel Duty Cycle =
Bottom N-Channel Duty Cycle =
V
V
VV
V
OUT
IN
IN OUT
IN
–
The MOSFET power dissipations at maximum output
current are:
P
V
V
IRV
ICf
P
VV
V
IR
TOP
OUT
IN
OUT MAX T DS ON IN
OUT MAX RSS
BOT
IN OUT
IN
OUT MAX T DS ON
=+
=
••• •
•••
–
•••
() ()
()
() ()
22
2
2ρ
ρ
Both MOSFETs have I
2
R losses and the P
TOP
equation
includes an additional term for transition losses, which are
largest at high input voltages. The bottom MOSFET losses
are greatest at high input voltage or during a short-circuit
when the bottom duty cycle is 100%.
The LTC3809 utilizes a non-overlapping, anti-shoot-
through gate drive control scheme to ensure that the
P- and N-channel MOSFETs are not turned on at the same
time. To function properly, the control scheme requires
that the MOSFETs used are intended for DC/DC switching
applications. Many power MOSFETs, particularly P-channel
MOSFETs, are intended to be used as static switches and
therefore are slow to turn on or off.
Reasonable starting criteria for selecting the P-channel
MOSFET are that it must typically have a gate charge (Q
G
)
less than 25nC to 30nC (at 4.5V
GS
) and a turn-off delay
(t
D(OFF)
) of less than approximately 140ns. However, due
to differences in test and specifi cation methods of various
MOSFET manufacturers, and in the variations in Q
G
and
t
D(OFF)
with gate drive (V
IN
) voltage, the P-channel MOSFET
ultimately should be evaluated in the actual LTC3809
application circuit to ensure proper operation.
Shoot-through between the P-channel and N-channel
MOSFETs can most easily be spotted by monitoring the
input supply current. As the input supply voltage increases,
if the input supply current increases dramatically, then the
likely cause is shoot-through. Note that some MOSFETs
JUNCTION TEMPERATURE (°C)
–50
R
T
NORMALIZED ON RESISTANCE
1.0
1.5
150
3809 F02
0.5
0
0
50
100
2.0
Figure 2. R
DS(ON)
vs Temperature
LTC3809
14
3809fc
that do not work well at high input voltages (e.g., V
IN
>
5V) may work fi ne at lower voltages (e.g., 3.3V).
Selecting the N-channel MOSFET is typically easier, since
for a given R
DS(ON)
, the gate charge and turn-on and turn-off
delays are much smaller than for a P-channel MOSFET.
Operating Frequency and Synchronization
The choice of operating frequency, f
OSC
, is a trade-off
between effi ciency and component size. Low frequency
operation improves effi ciency by reducing MOSFET
switching losses, both gate charge loss and transition
loss. However, lower frequency operation requires more
inductance for a given amount of ripple current.
The internal oscillator for the LTC3809’s controller runs
at a nominal 550kHz frequency when the PLLLPF pin is
left fl oating and the SYNC/MODE pin is not confi gured
for spread spectrum operation. Pulling the PLLLPF to
V
IN
selects 750kHz operation; pulling the PLLLPF to GND
selects 300kHz operation.
Alternatively, the LTC3809 will phase-lock to a clock
signal applied to the SYNC/MODE pin with a frequency
between 250kHz and 750kHz (see Phase-Locked Loop
and Frequency Synchronization).
To further reduce EMI, the nominal 550kHz frequency will
be spread over a range with frequencies between 460kHz
and 635kHz when spread spectrum modulation is enabled
(see Spread Spectrum Modulation with SYNC/MODE and
PLLLPF Pins).
Inductor Value Calculation
Given the desired input and output voltages, the inductor
value and operating frequency, f
OSC
, directly determine
the inductor’s peak-to-peak ripple current:
I
V
V
VV
fL
RIPPLE
OUT
IN
IN OUT
OSC
= •
–
•
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Thus, highest effi ciency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
that is about 40% of I
OUT(MAX)
. Note that the largest ripple
current occurs at the highest input voltage. To guarantee
that ripple current does not exceed a specifi ed maximum,
the inductor should be chosen according to:
L
VV
fI
V
V
IN OUT
OSC RIPPLE
OUT
IN
≥
–
•
•
Burst Mode Operation Considerations
The choice of R
DS(ON)
and inductor value also determines
the load current at which the LTC3809 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
I
V
R
BURST PEAK
SENSE MAX
DS ON
()
()
()
•=
Δ
1
4
The corresponding average current depends on the
amount of ripple current. Lower inductor values (higher
I
RIPPLE
) will reduce the load current at which Burst Mode
operation begins.
The ripple current is normally set so that the inductor cur-
rent is continuous during the burst periods. Therefore,
I
RIPPLE
≤ I
BURST(PEAK)
This implies a minimum inductance of:
L
VV
fI
V
V
MIN
IN OUT
OSC BURST PEAK
OUT
IN
≤
–
•
•
()
A smaller value than L
MIN
could be used in the circuit,
although the inductor current will not be continuous
during burst periods, which will result in slightly lower
effi ciency. In general, though, it is a good idea to keep
I
RIPPLE
comparable to I
BURST(PEAK)
.
Inductor Core Selection
Once the value of L is known, the type of inductor must
be selected. High effi ciency converters generally cannot
a f f o r d t h e c or e l o s s f o u nd i n l o w c o s t p o w d er e d i r o n c o r e s,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mμ
®
cores. Ac tual core loss is independent of core
APPLICATIONS INFORMATION
LTC3809
15
3809fc
size for a fi xed inductor value, but is very dependent on
the inductance selected. As inductance increases, core
losses go down. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard”, which means that
i n d u c t a n c e c o l l a p s e s a b r u p t l y w h e n t h e p e a k d e s i g n c u r r e n t
is exceeded. Core saturation results in an abrupt increase
in inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
Molypermalloy (from Magnetics, Inc.) is a very good,
low loss core material for toroids, but is more expensive
than ferrite. A reasonable compromise from the same
manufacturer is Kool Mμ. Toroids are very space effi cient,
especially when several layers of wire can be used, while
inductors wound on bobbins are generally easier to sur-
face mount. However, designs for surface mount that do
not increase the height signifi cantly are available from
Coiltronics, Coilcraft, Dale and Sumida.
Schottky Diode Selection (Optional)
The schottky diode D in Figure 11 conducts current dur-
ing the dead time between the conduction of the power
MOSFETs. This prevents the body diode of the bottom
N-channel MOSFET from turning on and storing charge
during the dead time, which could cost as much as 1%
in effi ciency. A 1A Schottky diode is generally a good
size for most LTC3809 applications, since it conducts
a relatively small average current. Larger diode results
in additional transition losses due to its larger junction
capacitance. This diode may be omitted if the effi ciency
loss can be tolerated.
C
IN
and C
OUT
Selection
In continuous mode, the source current of the P-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:
C
IN
Re •
•–
/
quiredI I
VVV
V
RMS MAX
OUT IN OUT
IN
≈
()
12
This formula has a maximum value at V
IN
= 2V
OUT
,
where I
RMS
= I
OUT
/2. This simple worst-case condition
is commonly used for design because even signifi cant
deviations do not offer much relief. Note that capacitor
manufacturer’s ripple current ratings are often based on
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 be
paralleled to meet the size or height requirements in the
design. Due to the high operating frequency of the LTC3809,
ceramic capacitors can also be used for C
IN
. Always consult
the manufacturer if there is any question.
The selection of C
OUT
is driven by the effective series
resistance (ESR). Typically, once the ESR requirement
is satisfi ed, the capacitance is adequate for fi ltering. The
output ripple (ΔV
OUT
) is approximated by:
Δ≈ +
⎛
⎝
⎜
⎞
⎠
⎟
V I ESR
fC
OUT RIPPLE
OUT
•
••
1
8
where f is the operating frequency, C
OUT
is the output
capacitance and I
RIPPLE
is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since I
RIPPLE
increase with input voltage.
Setting Output Voltage
The LTC3809 output voltage is set by an external feed-
back resistor divider carefully placed across the output,
as shown in Figure 3. The regulated output voltage is
determined by:
VV
R
R
OUT
B
A
=+
⎛
⎝
⎜
⎞
⎠
⎟
06 1.•
APPLICATIONS INFORMATION
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LTC3809EMSE#TRPBF

Mfr. #:
Buy LTC3809EMSE#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators No Rsense, Low EMI DC/DC Controller in MSE
Lifecycle:
New from this manufacturer.
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