Datasheet

LTC3809
12
3809fc
APPLICATIONS INFORMATION
The typical LTC3809 application circuit is shown in Fig-
ure 10. External component selection for the controller
is driven by the load requirement and begins with the
selection of the inductor and the power MOSFETs.
Power MOSFET Selection
The LTC3809’s controller requires two external power
MOSFETs: a P-channel MOSFET for the topside (main)
switch and a N-channel MOSFET for the bottom (synchro-
nous) switch. The main selection criteria for the power
MOSFETs are the breakdown voltage V
BR(DSS)
, threshold
voltage V
GS(TH)
, on-resistance R
DS(ON)
, reverse transfer
capacitance C
RSS
, turn-off delay t
D(OFF)
and the total gate
charge Q
G
.
The gate drive voltage is the input supply voltage. Since
the LTC3809 is designed for operation down to low input
voltages, a sublogic level MOSFET (R
DS(ON)
guaranteed at
V
GS
= 2.5V) is required for applications that work close to
this voltage. When these MOSFETs are used, make sure that
the input supply to the LTC3809 is less than the absolute
maximum MOSFET V
GS
rating, which is typically 8V.
The P-channel MOSFET’s on-resistance is chosen based
on the required load current. The maximum average load
current I
OUT(MAX)
is equal to the peak inductor current
minus half the peak-to-peak ripple current I
RIPPLE
. The
LTC3809’s current comparator monitors the drain-to-
source voltage V
DS
of the top P-channel MOSFET, which
is sensed between the V
IN
and SW pins. The peak induc-
tor current is limited by the current threshold, set by the
voltage on the I
TH
pin, of the current comparator. The
voltage on the I
TH
pin is internally clamped, which limits
the maximum current sense threshold ΔV
SENSE(MAX)
to
approximately 125mV when IPRG is fl oating (85mV when
IPRG is tied low; 204mV when IPRG is tied high).
The output current that the LTC3809 can provide is given
by:
I
V
R
I
OUT MAX
SENSE MAX
DS ON
RIPPLE
()
()
()
–=
Δ
2
where I
RIPPLE
is the inductor peak-to-peak ripple current
(see Inductor Value Calculation).
A reasonable starting point is setting ripple current I
RIPPLE
to be 40% of I
OUT(MAX)
. Rearranging the above equation
yields:
R
V
I
for Duty Cycle
DS ON MAX
SENSE MAX
OUT MAX
()
()
()
•%=
Δ
<
5
6
20
However, for operation above 20% duty cycle, slope
compensation has to be taken into consideration to select
the appropriate value of R
DS(ON)
to provide the required
amount of load current:
RSF
V
I
DS ON MAX
SENSE MAX
OUT MAX
()
()
()
••=
Δ
5
6
where SF is a scale factor whose value is obtained from
the curve in Figure 1.
These must be further derated to take into account the
signifi cant variation in on-resistance with temperature.
The following equation is a good guide for determining the
required R
DS(ON)MAX
at 25°C (manufacturer’s specifi ca-
tion), allowing some margin for variations in the LTC3809
and external component values:
RSF
V
I
DS ON MAX
SENSE MAX
OUT MAX T
()
()
()
•.• •
•
=
Δ
5
6
09
ρ
The ρ
T
is a normalizing term accounting for the temperature
variation in on-resistance, which is typically about 0.4%/°C,
as shown in Figure 2. Junction-to-case temperature T
JC
is
about 10°C in most applications. For a maximum ambi-
ent temperature of 70°C, using ρ
80°C
~ 1.3 in the above
equation is a reasonable choice.
The N-channel MOSFET’s on resistance is chosen based
on the short-circuit current limit (I
SC
). The LTC3809’s
short-circuit current limit comparator monitors the drain-
to-source voltage V
DS
of the bottom N-channel MOSFET,
which is sensed between the GND and SW pins. The