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參數(shù)資料
| 型號(hào)： | 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)： | 12/13頁 |
| 文件大?。?/td> | 497K |
| 代理商： | SI786 |
Si786
Vishay Siliconix
S-60752—Rev. G, 05-Apr-99
12
FaxBack 408-970-5600, request 70189
www.siliconix.com
OPERATIONAL MODES
PWM Mode
The 3.3-V and 5-V Buck controllers operate in continuous-
current PWM mode when the load demands more than
approximately 25% of the maximum current (see typical
curves). The duty cycle can be approximated as Duty_Cycle =
V
OUT
/V
IN
.
In this mode, the inductor current is continuous; in the first half
of the cycle, the current slopes up when the high-side
MOSFET conducts and then, in the second half, slopes back
down when the inductor is providing energy to the output
capacitor and load. As current enters the inductor in the first
half-cycle, it is also continuing through to the load; hence, the
load is receiving continuous current from the inductor. By
using this method, output ripple is minimized and smaller
form-factor inductors can be used. The output capacitor’s
ESR has the largest effect on output ripple. It is typically under
50 mV; the worst case condition is under light load with higher
input battery voltage.
Pulse-Skipping Mode
When the load requires less than 25% of its maximum, the
Si786 enters a mode which drives the gate for one clock cycle
and skips the majority of the remaining cycles. Pulse-skipping
mode cuts down on the switching losses, the dominant power
consumer at low current levels.
In the region between pulse-skipping mode and PWM mode,
the controller may transition between the two modes,
delivering spurts of pulses. This may cause the current
waveform to look irregular, but will not overly affect the ripple
voltage. Even in this transitional mode efficiency will stay high.
Current Limit
The current through an external resistor, is constantly
monitored to protect against over-current. A low value resistor
is placed in series with the inductor. The voltage across it is
measured by connecting it between CS_ and FB_. If this
voltage is larger than 100 mV, the high-side MOSFET drive is
shut down. Eliminating over-currents protects the MOSFET,
the load and the power source. Typical values for the sense
resistors with a 3-A load will be 25 m
.
Oscillator and SYNC
There are two ways to set the Si786 oscillator frequency: by
using an external SYNC signal, or using the internal oscillator.
The SYNC pin can be driven with an external CMOS level
signal with frequency from 240 kHz and 350 kHz to
synchronize to the internal oscillator. Tying SYNC to either V
L
or GND sets the frequency to 200 kHz and to REF sets the
frequency to 300 kHz.
Operation at 300 kHz is typically used to minimize output
passive component sizes. Slower switching speeds of
200 kHz may be needed for lower input voltages.
Internal V
L
and REF
A 5-V linear regulator supplies power to the internal logic
circuitry. The regulator is available for external use from pin
V
L
, able to source 5 mA. A 4.7-μF capacitor should be
connected between V
L
and GND. To increase efficiency, when
the 5 V switching supply has voltage greater than 4.5 V, V
L
is
internally switched over to the output of the 5-V switching
supply and the linear regulator is turned off.
The 5-V linear regulator provides power to the internal 3.3-V
bandgap reference (REF). The 3.3-V reference can supply
5 mA to an external load, connected to pin REF. Between
REF and GND connect a capacitor, 0.22 μF plus 1 μF per mA
of load current. The switching outputs will vary with the
reference; therefore, placing a load on the REF pin will cause
the main outputs to decrease slightly, within the specified
regulation tolerance.
V
L
and REF supplies stay on as long as V+ is greater than
4.5 V, even if the switching supplies are not enabled. This
feature is necessary when using the micropower regulators to
keep memory alive during shutdown.
Both linear regulators can be connected to their respective
switching supply outputs. For example, REF would be tied to
the output of the 3.3 V and V
L
to 5 V. This will keep the main
supplies up in standby mode, provided that each load current
in shutdown is not larger than 5 mA.
Fault Protection
The 3.3 V and 5 V switching controllers as well as the
comparators are shut down when one of the linear regulators
drops below 85% of its nominal value; that is, shut down will
occur when V
L
< 4.0 V or REF < 2.8 V.
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