SI786LSG-E3【PDF 12/16 页】IC REG QD BUCK/LINEAR 28SSOP

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Sheet目录46

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www.vishay.com

Document Number: 70189

S-40807-Rev. J, 26-Apr-04

Vishay Siliconix

Si786

Product is End of Life 3/2014

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

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 capacitors

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 domi-

nant power consumer at low current levels.

In the region between pulse-skipping mode and PWM mode,

the controller may transition between the two modes, deliv-

ering 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 moni-

tored 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 volt-

age 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?

Figure 4. Boost Supply for Gate Drivers

Level

Translator

DH_

BST_

LX_

DL_

PWM

BATTERY

INPUT

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