LTC3879
11
3879f
APPLICATIONS INFORMATION
The basic LTC3879 application circuit is shown on the fi rst
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 specifi cations.
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.
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)
.
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 specifi ed 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
Max V Sense Voltage
I
DS ON MAX
DS
OT
()( )
•
=
ρ
The ρ
T
term is a normalization factor (unity at 25°C)
accounting for the signifi cant 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
V
V
D
VV
V
TOP
OUT
IN
BOT
IN OUT
IN
=
=
–
Figure 1. R
DS(ON)
vs Temperature
JUNCTION TEMPERATURE (°C)
–50
R
T
NORMALIZED ON-RESISTANCE (Ω)
1.0
1.5
150
3879 F01
0.5
0
0
50
100
2.0
