Si9110/9111

Vishay Siliconix

www.vishay.com

6

Document Number: 70004

S-42037—Rev. H, 15-Nov-04

DETAILED DESCRIPTION (CONT’D)

Applications which use a separate external reference, such as

non-isolated converter topologies and circuits employing

optical coupling in the feedback loop, do not require a trimmed

voltage reference with 1% accuracy. The Si9111

accommodates the requirements of these applications at a

lower cost, by leaving the reference voltage untrimmed. The

10% accurate reference thus provided is sufficient to establish

a dc bias point for the error amplifier.

Error Amplifier

Closed-loop regulation is provided by the error amplifier, which

is intended for use with “around-the-amplifier” compensation.

A MOS differential input stage provides for low input current.

connected to the output of the reference supply and should be

bypassed with a small capacitor to ground.

Oscillator Section

The oscillator consists of a ring of CMOS inverters, capacitors,

and a capacitor discharge switch. Frequency is set by an

external resistor between the OSC IN and OSC OUT pins.

(See Figure 5 for details of resistor value vs. frequency.) The

oscillator operation. A frequency divider in the logic section

limits switch duty cycle to v 50% by locking the switching

frequency to one half of the oscillator frequency.

Remote synchronization is accomplished by capacitive

coupling of a positive SYNC pulse into the OSC IN (pin 8)

terminal. For a 5-V pulse amplitude and 0.5-ms pulse width,

typical values would be 100 pF in series with 3 kW to pin 8.

SHUTDOWN and RESET

SHUTDOWN (pin 11) and RESET (pin 12) are intended for

overriding the output MOSFET switch via external control

logic. The two inputs are fed through a latch preceding the

output switch. Depending on the logic state of RESET,

SHUTDOWN can be either a latched or unlatched input. The

output is off whenever SHUTDOWN is low. By simultaneously

having SHUTDOWN and RESET low, the latch is set and

SHUTDOWN has no effect until RESET goes high. The truth

table for these inputs is given in Table 1.

Table 1:

Truth Table for the SHUTDOWN and RESET Pins

SHUTDOWN

RESET

Output

H

H

Normal Operation

H

Normal Operation (No Change)

L

H

Off (Not Latched)

L

L

Off (Latched)

L

Off (Latched, No Change)

Both pins have internal current source pull-ups and should be

left disconnected when not in use. An added feature of the

current sources is the ability to connect a capacitor and an

open-collector driver to the SHUTDOWN or RESET pins to

provide variable shutdown time.

Output Driver

The push-pull driver output has a typical on-resistance of 20 W.

Maximum switching times are specified at 75 ns for a 500-pF

load. This is sufficient to directly drive MOSFETs such as the

2N7004, 2N7005, IRFD120 and IRFD220. Larger devices can

be driven, but switching times will be longer, resulting in higher

switching losses. In order to drive large MOSPOWER devices,

it is necessary to use an external driver IC, such as the

Vishay Siliconix D469A. The D469A can switch very large

devices such as the SMM20N50 (500 V, 0.3 W) in

approximately 100 ns.