ISL6721 datasheet(12/21 Pages) INTERSIL

12 / 21 page

FN9110.4

April 13, 2007

condition clears and the soft-start voltage is below the reset

threshold, a soft-start cycle begins.

Ground Plane Requirements

Careful layout is essential for satisfactory operation of the

device. A good ground plane must be employed. A unique

section of the ground plane must be designated for high di/dt

currents associated with the output stage. Power ground

(PGND) can be separated from the logic ground (LGND) and

connected at a single point. VC should be bypassed directly

to PGND with good high frequency capacitors. The return

connection for input power and the bulk input capacitor

should be connected to the PGND ground plane.

Reference Design

The Typical Application Schematic features the ISL6721 in a

conventional dual output 10W discontinuous mode flyback

DC/DC converter. The ISL6721EVAL1 demonstration unit

implements this design and is available for evaluation.

The input voltage range is from 36VDC to 75VDC, and the

two outputs are 3.3V @ 2.5A and 1.8V @ 1.0A. Cross

regulation is achieved using the weighted sum of the two

outputs.

Circuit Element Descriptions

The converter design may be broken down into the following

functional blocks:

Input Storage and Filtering Capacitance: C1, C2, C3

Isolation Transformer: T1

Primary voltage Clamp: CR6, R24, C18

Start Bias Regulator: R1, R2, R6, Q3, VR1

Operating Bias and Regulator: R25, Q2, D1, C5, CR2, D2

Main MOSFET Power Switch: Q1

Current Sense Network: R4, R3, R23, C4

Feedback Network:, R13, R15, R16, R17, R18, R19, R20,

R26, R27, C13, C14, U2, U3

Control Circuit:C7, C8, C9, C10, C11, C12, R5, R6, R8, R9,

R10, R11, R12, R14, R22

Output Rectification and Filtering: CR4, CR5, C15, C16, C19,

C20, C21, C22

Secondary Snubber: R21, C17

Design Criteria

The following design requirements were selected:

Switching Frequency, Fsw: 200kHz

VIN: 36V to 75V

VOUT(1): 3.3V @ 2.5A

VOUT(2): 1.8V @ 1.0A

VOUT(BIAS): 12V @ 50mA

POUT: 10W

Efficiency: 70%

Maximum Duty Cycle, DMAX: 0.45

Transformer Design

The design of a flyback transformer is a non-trivial affair. It is

an iterative process which requires a great deal of

experience to achieve the desired result. It is a process of

many compromises, and even experienced designers will

produce different designs when presented with identical

requirements. The iterative design process is not presented

here for clarity.

The abbreviated design process follows:

• Select a core geometry suitable for the application.

Constraints of height, footprint, mounting preference, and

operating environment will affect the choice.

• Select suitable core material(s).

• Select maximum flux density desired for operation.

• Select core size. Core size will be dictated by the

capability of the core structure to store the required

energy, the number of turns that have to be wound, and

the wire gauge needed. Often the window area (the space

used for the windings) and power loss determine the final

core size. For flyback transformers, the ability to store

energy is the critical factor in determining the core size.

The cross sectional area of the core and the length of the

air gap in the magnetic path determine the energy storage

capability.

• Determine maximum desired flux density. Depending on

the frequency of operation, the core material selected, and

the operating environment, the allowed flux density must

be determined. The decision of what flux density to allow

is often difficult to determine initially. Usually the highest

flux density that produces an acceptable design is used,

but often the winding geometry dictates a larger core than

is required based on flux density and energy storage

calculations.

• Determine the number of primary turns.

• Determine the turns ratio.

• Select the wire gauge for each winding.

• Determine winding order and insulation requirements.

• Verify the design.

Input Power:

Pout/Efficiency = 14.3W (use 15W)

Max On Time: tON(MAX) = DMAX/Fsw = 2.25μs

Average Input Current: IAVG(IN) = PIN/VIN(MIN) = 0.42A

ISL6721

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