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參數(shù)資料
| 型號： | LTC1778-1 |
| 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)： | 11/24頁 |
| 文件大?。?/td> | 303K |
| 代理商： | LTC1778-1 |
11
LTC1778/LTC1778-1
1778fa
APPLICATIOU
with temperature, typically about 0.4%/
°
C as shown in
Figure 2. 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 LTC1778 is operating in
continuous mode, the duty cycles for the MOSFETs are:
W
U
U
D
V
V
D
V
V
V
TOP
OUT
IN
BOT
IN
OUT
IN
=
=
–
The resulting power dissipation in the MOSFETs at maxi-
mum output current are:
P
TOP
= D
TOP
I
OUT(MAX)2
ρ
T(TOP)
R
DS(ON)(MAX)
+ k V
IN2
I
OUT(MAX)
C
RSS
f
P
BOT
= D
BOT
I
OUT(MAX)2
ρ
T(BOT)
R
DS(ON)(MAX)
Both MOSFETs have I
2
R losses and the top MOSFET
includes an additional term for transition losses, which are
largest at high input voltages. The constant k = 1.7A
–1
can
be used to estimate the amount of transition loss. The
bottom MOSFET losses are greatest when the bottom duty
cycle is near 100%, during a short-circuit or at high input
voltage.
Operating Frequency
The choice of operating frequency is a tradeoff between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC1778 applications is deter-
mined implicitly by the one-shot timer that controls the
on-time t
ON
of the top MOSFET switch. The on-time is set
by the current into the I
ON
pin and the voltage at the V
ON
pin (LTC1778-1) according to:
t
V
I
pF
ON
VON
ION
=
(
)
10
V
ON
defaults to 0.7V in the LTC1778.
Tying a resistor R
ON
from V
IN
to the I
ON
pin yields an on-
time inversely proportional to V
IN
. For a step-down con-
verter, this results in approximately constant frequency
operation as the input supply varies:
V
V
R
pF
VON
ON
(
10
To hold frequency constant during output voltage changes,
tie the V
ON
pin to V
OUT
or to a resistive divider from V
OUT
when V
OUT
> 2.4V. The V
ON
pin has internal clamps that
limit its input to the one-shot timer. If the pin is tied below
0.7V, the input to the one-shot is clamped at 0.7V. Simi-
larly, if the pin is tied above 2.4V, the input is clamped at
2.4V. In high V
OUT
applications, tying V
ON
to INTV
CC
so
that the comparator input is 2.4V results in a lower value
for R
ON
. Figures 3a and 3b show how R
ON
relates to
switching frequency for several common output voltages.
f
H
OUT
Z
=
)[
]
R
ON
(k
)
100
100
S
1000
1000
10000
1778 F03a
V
OUT
= 3.3V
V
OUT
= 1.5V
V
OUT
= 2.5V
R
ON
(k
)
100
100
S
1000
1000
10000
1778 F03b
V
OUT
= 3.3V
V
OUT
= 12V
V
OUT
= 5V
Figure 3a. Switching Frequency vs R
ON
for the LTC1778 and LTC1778-1 (V
ON
= 0V)
Figure 3b. Switching Frequency vs R
ON
for the LTC1778-1 (V
ON
= INTV
CC
)
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| LTC1778EGN#PBF | 功能描述:IC REG CTRLR BUCK PWM CM 16-SSOP RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標準包裝:2,500 系列:- PWM 型:電流模式 輸出數(shù):1 頻率 - 最大:500kHz 占空比:96% 電源電壓:4 V ~ 36 V 降壓:無 升壓:是 回掃:無 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:無 工作溫度:-40°C ~ 125°C 封裝/外殼:24-WQFN 裸露焊盤 包裝:帶卷 (TR) |
| LTC1778EGN#TR | 功能描述:IC REG CTRLR BUCK PWM CM 16-SSOP RoHS:否 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標準包裝:2,500 系列:- PWM 型:電流模式 輸出數(shù):1 頻率 - 最大:500kHz 占空比:96% 電源電壓:4 V ~ 36 V 降壓:無 升壓:是 回掃:無 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:無 工作溫度:-40°C ~ 125°C 封裝/外殼:24-WQFN 裸露焊盤 包裝:帶卷 (TR) |
| LTC1778EGN#TRPBF | 功能描述:IC REG CTRLR BUCK PWM CM 16-SSOP RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標準包裝:2,500 系列:- PWM 型:電流模式 輸出數(shù):1 頻率 - 最大:500kHz 占空比:96% 電源電壓:4 V ~ 36 V 降壓:無 升壓:是 回掃:無 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:無 工作溫度:-40°C ~ 125°C 封裝/外殼:24-WQFN 裸露焊盤 包裝:帶卷 (TR) |
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