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參數(shù)資料
| 型號(hào)： | 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(cè)電阻范圍步降控制器 |
| 文件頁數(shù)： | 15/24頁 |
| 文件大?。?/td> | 303K |
| 代理商： | LTC1778-1 |
15
LTC1778/LTC1778-1
1778fa
INTV
CC
Regulator
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC1778. The INTV
CC
pin can supply up to
50mA RMS and must be bypassed to ground with a
minimum of 4.7
μ
F low ESR tantalum capacitor. Good
bypassing is necessary to supply the high transient cur-
rents required by the MOSFET gate drivers. Applications
using large MOSFETs with a high input voltage and high
frequency of operation may cause the LTC1778 to exceed
its maximum junction temperature rating or RMS current
rating. Most of the supply current drives the MOSFET
gates unless an external EXTV
CC
source is used. In con-
tinuous mode operation, this current is I
GATECHG
= f(Q
g(TOP)
+ Q
g(BOT)
). The junction temperature can be estimated
from the equations given in Note 2 of the Electrical
Characteristics. For example, the LTC1778CGN is limited
to less than 14mA from a 30V supply:
T
J
= 70
°
C + (14mA)(30V)(130
°
C/W) = 125
°
C
For larger currents, consider using an external supply with
the EXTV
CC
pin.
EXTV
CC
Connection
The EXTV
CC
pin can be used to provide MOSFET gate drive
and control power from the output or another external
source during normal operation. Whenever the EXTV
CC
pin is above 4.7V the internal 5V regulator is shut off and
an internal 50mA P-channel switch connects the EXTV
CC
pin to INTV
CC
. INTV
CC
power is supplied from EXTV
CC
until
this pin drops below 4.5V. Do not apply more than 7V to
the EXTV
CC
pin and ensure that EXTV
CC
≤
V
IN
. The follow-
ing list summarizes the possible connections for EXTV
CC
:
1. EXTV
CC
grounded. INTV
CC
is always powered from the
internal 5V regulator.
2. EXTV
CC
connected to an external supply. A high effi-
ciency supply compatible with the MOSFET gate drive
requirements (typically 5V) can improve overall
efficiency.
3. EXTV
CC
connected to an output derived boost network.
The low voltage output can be boosted using a charge
pump or flyback winding to greater than 4.7V. The system
APPLICATIOU
W
U
U
will start-up using the internal linear regulator until the
boosted output supply is available.
External Gate Drive Buffers
The LTC1778 drivers are adequate for driving up to about
30nC into MOSFET switches with RMS currents of 50mA.
Applications with larger MOSFET switches or operating at
frequencies requiring greater RMS currents will benefit
from using external gate drive buffers such as the LTC1693.
Alternately, the external buffer circuit shown in Figure 7
can be used. Note that the bipolar devices reduce the
signal swing at the MOSFET gate, and benefit from an
increased EXTV
CC
voltage of about 6V.
Figure 7. Optional External Gate Driver
Q1
FMMT619
GATE
OF M1
TG
BOOST
SW
Q2
FMMT720
Q3
FMMT619
GATE
OF M2
BG
1778 F07
INTV
CC
PGND
Q4
FMMT720
10
10
Soft-Start and Latchoff with the RUN/SS Pin
The RUN/SS pin provides a means to shut down the
LTC1778 as well as a timer for soft-start and overcurrent
latchoff. Pulling the RUN/SS pin below 0.8V puts the
LTC1778 into a low quiescent current shutdown (I
Q
<
30
μ
A). Releasing the pin allows an internal 1.2
μ
A current
source to charge up the external timing capacitor C
SS
. If
RUN/SS has been pulled all the way to ground, there is a
delay before starting of about:
(
1 2
.
When the voltage on RUN/SS reaches 1.5V, the LTC1778
begins operating with a clamp on I
TH
of approximately
0.9V. As the RUN/SS voltage rises to 3V, the clamp on I
TH
is raised until its full 2.4V range is available. This takes an
additional 1.3s/
μ
F, during which the load current is folded
back until the output reaches 75% of its final value. The pin
can be driven from logic as shown in Figure 7. Diode D1
t
V
AC
μ
s F C
1 3
.
/
DELAY
SS
SS
=
=
)
1 5
.
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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(biāo)準(zhǔn)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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