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LTC3809EDD-1#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LTC3809-1
13
38091fc
The short-circuit current sense threshold ΔV
SC
is set
approximately 90mV when IPRG is fl oating (60mV when
IPRG is tied low; 150mV when IPRG is tied high). The
on-resistance 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
LTC3809-1 is operating in continuous mode, the duty
cycles for the MOSFETs are:
APPLICATIONS INFORMATION
Top P-Channel Duty Cycle =
V
Bottom N-
OUT
V
IN
CChannel Duty Cycle =
V
IN
− V
V
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 LTC3809-1 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 LTC3809-1
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
Figure 2. R
DS(ON)
vs Temperature
JUNCTION TEMPERATURE (°C)
–50
1.0
1.5
150
38091 F02
0.5
0
0
50
100
2.0
LTC3809-1
14
38091fc
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.
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 LTC3809-1 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
APPLICATIONS INFORMATION
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 current
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. Actual core loss is independent of core size for a
fi xed inductor value, but is very dependent on the induc-
tance 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
inductance collapses abruptly when the peak design current
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!
LTC3809-1
15
38091fc
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price vs size requirements and any
radiated fi eld/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
Toko and Sumida.
Schottky Diode Selection (Optional)
The schottky diode D in Figure 9 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 LTC3809-1 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
APPLICATIONS INFORMATION
This formula has a maximum value at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/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 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 LTC3809-1, 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 LTC3809-1 output voltage is set by an external
feedback 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.•
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LTC3809EDD-1#PBF

Mfr. #:
Buy LTC3809EDD-1#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators No Rsense, Low EMI DC/DC Controller in DFN
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