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參數(shù)資料
| 型號(hào)： | LTC3770 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Synchronous Controller with Margining, Tracking and PLL |
| 中文描述： | 同步控制器裕，跟蹤和PLL |
| 文件頁數(shù)： | 13/24頁 |
| 文件大?。?/td> | 302K |
| 代理商： | LTC3770 |
13
LTC3770
3770f
When there is no R
ON
resistor connected to the I
ON
pin, the
on-time t
ON
is theoretically infinite, which in turn could
damage the converter. To prevent this, the LTC3770 will
detect this fault condition and provide a minimum I
ON
current of 5
μ
A to 10
μ
A.
Changes in the load current magnitude will cause fre-
quency shift. Parasitic resistance in the MOSFET switches
and inductor reduce the effective voltage across the induc-
tance, resulting in increased duty cycle as the load current
increases. By lengthening the on-time slightly as current
increases, constant frequency operation can be main-
tained. 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 applica-
tion. 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 3a. 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 3b.
Minimum Off-Time and Dropout Operation
The minimum off-time t
OFF(MIN)
is the smallest amount of
time that the LTC3770 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 4.
C
0.01
μ
F
R
100k
R
30k
C
C
V
OUT
R
C
(3a)
(3b)
V
ON
I
TH
LTC3770
C
0.01
μ
F
R
VON2
10k
Q1
2N5087
R
VON1
10k
C
C
3770 F03
V
OUT
INTV
CC
R
C
V
ON
I
TH
LTC3770
Figure 3. 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
3770 F04
1.0
DROPOUT
REGION
S
Figure 4. Maximum Switching Frequency vs Duty Cycle
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
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of I
OUT(MAX)
. The largest ripple current
occurs at the highest V
IN
. To guarantee that ripple current
does not exceed a specified maximum, the inductance
APPLICATIU
W
U
U
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| LTC3770EUH#PBF | 功能描述:IC REG CTRLR BUCK PWM CM 32-QFN 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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