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參數(shù)資料
| 型號： | LTC1778 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Wide Operating Range, No RSENSE Step-Down Controller |
| 中文描述： | 寬工作，無檢測電阻范圍步降控制器 |
| 文件頁數(shù)： | 10/24頁 |
| 文件大小： | 303K |
| 代理商： | LTC1778 |
10
LTC1778/LTC1778-1
1778fa
APPLICATIOU
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
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 specification.
W
U
U
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
ON
pin that allows the on-time to be adjusted. Tying the
V
ON
pin high results in lower values for R
ON
which is useful
in high V
OUT
applications. The V
ON
pin also provides a
means to adjust the on-time to maintain constant fre-
quency operation in applications where V
OUT
changes and
to correct minor frequency shifts with changes in load
current. Finally, the V
ON
pin can be used to provide
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
RNG
Pin
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
RNG
pin and is equal to approxi-
mately (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 LTC1778 and external compo-
nent values and a good guide for selecting the sense
resistance is:
R
V
I
OUT MAX
(
SENSE
RNG
=
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 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
(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 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
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
ρ
T
)(
)
=
The
ρ
T
term is a normalization factor (unity at 25
°
C)
accounting for the significant variation in on-resistance
Figure 2. R
DS(ON)
vs. Temperature
JUNCTION TEMPERATURE (
°
C)
–50
ρ
T
1.0
1.5
150
1778 F02
0.5
0
0
50
100
2.0
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