LTC3770EUH#TR Linear Technology, LTC3770EUH#TR Datasheet - Page 11

• Manufacturer Part Number: LTC3770EUH#TR
• Manufacturer: Linear Technology

Specifications of LTC3770EUH#TR

• Lead Free Status / RoHS Status: Not Compliant

APPLICATIONS INFORMATION
The basic LTC3770 application circuit is shown in Figure 12.
External component selection is primarily determined by
the maximum load current and begins with the selection
of the sense resistance and power MOSFET switches. The
LTC3770 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
large RMS current into the converter and C
with low enough ESR to meet the output voltage ripple
and transient specifi cation.
Maximum Sense Voltage and V
Inductor current is determined by measuring the voltage
across a sense resistance that appears between the SENSE
(PGND on G package) and SENSE
pins. The maximum sense voltage is set by the voltage
applied to the V
(0.133)V
the inductor current valleys to exceed (0.133)V
In practice, one should allow some margin for variations
in the LTC3770 and external component values and a good
guide for selecting the sense resistance is:
An external resistive divider from INTV
to set the voltage of the V
resulting in nominal sense voltages of 50mV to 200mV.
Additionally, the V
in which case the nominal sense voltage defaults to 50mV
or 200mV, respectively. The maximum allowed sense volt-
age is about 1.33 times this nominal value. However tying
the V
current comparator offset is now a bigger proportion of
the total sense voltage.
Connecting the SENSE
The LTC3770 comes in UH and G packages. The UH package
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
R
SENSE
RNG
RNG
pin to ground is not recommended because the
=
. The current mode control loop will not allow
10 •
IN
RNG
I
V
RNG
is selected for its ability to handle the
OUT MAX
RNG
pin and is equal to approximately
(
pin can be tied to SGND or INTV
+
and SENSE
RNG
)
pin between 0.5V and 2V
RNG
+
–
(SW on G package)
Pin
Pins
CC
OUT
can be used
RNG
is chosen
/R
SENSE
+
and
CC
–
.
SENSE
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
This improves effi ciency, but one must carefully choose
the MOSFET on-resistance as discussed below.
Power MOSFET Selection
The LTC3770 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
transfer capacitance C
The gate drive voltage is set by the 5V INTV
Consequently, logic-level threshold MOSFETs must be used
in LTC3770 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
additional margin is required to accommodate the rise in
MOSFET on-resistance with temperature:
R
DS ON MAX
(
–
pins to the top and bottom of the sense resistor.
)(
+
2.0
1.5
1.0
0.5
pin to the SW pin and SENSE
0
Figure 1. R
–50
)
=
(GS)TH
R
JUNCTION TEMPERATURE (°C)
SENSE
DS(ON)(MAX)
RSS
ρ
0
T
DS(ON)
, on-resistance R
and maximum current I
50
vs Temperature
at 25°C. In this case,
100
LTC3770
3770 F01
–
DS(ON)
pin to PGND.
150
CC
, reverse
DS(MAX)
(BR)DSS
11
supply.
3770fc
.
,