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參數(shù)資料
| 型號： | LTC3442EDE |
| 廠商： | Linear Integrated Systems |
| 英文描述： | Micropower Synchronous Buck-Boost DC/DC Converter with Automatic Burst Mode Operation |
| 中文描述： | 微功率同步降壓升壓與自動突發(fā)模式的DC / DC轉(zhuǎn)換器操作 |
| 文件頁數(shù)： | 9/20頁 |
| 文件大小： | 332K |
| 代理商： | LTC3442EDE |
LTC3442
9
3442f
Reverse Current Limit
During fixed frequency operation, the LTC3442 operates
in forced continuous conduction mode. The reverse cur-
rent limit amplifier monitors the inductor current from
the output through switch D. Once the negative inductor
current exceeds 500mA typical, the IC will shut off switch D.
Four-Switch Control
Figure 1 shows a simplified diagram of how the four
internal switches are connected to the inductor, V
IN
, V
OUT
and GND. Figure 2 shows the regions of operation for the
LTC3442 as a function of the internal control voltage, V
CI
.
Depending on the control voltage, the IC will operate in
either buck, buck/boost or boost mode. The V
CI
voltage is
a level shifted voltage from the output of the error amp (V
C
)
(see Figure 5). The four power switches are properly
phased so the transfer between operating modes is con-
tinuous, smooth and transparent to the user. When V
IN
approaches V
OUT
the buck/boost region is reached where
the conduction time of the four switch region is typically
150ns. Referring to Figures 1 and 2, the various regions of
operation will now be described.
Buck Region (V
IN
> V
OUT
)
Switch D is always on and switch C is always off during this
mode. When the internal control voltage, V
CI
, is above
voltage V1, output A begins to switch. During the off-time
of switch A, synchronous switch B turns on for the
remainder of the time. Switches A and B will alternate
similar to a typical synchronous buck regulator. As the
control voltage increases, the duty cycle of switch A
increases until the maximum duty cycle of the converter in
buck mode reaches DMAX_BUCK, given by:
DMAX_BUCK = 100 – D4
SW
%
where D4
SW
= duty cycle % of the four switch range.
D4
SW
= (150ns f) 100 %
where f = operating frequency, Hz.
Beyond this point the “four switch,” or buck/boost region
is reached.
Buck/Boost or Four Switch (V
IN
~ V
OUT
)
When the internal control voltage, V
CI
, is above voltage V2,
switch pair AD remain on for duty cycle DMAX_BUCK, and
the switch pair AC begins to phase in. As switch pair AC
phases in, switch pair BD phases out accordingly. When
the V
CI
voltage reaches the edge of the buck/boost range,
at voltage V3, the AC switch pair completely phase out the
BD pair, and the boost phase begins at duty cycle D4
SW
.
The input voltage, V
IN
, where the four switch region begins
is given by:
V
V
150
ns )
IN
OUT
=
1
–(
The point at which the four switch region ends is given by:
V
IN
= V
OUT
(1 – D) = V
OUT
(1 – 150ns f) V
4
SW1
6
SW2
PMOS A
NMOS B
9
V
IN
PMOS D
NMOS C
3442 F01
8
V
OUT
Figure 1. Simplified Diagram of Output Switches
Figure 2. Switch Control vs Internal Control Voltage, V
CI
88%
D
MAX
BOOST
D
BOOST
D
BUCK
DUTY
CYCLE
0%
V4 (
≈
2.05V)
V3 (
≈
1.65V)
BOOST REGION
BUCK REGION
BUCK/BOOST REGION
V2 (
≈
1.55V)
V1 (
≈
0.9V)
3442 F02
A ON, B OFF
PWM CD SWITCHES
D ON, C OFF
PWM AB SWITCHES
FOUR SWITCH PWM
INTERNAL
CONTROL
VOLTAGE, V
CI
OPERATIOU
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