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參數(shù)資料
| 型號： | Si786 |
| 廠商： | Vishay Intertechnology,Inc. |
| 英文描述： | Dual-Output Power-Supply Controller(用于便攜電腦電源轉(zhuǎn)換的雙輸出步降轉(zhuǎn)換器) |
| 中文描述： | 雙輸出電源控制器（用于便攜電腦電源轉(zhuǎn)換的雙輸出步降轉(zhuǎn)換器） |
| 文件頁數(shù)： | 8/13頁 |
| 文件大小： | 497K |
| 代理商： | SI786 |
Si786
Vishay Siliconix
S-60752—Rev. G, 05-Apr-99
8
FaxBack 408-970-5600, request 70189
www.siliconix.com
DESCRIPTION OF OPERATION
The Si786 is a dual step-down converter, which takes a 5.5-V
to 30-V input and supplies power via two PWM controllers
(Figure 1). These 5-V and 3.3-V supplies run on an optional
300-kHz or 200-kHz internal oscillator, or an external sync
signal. Amount of output current is limited by external
components, but can deliver greater than 6 A on either supply.
As well as these two main Buck controllers, additional loads
can be driven from two micropower linear regulators, one 5 V
(V
L
) and the other 3.3 V (REF) (Figure 2). These supplies are
each rated to deliver 5 mA. If the linear regulator circuits fall
out of regulation, both Buck controllers are shut down.
Two voltage comparators with adjustable output voltages are
included in the Si786. They can be used for gate drive in load
switching applications, where n-channel MOSFETs are used.
Logic level voltages can be generated as well, for instance to
serve as μP interfacing (e.g. a Power-good signal).
3.3-V Switching Supply
The 3.3-V supply is regulated by a current-mode PWM
controller in conjunction with several externals: two n-channel
MOSFETs, a rectifier, an inductor and output capacitors (see
Figure 1). The gate drive supplied by DH
3
needs to be greater
than V
L
, so it is provided by the bootstrap circuit consisting of
a 100-nF capacitor and diode connected to BST
3
.
A low-side switching MOSFET connected to DL
3
increases
efficiency by reducing the voltage across the rectifier diode. A
low value sense resistor in series with the inductor sets the
maximum current limit, to disallow current overloads at power-
on or in short-circuit situations.
The soft-start feature on the Si786 is capacitor programmable;
pin SS
3
functions as a constant current source to the external
capacitor connected to GND. Excess currents at power-on are
avoided, and power-supplies can be sequenced with different
turn-on delay times by selecting the correct capacitor value.
5-V Switching Supply
The 5-V supply is regulated by a current-mode PWM
controller which is nearly the same as the 3.3-V output. The
dropout voltage across the 5-V supply, as shown in the
schematic in Figure 1, is 400 mV (typ) at 2 A. If the voltage at
V+ falls, nearing 5 V, the 5-V supply will lower as well, until the
V
L
linear regulator output falls below the 4-V undervoltage
lockout threshold. Below this threshold, the 5-V controller is
shut off.
The frequency of both PWM controllers is set at 300 kHz
when the SYNC pin is tied to REF. Connecting SYNC to either
GND or V
L
sets the frequency at 200 kHz.
3.3-V and 5-V Switching Controllers
Each PWM controller on the Si786 is identical with the
exception of the preset output voltages. The controllers only
share three functional blocks (Figure 2): the oscillator, the
voltage reference (REF) and the 5-V logic supply (V
L
). The
3.3-V and 5-V controllers are independently enabled with pins
ON
3
and ON
5
, respectively. The PWMs are a direct-summing
type, without the typical integrating error amplifier along with
the phase shift which is a side effect of this type of topology.
Feedback compensation is not needed, as long as the output
capacitance and its ESR requirements are met, according to
the
Design Considerations
section of this data sheet.
The main PWM comparator is an open loop device which is
comprised of three comparators summing four signals: the
feedback voltage error signal, current sense signal, slope-
compensation ramp and voltage reference as shown in
Figure 3. This method of control comes closer to the ideal of
maintaining the output voltage on a cycle-by-cycle basis.
When the load demands high current levels, the controller is in
full PWM mode. Every cycle from the oscillator asserts the
output latch and drives the gate of the high-side MOSFET for
a period determined by the duty cycle (approximately V
OUT
/
V
IN
100%) and the frequency. The high-side switch turns off,
setting the synchronous rectifier latch and 60ns later, the
rectifier MOSFET turns on. The low-side switch stays on until
the start of the next clock cycle in continuous mode, or until
the inductor current becomes positive again in discontinuous
mode. In over-current situations, where the inductor current is
greater than the 100-mV current-limit threshold, the high-side
latch is reset and the high-side gate drive is shut off.
During low-current load requirements, the inductor current will
not deliver the 25-mV minimum current threshold. The
Minimum Current comparator signals the PWM to enter pulse-
skipping mode when the threshold has not been reached.
Pulse-skipping mode skips pulses to reduce switching losses,
the losses which decrease efficiency the most at light load.
Entering this mode causes the minimum current comparator
to reset the high-side latch at the beginning of each oscillator
cycle.
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| SI7860ADP | 制造商:VISHAY 制造商全稱:Vishay Siliconix 功能描述:N-Channel Reduced, Fast Switching MOSFET |
| SI7860ADP-T1-E3 | 功能描述:MOSFET 30V 16A 4.8W 9.5mohm @ 10V RoHS:否 制造商:STMicroelectronics 晶體管極性:N-Channel 汲極/源極擊穿電壓:650 V 閘/源擊穿電壓:25 V 漏極連續(xù)電流:130 A 電阻汲極/源極 RDS（導(dǎo)通）:0.014 Ohms 配置:Single 最大工作溫度: 安裝風(fēng)格:Through Hole 封裝 / 箱體:Max247 封裝:Tube |
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