LTC3722-2_15 Datasheet, PDF(17/28 Page) Linear Technology – Synchronous Dual Mode Phase Modulated Full Bridge Controllers

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LTC3722-2_15 Datasheet, PDF (17/28 Pages) Linear Technology – Synchronous Dual Mode Phase Modulated Full Bridge Controllers

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LTC3722-1/LTC3722-2

OPERATION

is designed so that the positive voltage square wave is

produced while the output inductor is freewheeling. An

advantage of this technique over the previous is that it

does not require a separate filter inductor and since the

voltage is derived from the well regulated output voltage, it

is also well controlled. One disadvantage is that this wind-

ing will require the same safety isolation that is required

for the main transformer. Another disadvantage is that a

much larger VCC filter capacitor is needed, since it does

not generate a voltage as the output is first starting up,

or during short-circuit conditions.

Programming the LTC3722-1/LTC3722-2 Oscillator

The high accuracy LTC3722-1/LTC3722-2 oscillator circuit

provides flexibility to program the switching frequency,

slope compensation, and synchronization with minimal

external components. The LTC3722-1/LTC3722-2 oscillator

circuitry produces a 2.2V peak-to-peak amplitude ramp

waveform on CT and a narrow pulse on SYNC that can be

used to synchronize other PWM chips. Typical maximum

duty cycles of 98.5% are obtained at 300kHz and 96% at

1MHz. A compensating slope current is derived from the

oscillator ramp waveform and sourced out of CS.

The desired amount of slope compensation is selected

with single external resistor. A capacitor to GND on CT

programs the switching frequency. The CT ramp discharge

current is internally set to a high value (>10mA). The dedi-

cated SYNC I/O pin easily achieves synchronization. The

LTC3722-1/LTC3722-2 can be set up to either synchronize

other PWM chips or be synchronized by another chip or

external clock source. The 1.8V SYNC threshold allows the

LTC3722-1/LTC3722-2 to be synchronized directly from

all standard 3V and 5V logic families.

Design Procedure:

1. Choose CT for the desired oscillator frequency. The

switching frequency selected must be consistent with

the power magnetics and output power level. In general,

increasing the switching frequency will decrease the

maximum achievable output power, due to limitations

of maximum duty cycle imposed by transformer core

reset and ZVS. Remember that the tranformer fre-

quency is one-half that of the oscillator.

(13.4 k â¢

fOSC )

Example: Desired fOSC = 330kHz

CT = 1/(13.4k â¢ fOSC) = 226pF, choose closest standard

value of 220pF. A 5% or better tolerance multilayer NPO

or X7R ceramic capacitor is recommended for best

performance.

2. The LTC3722-1/LTC3722-2 can either synchronize other

PWMs, or be synchronized to an external frequency

source or PWM chip (see Figure 8 for details).

CT OF SLAVE(S) IS

1.25 CT OF MASTER.

LTC3722

SYNC

5.1k

MASTER

5.1k

â¢

â¢ 5.1k

â¢

UP TO

5 SLAVES

SYNC

LTC3722

SLAVES

372212 F08a

Figure 8a. SYNC Output (Master Mode)

EXTERNAL

FREQUENCY

SOURCE

AMPLITUDE > 1.8V

100ns < PW < 0.4/Æ

SYNC

LTC3722

5.1k

372212 F08b

Figure 8b. SYNC Input from an External Source

For more information www.linear.com/LTC3722

372212fb

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