Datasheet

LTC3809-1
12
38091fc
The typical LTC3809-1 application circuit is shown in Figure
8. 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 LTC3809-1’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 LTC3809-1 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 LTC3809-1 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
LTC3809-1’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 inductor
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 LTC3809-1 can provide is
given by:
I
V
R
I
OUT MAX
SENSE MAX
DS ON
RIPPLE
()
()
()
–=
Δ
2
APPLICATIONS INFORMATION
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 C
DS ON MAX
SENSE MAX
OUT MAX
()
()
()
•=
Δ
5
6
yycle <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 re-
quired R
DS(ON)MAX
at 25°C (manufacturer’s specifi cation),
allowing some margin for variations in the LTC3809-1 and
external component values:
RSF
V
I
DS ON MAX
SENSE MAX
OUT MAX
()
()
()
•.• •
•
=
Δ
5
6
09
ρρ
T
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-
1’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.