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參數(shù)資料
| 型號： | 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)： | 12/24頁 |
| 文件大小： | 302K |
| 代理商： | LTC3770 |
12
LTC3770
3770f
with temperature, typically about 0.4%/
°
C as shown in
Figure 1. 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 LTC3770 is operating in
continuous mode, the duty cycles for the MOSFETs are:
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 LTC3770 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 out of the I
ON
pin and the voltage at the V
ON
pin according to:
t
V
I
pF
ON
VON
ION
=
(
)
10
Tying a resistor R
ON
to SGND from the I
ON
pin yields an on-
time inversely proportional to 1/3 V
IN
. The current out of
the I
ON
pin is:
V
R
ON
For a step-down converter, this results in approximately
constant frequency operation as the input supply varies:
V
V
R
pF
VON
ON
(
3
10
To hold frequency constant during output voltage changes,
tie the V
ON
pin to V
OUT
. The V
ON
pin has internal clamps
that limit its input to the one-shot timer. If the pin is tied
below 0.6V, the input to the one-shot is clamped at 0.6V.
Similarly, if the pin is tied above 4.8V, the input is clamped
at 4.8V. In high V
OUT
applications, tie V
ON
to INTV
CC
.
Figures 2a and 2b show how R
ON
relates to switching
frequency for several common output voltages.
I
ION
IN
=
3
f
H
OUT
Z
=
)[
]
R
ON
(k
)
100
100
S
1000
1000
3770 F02a
V
OUT
= 3.3V
V
OUT
= 1.5V
V
OUT
= 2.5V
R
ON
(k
)
10
100
S
1000
100
1000
3770 F02b
V
OUT
= 3.3V
V
OUT
= 12V
V
OUT
= 5V
Figure 2a. Switching Frequency vs R
ON
(V
ON
= 0V)
Figure 2b. Switching Frequency vs R
ON
(V
ON
= INTV
CC
)
APPLICATIU
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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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