LTC3709_15 Datasheet, PDF (12/24 Pages) Linear Technology – Fast 2-Phase, No RSENSE Synchronous DC/DC Controller with Tracking/Sequencing

LTC3709
APPLICATIO S I FOR ATIO
Connecting the SENSE⁺ and SENSE^– Pins
The LTC3709 provides the user with an optional method to
sense current through a sense resistor instead of using the
R_DS(ON) of the synchronous MOSFET. When using a sense
resistor, it is placed between the source of the synchro-
nous MOSFET and ground. To measure the voltage across
this resistor, connect the SENSE⁺ pin to the source end of
the resistor and the SENSE^– pin to the other end of the
resistor. The SENSE⁺ and SENSE^– pin connections pro-
vide the Kelvin connections, ensuring accurate voltage
measurement across the resistor. Using a sense resistor
provides a well-defined current limit, but adds cost and
reduces efficiency. Alternatively, one can use the synchro-
nous MOSFET as the current sense element by simply
connecting the SENSE⁺ pin to the switch node SW and the
SENSE^– pin to the source of the synchronous MOSFET,
eliminating the sense resistor. This improves efficiency,
but one must carefully choose the MOSFET on-resistance
as discussed in the Power MOSFET Selection section.
Power MOSFET Selection
The LTC3709 requires four external N-channel power
MOSFETs, two for the top (main) switches and two for the
bottom (synchronous) switches. 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 and maximum current
I_DS(MAX).
The gate drive voltage is set by the 5V DRV_CC supply.
Consequently, logic-level threshold MOSFETs must be
used in LTC3709 applications. If the driver’s voltage is
expected to drop below 5V, then sub-logic level threshold
MOSFETs should be used.
When the bottom MOSFETs are used as the current sense
elements, particular attention must be paid to their on-
resistance. MOSFET on-resistance is typically specified
with a maximum value R_DS(ON)(MAX) at 25°C. In this case
additional margin is required to accommodate the rise in
MOSFET on-resistance with temperature:
R_DS(ON)(MAX)
=
R_SENSE
ρ_T
12
2.0
1.5
1.0
0.5
0
– 50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3709 F01
Figure 1. R_DS(ON) vs Temperature
The ρ_T term is a normalization factor (unity at 25°C)
accounting for the significant variation in on-resistance
with temperature, typically about 0.4%/°C. Junction-to-
case temperature is about 20°C in most applications. For
a maximum junction temperature of 100°C, using a value
ρ_100°C = 1.3 is reasonable (Figure 1).
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and
the load current. When the LTC3709 is operating in
continuous mode, the duty cycles for the MOSFETs are:
D_TOP
=
V_OUT
V_IN
D_BOT
=
V_IN
– V_OUT
V_IN
The maximum power dissipation in the MOSFETs per
channel is:
P_TOP
=
D_TOP
•
⎛
⎜
⎝
I_OUT(MAX)
2
⎞²
⎟
⎠
•
ρ_T(TOP)
•
R_DS(ON)(MAX)
+
(0.5)
•
V_IN2
•
⎛
⎝⎜
I_OUT
2
⎞
⎠⎟
•
C_RSS
•
f
•
( ) ^⎛
R_{DS(ON)_DRV ⎝⎜⎜}
1
DRV_CC – V_GS(TH)
+
1
V_GS(TH)
⎞
⎠⎟⎟
P_BOT
=
D_BOT
•
⎛
⎜
⎝
I_OUT(MAX)
2
⎞²
⎟
⎠
•
ρ_T(BOT)
•
R_DS(ON)(MAX)
3709fb