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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)： | 9/24頁 |
| 文件大?。?/td> | 303K |
| 代理商： | LTC1778-1 |
9
LTC1778/LTC1778-1
1778fa
OPERATIOU
Main Control Loop
The LTC1778 is a current mode controller for DC/DC
step-down converters. In normal operation, the top
MOSFET is turned on for a fixed interval determined by a
one-shot timer OST. When the top MOSFET is turned off,
the bottom MOSFET is turned on until the current com-
parator I
CMP
trips, restarting the one-shot timer and initi-
ating the next cycle. Inductor current is determined by
sensing the voltage between the PGND and SW pins using
the bottom MOSFET on-resistance . The voltage on the I
TH
pin sets the comparator threshold corresponding to in-
ductor valley current. The error amplifier EA adjusts this
voltage by comparing the feedback signal V
FB
from the
output voltage with an internal 0.8V reference. If the load
current increases, it causes a drop in the feedback voltage
relative to the reference. The I
TH
voltage then rises until the
average inductor current again matches the load current.
At low load currents, the inductor current can drop to zero
and become negative. This is detected by current reversal
comparator I
REV
which then shuts off M2, resulting in
discontinuous operation. Both switches will remain off
with the output capacitor supplying the load current until
the I
TH
voltage rises above the zero current level (0.8V) to
initiate another cycle. Discontinuous mode operation is
disabled by comparator F when the FCB pin is brought
below 0.8V, forcing continuous synchronous operation.
The operating frequency is determined implicitly by the
top MOSFET on-time and the duty cycle required to
maintain regulation. The one-shot timer generates an on-
time that is proportional to the ideal duty cycle, thus
holding frequency approximately constant with changes
in V
IN
. The nominal frequency can be adjusted with an
external resistor R
ON
.
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback voltage
exits a
±
7.5% window around the regulation point.
Furthermore, in an overvoltage condition, M1 is turned off
and M2 is turned on and held on until the overvoltage
condition clears.
Foldback current limiting is provided if the output is
shorted to ground. As V
FB
drops, the buffered current
threshold voltage I
THB
is pulled down by clamp Q3 to a 1V
level set by Q4 and Q6. This reduces the inductor valley
current level to one sixth of its maximum value as V
FB
approaches 0V.
Pulling the RUN/SS pin low forces the controller into its
shutdown state, turning off both M1 and M2. Releasing
the pin allows an internal 1.2
μ
A current source to charge
up an external soft-start capacitor C
SS
. When this voltage
reaches 1.5V, the controller turns on and begins switch-
ing, but with the I
TH
voltage clamped at approximately
0.6V below the RUN/SS voltage. As C
SS
continues to
charge, the soft-start current limit is removed.
INTV
CC
/EXTV
CC
Power
Power for the top and bottom MOSFET drivers and most
of the internal controller circuitry is derived from the
INTV
CC
pin. The top MOSFET driver is powered from a
floating bootstrap capacitor C
B
. This capacitor is re-
charged from INTV
CC
through an external Schottky diode
D
B
when the top MOSFET is turned off. When the EXTV
CC
pin is grounded, an internal 5V low dropout regulator
supplies the INTV
CC
power from V
IN
. If EXTV
CC
rises
above 4.7V, the internal regulator is turned off, and an
internal switch connects EXTV
CC
to INTV
CC
. This allows
a high efficiency source connected to EXTV
CC
, such as an
external 5V supply or a secondary output from the
converter, to provide the INTV
CC
power. Voltages up to
7V can be applied to EXTV
CC
for additional gate drive. If
the input voltage is low and INTV
CC
drops below 3.5V,
undervoltage lockout circuitry prevents the power
switches from turning on.
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| LTC1778EGN | 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#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) |
| LTC1778EGN-1 | 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) |
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