LTC1922-1_15 Datasheet, PDF(21/24 Page) Linear Technology – Synchronous Phase Modulated Full-Bridge Controller

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LTC1922-1_15 Datasheet, PDF (21/24 Pages) Linear Technology – Synchronous Phase Modulated Full-Bridge Controller

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OPERATIO

slower. A linearized SPICE macromodel of the control loop

is very helpful tool to quickly evaluate the frequency

response of various compensation networks.

Polymer Electrolytic (see Figure 12) 1/(2ÏCCRI) sets a

low frequency pole. 1/(2ÏCCRF) sets the low frequency

zero. The zero frequency should coincide with the worst-

case lowest output pole frequency. The pole frequency

and mid frequency gain (RF/RI) should be set such so that

the loop crosses over zero dB with a â1 slope at a

frequency lower than (fSW/8). Use a bode plot to graphi-

cally display the frequency response. An optional higher

frequency pole set by CP2 and Rf is used to attenuate

switching frequency noise.

VOUT

COMP

VOUT

CP2

OPTIONAL

OPTO

CO RI

REF

COLL

ESR RD 2.5V

LT1431 OR EQUIVALENT

PRECISION ERROR

AMP AND REFERENCE

1922 F12

Figure 12. Compensation for Polymer Electrolytic

Aluminum Electrolytic (see Figure 12) the goal of this

compensator will be to cross over the output minimum

pole frequency. Set a low frequency pole with CC and RIN

at a frequency that will cross over the loop at the output

pole minimum F, place the zero formed by CC and Rf at the

output pole F.

LTC1922-1

Current Doubler

The current doubler secondary employs two output induc-

tors that equally share the output load current. The trans-

former secondary is not center-tapped. This configuration

provides 2Ã higher output current capability compared to

similarly sized single output inductor modules, hence the

name. Each output inductor is twice the inductance value

as the equivalent single inductor configuration and the

transformer turns ratio is 1/2 that of a single inductor

secondary. The drive to the inductors is 180 degrees out

of phase which provides partial ripple current cancellation

in the output capacitor(s). Reduced capacitor ripple cur-

rent lowers output voltage ripple and enhances the

capacitorsâs reliability. The amount of ripple cancellation

is related to duty cycle (see Figure 13). Although the

current doubler requires an additional inductor, the induc-

tor core volume is proportional to LI2, thus the size penalty

is small. The transformer construction is simplified with-

out a center-tap winding and the turns ratio is reduced by

1/2 compared to a conventional full wave rectifier configu-

ration.

NOTE: INDUCTOR(S) DUTY CYCLE

IS LIMITED TO 50% WITH CURRENT

DOUBLER PHASE SHIFT CONTROL.

NORMALIZED

OUTPUT RIPPLE

CURRENT

ATTENUATION

Synchronous Rectification

The LTC1922-1 produces the precise timing signals nec-

essary to control current doubler secondary side synchro-

nous MOSFETs on OUTE and OUTF. Synchronous rectifi-

ers are used in place of Schottky or Silicon diodes on the

secondary side of the power supply. As MOSFET RDS(ON)

levels continue to drop, significant efficiency improve-

ments can be realized with synchronous rectification,

provided that the MOSFET switch timing is optimized. An

additional benefit realized with synchronous rectifiers is

bipolar output current capability. These characteristics

improve transient response, particularly overshoot, and

improve ZVS ability at light loads.

0.25

0.5

DUTY CYCLE

1922 â¢ F13

Figure 13. Ripple Current Cancellation vs Duty Cycle

Synchronous rectification of the current doubler second-

ary requires two ground referenced N-channel MOSFETs.

The timing of the LTC1922-1 drive signals is shown in the

Timing Diagram. Synchronous rectifier turn-on is inter-

nally delayed by the LTC1922-1 after OUT (C or D)

turn-offâjust after the end of a power cycle. Synchronous

rectifier turn-off occurs coincident with OUT (A or B)

turn-off. This gives a passive transition time margin before

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