Datasheet

14
LTC3776
3776fa
The typical LTC3776 application circuit is shown in
Figure 11. External component selection for each of the
LTC3776’s controllers is driven by the load requirement
and begins with the selection of the inductor (L) and the
power MOSFETs (MP and MN).
Power MOSFET Selection
Each of the LTC3776’s two controllers requires two exter-
nal power MOSFETs: a P-channel MOSFET for the topside
(main) switch and an N-channel MOSFET for the bottom
(synchronous) switch. Important parameters 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
LTC3776 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 LTC3776 is less than the abso-
lute 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
output load current I
OUT(MAX)
is equal to the peak inductor
current minus half the peak-to-peak ripple current I
RIPPLE
.
The LTC3776’s current comparator monitors the drain-to-
source voltage V
DS
of the P-channel MOSFET, which is
sensed between the SENSE
+
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)
.
The output current that the LTC3776 can provide is given
by:
I
V
R
I
OUT MAX
SENSE MAX
DS ON
RIPPLE
()
()
()
–=
Δ
2
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
DS ON MAX
SENSE MAX
OUT MAX
()( )
()
()
•=
Δ
5
6
for Duty Cycle < 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 2.
These must be further derated to take into account the
significant 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 specifica-
tion), allowing some margin for variations in the LTC3776
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 tempera-
ture variation in on-resistance, which is typically about
0.4%/°C, as shown in Figure 5. Junction to case tempera-
ture T
JC
is about 10°C in most applications. For a maxi-
mum ambient temperature of 70°C, using ρ
80°C
~ 1.3 in
the above equation is a reasonable choice.
APPLICATIO S I FOR ATIO
WUUU
INPUT VOLTAGE (V)
2
0
INPUT CAPACITOR RMS CURRENT
0.2
0.6
0.8
1.0
2.0
1.4
4
6
7
3776 F04
0.4
1.6
1.8
1.2
35
8
9
10
SINGLE PHASE
DUAL CONTROLLER
2-PHASE
DUAL CONTROLLER
V
OUT1
= 2.5V/2A
V
OUT2
= 1.8V/2A
Figure 4. RMS Input Current Comparison