LTC3879_15 Datasheet, PDF (11/28 Pages) Linear Technology – Fast, Wide Operating Range No RSENSE Step-Down Controller

LTC3879
APPLICATIONS INFORMATION
The basic LTC3879 application circuit is shown on the first
page of this data sheet. External component selection is
largely determined by maximum load current and begins
with the selection of sense resistance and power MOSFET
switches. The LTC3879 uses the on-resistance of the syn-
chronous power MOSFET to determine the inductor current.
The desired ripple current and operating frequency largely
determines the inductor value. Finally, C_IN is selected for its
ability to handle the large RMS current into the converter,
and C_OUT is chosen with low enough ESR to meet output
voltage ripple and transient specifications.
Maximum V_DS Sense Voltage and V_RNG Pin
Inductor current is measured by sensing the bottom
MOSFET V_DS voltage that appears between the PGND
and SW pins. The maximum allowed V_DS sense voltage is
set by the voltage applied to the V_RNG pin and is approxi-
mately equal to (0.133)V_RNG. The current mode control
loop does not allow the inductor current valleys to exceed
(0.133)V_RNG. In practice, one should allow margin, to ac-
count for variations in the LTC3879 and external component
values. A good guide for setting V_RNG is:
V_RNG = 7.5 • (Maximum V_DS Sense Voltage)
An external resistive divider from INTV_CC can be used
to set the voltage on the V_RNG pin between 0.2V and 2V,
resulting in peak sense voltages between 26.6mV and
266mV. The wide peak voltage sense range allows for a
variety of applications and MOSFET choices. The V_RNG pin
can also be tied to either SGND or INTV_CC to force internal
defaults. When V_RNG is tied to SGND, the device operates
at a valley current sense threshld of 30mV typical. If V_RNG
is tied to INTV_CC, the device operates at a valley current
sense threshold of 75mV typical.
The gate drive voltages are set by the 5.3V INTV_CC supply.
Consequently, logic-level threshold MOSFETs must be used
in LTC3879 applications. If the input voltage is expected
to drop below 5V, then sub-logic level threshold MOSFETs
should be considered.
Using the bottom MOSFET as the current sense element
requires particular attention be paid to its on-resistance.
MOSFET on-resistance is typically specified with a maxi-
mum 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)
=
Max
V_DS Sense
I_O • ρ_T
Voltage
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, as shown in
Figure 1. For a maximum junction temperature of 100°C
using a value of ρ_T = 1.3 is reasonable.
The power dissipated by the top and bottom MOSFETs
depends upon their respective duty cycles and the load
current. When the LTC3879 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
2.0
1.5
Power MOSFET Selection
The LTC3879 requires two external N-channel power
MOSFETs, one for the top (main) switch and one 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), re-
verse transfer capacitance C_RSS and maximum current
I_DS(MAX).
1.0
0.5
0
–50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3879 F01
Figure 1. R_DS(ON) vs Temperature
3879f
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