LTC3737_15 Datasheet, PDF (12/24 Pages) Linear Technology – Dual 2-Phase, No RSENSE, DC/DC Controller with Output Tracking

LTC3737
APPLICATIO S I FOR ATIO
The typical LTC3737 application circuit is shown in Figure
1. External component selection for each of the LTC3737’s
controllers is driven by the load requirement and begins
with the selection of the inductor (L) and the power
MOSFET M1. Next, the output diode D1 is selected. Finally
C_IN and C_OUT are chosen.
Power MOSFET Selection
An external P-channel MOSFET must be selected for use
with each channel of the LTC3737. The main selection
criteria for the power MOSFET are the breakdown voltage
V_BR(DSS), threshold voltage V_GS(TH), on-resistance
R_DS(ON), reverse transfer capacitance C_RSS and the total
gate charge Q_G.
The gate drive voltage is the input supply voltage. Since the
LTC3737 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 LTC3737 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 induc-
tor current minus half the peak-to-peak ripple current,
I_RIPPLE. The LTC3737’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 ∆V_SENSE(MAX)
to approximately 125mV when IPRG is floating (85mV
when IPRG is tied low; 204mV when IPRG is tied high).
The output current that the LTC3737 can provide is given
by:
I_OUT(MAX)
=
∆V_SENSE(MAX)
R_DS(ON)
–
I_RIPPLE
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_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 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 LTC3737
and external component values:
R_DS(ON)(MAX)
=
5
6
•
0.9
•
SF
•
∆V_SENSE(MAX)
I_OUT(MAX) • ρ_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 5. Junction to case temperature T_JC is
2.0
1.5
1.0
0.5
0
– 50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3737 F05
Figure 5. R_DS(ON) vs Temperature
3737fa
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