參數(shù)資料
型號(hào)： 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(wú)檢測(cè)電阻范圍步降控制器
文件頁(yè)數(shù)： 12/24頁(yè)
文件大?。?/td> 303K
代理商： LTC1778
12
LTC1778/LTC1778-1
1778fa
Because the voltage at the I
ON
pin is about 0.7V, the
current into this pin is not exactly inversely proportional to
V
IN
, especially in applications with lower input voltages.
To correct for this error, an additional resistor R
ON2
connected from the I
ON
pin to the 5V INTV
CC
supply will
further stabilize the frequency.
R
V
VR
ON
ON
2
5
.
0 7
=
Changes in the load current magnitude will also cause
frequency shift. Parasitic resistance in the MOSFET
switches and inductor reduce the effective voltage across
the inductance, resulting in increased duty cycle as the
load current increases. By lengthening the on-time slightly
as current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the I
TH
pin to the V
ON
pin and V
OUT
. The values
required will depend on the parasitic resistances in the
specific application. A good starting point is to feed about
25% of the voltage change at the I
TH
pin to the V
ON
pin as
shown in Figure 4a. Place capacitance on the V
ON
pin to
filter out the I
TH
variations at the switching frequency. The
resistor load on I
TH
reduces the DC gain of the error amp
and degrades load regulation, which can be avoided by
using the PNP emitter follower of Figure 4b.
Minimum Off-time and Dropout Operation
The minimum off-time t
OFF(MIN)
is the smallest amount of
time that the LTC1778 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
MOSFET back off. This time is generally about 250ns. The
minimum off-time limit imposes a maximum duty cycle of
t
ON
/(t
ON
+ t
OFF(MIN)
). If the maximum duty cycle is reached,
due to a dropping input voltage for example, then the
output will drop out of regulation. The minimum input
voltage to avoid dropout is:
V
V
t
t
t
IN MIN
(
OUT
ON
OFF MIN
(
ON
)
)
=
+
A plot of maximum duty cycle vs frequency is shown in
Figure 5.
Inductor Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple
current:
=
I
V
f L
V
V
L
OUT
OUT
IN
1
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
APPLICATIOU
W
U
U
C
0.01
μ
F
R
VON2
100k
R
30k
C
C
V
OUT
R
C
(4a)
(4b)
V
ON
I
TH
LTC1778
C
0.01
μ
F
R
VON2
10k
Q1
2N5087
R
VON1
3k
10k
C
C
1778 F04
V
OUT
INTV
CC
R
C
V
ON
I
TH
LTC1778
Figure 4. Correcting Frequency Shift with Load Current Changes
2.0
1.5
1.0
0.5
0
0
0.25
DUTY CYCLE (V
OUT
/V
IN
)
0.50
0.75
1778 F05
1.0
DROPOUT
REGION
S
Figure 5. Maximum Switching Frequency vs Duty Cycle
相關(guān)PDF資料
PDF描述
LTC1779 250mA Low Dropout Current Mode Step-Down DC/DC Converter in SOT-23(2.5V輸出,250mA電流模式，低壓差,步降DC/DC變換器（SOT-23封裝）)
LTC1798CS8 Micropower Low Dropout References(可調(diào)輸出,微功耗，低壓差穩(wěn)壓器)
LTC1798CS8-5 Micropower Low Dropout References(5V輸出,微功耗，低壓差穩(wěn)壓器)
LTC1798CS8-2.5 Micropower Low Dropout References(2.5V輸出,微功耗，低壓差穩(wěn)壓器)
LTC1798CS8-3 Micropower Low Dropout References(3V輸出,微功耗，低壓差穩(wěn)壓器)
相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
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(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(wú) 升壓:是 回掃:無(wú) 反相:無(wú) 倍增器:無(wú) 除法器:無(wú) Cuk:無(wú) 隔離:無(wú) 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(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(wú) 升壓:是 回掃:無(wú) 反相:無(wú) 倍增器:無(wú) 除法器:無(wú) Cuk:無(wú) 隔離:無(wú) 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(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(wú) 升壓:是 回掃:無(wú) 反相:無(wú) 倍增器:無(wú) 除法器:無(wú) Cuk:無(wú) 隔離:無(wú) 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(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(wú) 升壓:是 回掃:無(wú) 反相:無(wú) 倍增器:無(wú) 除法器:無(wú) Cuk:無(wú) 隔離:無(wú) 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(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(wú) 升壓:是 回掃:無(wú) 反相:無(wú) 倍增器:無(wú) 除法器:無(wú) Cuk:無(wú) 隔離:無(wú) 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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