LTC3823
11
3823fd
APPLICATIONS INFORMATION
The basic LTC3823 application circuit is shown in
Figure 12. External component selection is primarily de-
termined by the maximum load current and begins with
the selection of the sense resistance and power MOSFET
switches. The LTC3823 uses either a sense resistor or
the on-resistance of the synchronous power MOSFET for
determining the inductor current. The desired amount of
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 the output voltage
ripple and transient specifi cation.
Maximum Sense Voltage and V
RNG
Pin
Inductor current is determined by measuring the volt-
age across a sense resistance that appears between the
SENSE
–
and SENSE
+
pins. The maximum sense voltage
is set by the voltage applied to the V
RNG
pin and is equal
to approximately (0.133)V
RNG
. The current mode control
loop will not allow the inductor current valleys to exceed
(0.133)V
RNG
/R
SENSE
. In practice, one should allow some
margin for variations in the LTC3823 and external com-
ponent values and a good guide for selecting the sense
resistance is:
R
V
I
SENSE
RNG
OUT MAX
=
10 •
()
An external resistive divider from INTV
CC
can be used
to set the voltage of the V
RNG
pin between 0.5V and 2V
resulting in nominal sense voltages of 50mV to 200mV.
Additionally, the V
RNG
pin can be tied to SGND or INTV
CC
in
which case the nominal sense voltage defaults to 50mV or
200mV, respectively. The maximum allowed sense voltage
is about 1.33 times this nominal value.
Connecting the SENSE
+
and SENSE
–
Pins
The IC can be used with or without a sense resistor. When
using a sense resistor, place it between the source of the
bottom MOSFET, M2, and PGND. Connect the SENSE
+
and
SENSE
–
pins to the top and bottom of the sense resistor.
Using a sense resistor provides a well defi ned current
limit, but adds cost and reduces effi ciency. Alternatively,
one can eliminate the sense resistor and use the bottom
MOSFET as the current sense element by simply connecting
the SENSE
+
pin to the SW pin and SENSE
–
pin to PGND.
This improves effi ciency, but one must carefully choose
the MOSFET on-resistance as discussed below.
Power MOSFET Selection
The LTC3823 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)
, reverse
transfer capacitance C
RSS
and maximum current I
DS(MAX)
.
The gate drive voltage is set by the 5V INTV
CC
supply.
Consequently, logic-level threshold MOSFETs must be used
in LTC3823 applications. If the input voltage is expected
to drop below 5V, then sub-logic level threshold MOSFETs
should be considered.
When the bottom MOSFET is used as the current sense
element, particular attention must be paid to its on-resis-
tance. MOSFET on-resistance is typically specifi ed 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
R
DS ON MAX
SENSE
T
()( )
=
ρ
The ρ
T
term is a normalization factor (unity at 25°C) ac-
counting 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 ρ
T
= 1.3 is reasonable.
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and the
load current. When the LTC3823 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
=
=
–
