Si786

Vishay Siliconix

www.vishay.com

12

Document Number: 70189

S-40807—Rev. J, 26-Apr-04

FIGURE 4.

Boost Supply for Gate Drivers

Level

Translator

DH_

BST_

LX_

DL_

PWM

BATTERY

INPUT

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

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 mW.