LTC3836 LINER [Linear Technology], LTC3836 Datasheet - Page 14

• Manufacturer Part Number: LTC3836
• Description: Dual 2-Phase, No RSENSETM Low VIN Synchronous Controller
• Manufacturer: LINER [Linear Technology]

LTC3836
APPLICATIONS INFORMATION
The typical LTC3836 application circuit is shown in
Figure 13. External component selection for each of the
LTC3836’s controllers is driven by the load requirement
and begins with the selection of the inductor (L) and the
power MOSFETs (M1 to M4).
Power MOSFET Selection
Each of the LTC3836’s two controllers requires two ex-
ternal N-channel power MOSFETs for the topside (main)
switch and the bottom (synchronous) switch. Important
parameters for the power MOSFETs are the breakdown
voltage V
R
t
The gate drive voltage is the input supply voltage. Since
the LTC3836 is designed for operation down to low input
voltages, a sublogic level MOSFET (R
at V
to this voltage.
The main MOSFET’s on-resistance is chosen based on the
required load current. The maximum average output load
current I
minus half the peak-to-peak ripple current I
LTC3836’s current comparator monitors the drain-to-
source voltage V
between the SENSE
rent is limited by the current threshold, set by the voltage
on the I
I
current sense threshold ΔV
122mV when IPRG is fl oating (82mV when IPRG is tied
low; 202mV when IPRG is tied high).
The output current that the LTC3836 can provide is
given by:
14
D(OFF)
TH
DS(ON)
I
OUT(MAX)
GS
pin is internally clamped, which limits the maximum
= 2.5V) is required for applications that work close
and the total gate charge Q
TH
, reverse transfer capacitance C
OUT(MAX)
BR(DSS)
pin of the current comparator. The voltage on the
=
, threshold voltage V
DS
V
SENSE(MAX)
R
is equal to the peak inductor current
+
of the main MOSFET, which is sensed
DS(ON)
and SW pins. The peak inductor cur-
SENSE(MAX)
–
I
RIPPLE
G
2
.
GS(TH)
DS(ON)
RSS
to approximately
, on-resistance
, turn-off delay
RIPPLE
guaranteed
. The
A reasonable starting point is setting ripple current I
to be 40% of I
yields:
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
amount of load current:
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
tion), allowing some margin for variations in the LTC3836
and external component values:
The
variation in on-resistance, which is typically about 0.4%/°C,
as shown in Figure 4. Junction to case temperature T
about 10°C in most applications. For a maximum ambi-
ent temperature of 70°C, using
equation is a reasonable choice.
The power dissipated in the top and bottom MOSFETs
strongly depends on their respective duty cycles and load
current. When the LTC3836 is operating in continuous
mode, the duty cycles for the MOSFETs are:
R
R
R
Bottom MOSFET Duty Cycle =
Top MOSFET Duty Cycle =
ρ
DS(ON)(MAX)
DS(ON)(MAX)
DS(ON)(MAX)
T
is a normalizing term accounting for the temperature
DS(ON)MAX
OUT(MAX)
=
=
=
5
6
5
6
5
6
•
• SF •
• 0.9 • SF •
at 25°C (manufacturer’s specifi ca-
. Rearranging the above equation
V
I
OUT(MAX)
SENSE(MAX)
DS(ON)
V
I
OUT(MAX)
V
SENSE(MAX)
V
OUT
I
ρ
IN
OUT(MAX)
V
to provide the required
80°C
V
IN
SENSE(MAX)
– V
V
≈ 1.3 in the above
IN
OUT
•
T
RIPPLE
JC
3836fa
is