LTC3836_15 Datasheet, PDF (14/30 Pages) Linear Technology – Dual 2-Phase, No RSENSETM Low VIN Synchronous Controller

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_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 LTC3836 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.
The main 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
LTC3836’s current comparator monitors the drain-to-
source voltage V_DS of the main MOSFET, which is sensed
between the SENSE⁺ and SW pins. The peak inductor cur-
rent 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 ∆V_SENSE(MAX) to approximately
122mV when IPRG is floating (82mV when IPRG is tied
low; 202mV when IPRG is tied high).
The output current that the LTC3836 can provide is
given by:
I_OUT(MAX)
=

V_SENSE(MAX)
R_DS(ON)
–
I_RIPPLE
2
A reasonable starting point is setting ripple current I_RIPPLE
to be 40% of I_OUT(MAX). Rearranging the above equation
yields:
R_DS(ON)(MAX)
=
5
6
•

V_SENSE(MAX)
I_OUT(MAX)
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:
R_DS(ON)(MAX)
=
5
6
•
SF
•

V_SENSE(MAX)
I_OUT(MAX)
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
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 LTC3836
and external component values:
R_DS(ON)(MAX)
=
5
6
•
0.9
•
SF
•

V_SENSE(MAX)
I_OUT(MAX) • _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 4. 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 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:
Top MOSFET Duty Cycle= ^VOUT
V_IN
Bottom MOSFET Duty Cycle= ^{VIN – VOUT}
V_IN
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