MIC9131YQS Просмотр технического описания (PDF) - Micrel

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MIC9131YQS

High-Voltage, High-Speed Telecom DC-to-DC Controller

Micrel 

MIC9131

TP_OSCILLATOR= tCHARGE + t DISCHARGE + tDELAY

Where tDELAY= 75ns

fS _

OSCILLATOR

=

TP _

1

OSCILLATOR

fS _ OUTPU

=

1

4

× fS _OSCILLATOR

The timing capacitor, COSC, should be an NPO ceramic or a

temperature stable film capacitor. Care must be taken when

using capacitor values less than 47pF. The high impedance of

a small value capacitor makes the OSC pin more susceptible

to switching noise. Also, the input capacitance of the OSC pin

and the stray capacitance of the board will have a noticeable

effect on the oscillator frequency.

SYNC 5

VBIAS 9

ROSC OSC 4

COSC

3V

11

AGND

75ns

1-shot

VOSC

VCC 2

1µF

SYNC 5

2N3904

VBIAS 9

4.7µF

4.7µF

3V

ROSC

1.6k

OSC 4

COSC

33pF

AGND 11

Micrel, Inc.

75ns

1-shot

Figure 5a

Oscillator Synchronization

The switching frequency of the MIC9131 can be synchronized

to an external oscillator or frequency source. Figure 6 shows

the relationship between the sync input, oscillator waveform

and gate drive output. The external frequency should be set

at least 15% greater than the free running oscillator frequency

to account for tolerances in the oscillator circuit and external

components. The positive edge of the sync signal resets

the oscillator. The sync pulse frequency, like the oscillator,

is four times the gate drive frequency. When an external

sync signal is applied, the peak amplitude of the oscillator

signal (pin 4) is less than when it is free running because

the oscillator signal is terminated before it reaches its 3V

(typical) amplitude. When not used, the sync pin should

be connected to ground to prevent noise from erroneously

resetting the oscillator.

Gate Drive

(pin 16)

tON

tPERIOD

Figure 5

Higher Switching Frequencies

The MIC9131 is capable of very high switching frequencies.

One of the limitations on the maximum frequency is the cur-

rent capability of the 5V regulator supplying the oscillator

and VBIAS. By powering VBIAS with an external source, e.g.

linear regulator much higher switching frequencies can be

achieved. A simple way of using an external current source

is to set an NPN as an emitter follower. Figure 5b shows the

MIC9131 oscillator frequency set to 4MHz using an external

NPN. The emitter followerj circuit allows the current to be

supplied by VCC while the voltage is regulated to a diode

drop below VBIAS. This configuration is quite stable over

temperature and voltage variations.

TIME (500ns/div)

Figure 6. Sync Waveform

Soft Start Circuit

The soft start is programmed by a capacitor on the soft start

pin. A 4µA current source charges up the capacitor. At power

up, the SS pin is discharged. Once the UVLO and enable

functions release the soft start circuit, the voltage of the ca-

pacitor increases. The active voltage range of the soft start

pin is from typically from 0.9V to 1.7V. The internal current

source increases the voltage on the soft start capacitor to

approximately 4V. The soft start pin and the current sense

voltage are connected to a comparator in the MIC9131. The

voltage from the soft start pin effectively limits the peak current

through the current sense resistor by prematurely terminating

the on-time of the gate drive output. Referring to Figure 1,

with the soft start voltage low, the duty cycle of the output is

at a minimum. As the soft start voltage increases, the duty

M9999-080206

12

August 2006

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