Document Number: 70190

S11-0975-Rev. G, 16-May-11

www.vishay.com

11

Vishay Siliconix

Si9130

This document is subject to change without notice.

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.

Soft-Start

To slowly bring up the 3.3 V and 5 V supplies, connect

starts a 4 A constant current source to charge these

capacitors to 4 V. As the voltage on these pins ramps up, so

does the current limit comparator threshold, to increase the

controller to reach the terminal over-current level within

10 µs.

Soft start helps prevent current spikes at turn-on and allows

separate

supplies

to

be

delayed

using

external

programmability.

Synchronous Rectifiers

Synchronous rectification replaces the Schottky rectifier with

a MOSFET, which can be controlled to increase the

efficiency of the circuit.

When the high-side MOSFET is switched off, the inductor will

try to maintain its current flow, inverting the inductor’s

polarity. The path of current then becomes the circuit made

of the Schottky diode, inductor and load, which will charge

the output capacitor. The diode has a 0.5 V forward voltage

drop, which contributes a significant amount of power loss,

decreasing efficiency. A low-side switch is placed in parallel

with the Schottky diode and is turned on just after the diode

low, the I*R voltage drop will not be as large as the diode,

which increases efficiency.

The low-side rectifier is shut off when the inductor current

drops to zero.

Shoot-through current is the result when both the high-side

and rectifying MOSFETs are turned on at the same time.

Break-before-make timing internal to the Si9130 manages

this potential problem. During the time when neither

MOSFET is on, the Schottky is conducting, so that the body

diode in the low-side MOSFET is not forced to conduct.

Synchronous rectification is always active when the Si9130

is powered-up, regardless of the operational mode.

Gate-Driver Boost

The high-side N-Channel drive is supplied by a flying-

capacitor boost circuit (see Figure 4). The capacitor takes a

the high-side MOSFET to provide gate enhancement. At

power-up, the low-side MOSFET pulls LX_ down to GND

and charges the BST_ capacitor connected to 5 V. During

the second half of the oscillator cycle, the controller drives

the gate of the high-side MOSFET by internally connecting

node BST_ to DH_. This supplies a voltage 5 V higher than

the battery voltage to the gate of the high-side MOSFET.

Oscillations on the gates of the high-side MOSFET in

discontinuous mode are a natural occurrence caused by the

LC network formed by the inductor and stray capacitance at

the LX_ pins. The negative side of the BST_ capacitor is

connected to the LX_ node, so ringing at the inductor is

translated through to the gate drive.

Figure 4. Boost Supply for Gate Drivers

Level

Translator

DH_

BST_

LX_

DL_

PWM

BATTERY

INPUT