LTC3879 Linear Technology Corporation, LTC3879 Datasheet - Page 11

• Manufacturer Part Number: LTC3879
• Description: Wide Operating Range No RSENSE Step-Down Controller
• Manufacturer: Linear Technology Corporation

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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
ability to handle the large RMS current into the converter,
and C
voltage ripple and transient specifi cations.
Maximum V
Inductor current is measured by sensing the bottom
MOSFET V
and SW pins. The maximum allowed V
set by the voltage applied to the V
mately equal to (0.133)V
loop does not allow the inductor current valleys to exceed
(0.133)V
count for variations in the LTC3879 and external component
values. A good guide for setting V
An external resistive divider from INTV
to set the voltage on the V
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
can also be tied to either SGND or INTV
defaults. When V
at a valley current sense threshld of 30mV typical. If V
is tied to INTV
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
threshold voltage V
verse transfer capacitance C
I
DS(MAX)
V
RNG
OUT
.
= 7.5 • (Maximum V
RNG
is chosen with low enough ESR to meet output
DS
DS
. In practice, one should allow margin, to ac-
CC
voltage that appears between the PGND
Sense Voltage and V
, the device operates at a valley current
RNG
GS(TH)
is tied to SGND, the device operates
RNG
RNG
, on-resistance R
DS
. The current mode control
RSS
pin between 0.2V and 2V,
Sense Voltage)
RNG
and maximum current
RNG
RNG
DS
pin and is approxi-
IN
CC
is:
is selected for its
CC
sense voltage is
to force internal
Pin
can be used
DS(ON)
BR(DSS)
RNG
, re-
RNG
pin
,
The gate drive voltages are set by the 5.3V INTV
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
margin is required to accommodate the rise in MOSFET
on-resistance with temperature.
The ρ
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
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:
R
D
D
DS ON MAX
BOT
TOP
T
(
term is a normalization factor (unity at 25°C)
=
=
)(
2.0
1.5
1.0
0.5
V
V
0
V
–50
OUT
IN
DS(ON)(MAX)
IN
Figure 1. R
–
V
)
IN
V
=
T
OUT
JUNCTION TEMPERATURE (°C)
Max V
= 1.3 is reasonable.
0
DS(ON)
at 25°C. In this case additional
DS
50
I
vs Temperature
O
Sense Voltage
•
ρ
T
100
LTC3879
3879 F01
150
CC
11
supply.
3879f