LTC1778EGN Linear Technology, LTC1778EGN Datasheet - Page 10

• Manufacturer Part Number: LTC1778EGN
• Description: IC CONTROLLR STP-DWN SYNC 16SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1778EGN

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Current - Output: 2A
• Voltage - Input: 4 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Output: -
• Power - Output: -
• Frequency - Switching: -

APPLICATIO S I FOR ATIO
LTC1778/LTC1778-1
The basic LTC1778 application circuit is shown in
Figure 1. 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 LTC1778 uses the on-resistance of the
synchronous power MOSFET for determining the induc-
tor current. The desired amount of ripple current and
operating frequency largely determines the inductor value.
Finally, C
RMS current into the converter and C
low enough ESR to meet the output voltage ripple and
transient specification.
Choosing the LTC1778 or LTC1778-1
The LTC1778 has an open-drain PGOOD output that
indicates when the output voltage is within ±7.5% of the
regulation point. The LTC1778-1 trades the PGOOD pin for
a V
V
in high V
means to adjust the on-time to maintain constant fre-
quency operation in applications where V
to correct minor frequency shifts with changes in load
current. Finally, the V
additional current limiting in positive-to-negative convert-
ers and as a control input to synchronize the switching
frequency with a phase locked loop.
Maximum Sense Voltage and V
Inductor current is determined by measuring the voltage
across a sense resistance that appears between the PGND
and SW pins. The maximum sense voltage is set by the
voltage applied to the V
mately (0.133)V
not allow the inductor current valleys to exceed
(0.133)V
margin for variations in the LTC1778 and external compo-
nent values and a good guide for selecting the sense
resistance is:
An external resistive divider from INTV
set the voltage of the V
10
ON
R
ON
pin high results in lower values for R
SENSE
pin that allows the on-time to be adjusted. Tying the
RNG
IN
OUT
=
is selected for its ability to handle the large
/R
10 •
applications. The V
SENSE
RNG
I
V
OUT MAX
U
RNG
. The current mode control loop will
. In practice, one should allow some
(
ON
RNG
RNG
U
)
pin can be used to provide
pin and is equal to approxi-
pin between 0.5V and 2V
RNG
ON
W
pin also provides a
Pin
OUT
CC
ON
OUT
can be used to
is chosen with
which is useful
changes and
U
resulting in nominal sense voltages of 50mV to 200mV.
Additionally, the V
in which case the nominal sense voltage defaults to 70mV
or 140mV, respectively. The maximum allowed sense
voltage is about 1.33 times this nominal value.
Power MOSFET Selection
The LTC1778 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 LTC1778 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-
resistance. MOSFET on-resistance is typically specified
with a maximum value R
additional margin is required to accommodate the rise in
MOSFET on-resistance with temperature:
The ρ
accounting for the significant variation in on-resistance
R
DS ON MAX
T
(
term is a normalization factor (unity at 25°C)
)(
2.0
1.5
1.0
0.5
0
– 50
Figure 2. R
)
=
RNG
JUNCTION TEMPERATURE (°C)
(GS)TH
R
0
SENSE
RSS
ρ
pin can be tied to SGND or INTV
T
DS(ON)
DS(ON)(MAX)
, on-resistance R
and maximum current I
50
vs. Temperature
100
at 25°C. In this case,
1778 F02
DS(ON)
150
CC
, reverse
DS(MAX)
(BR)DSS
supply.
1778fb
CC
,
.