LTC4267CDHC datasheet(26/32 Pages) LINEAR

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LTC4267

4267fc

transformer voltage is higher than the PSE voltage, the

LTC4267 switching regulator will draw power from the

transformer. In this situation, it is necessary to address the

issue of power cycling that may occur if a PSE is present.

The PSE will detect the PD and apply power. If the switcher

is being powered by the wall transformer, then the PD will

not meet the minimum load requirement and the PSE will

subsequently remove power. The PSE will again detect

the PD and power cycling will start. With a transformer

voltage above the PSE voltage, it is necessary to either

disable the signature, as shown in option 2, or install a

minimum load on the output of the LTC4267 interface to

prevent power cycling.

The third option also applies power directly to the LTC4267

switching regulator, bypassing the LTC4267 interface

controller and omitting diode D9. With the diode omit-

ted, the transformer voltage is applied to the LTC4267

interface controller in addition to the switching regulator.

For this reason, it is necessary to ensure that the trans-

former maintain the voltage between 38V and 57V to keep

the LTC4267 interface controller in its normal operating

range. The third option has the advantage of automatically

disabling the 25kΩ signature resistor when the external

voltage exceeds the PSE voltage.

Power-Up Sequencing the LTC4267

The LTC4267 consists of two functional cells, the PD

interface and the switching regulator, and the power up

sequencing of these two cells must be carefully considered.

The PD designer should ensure that the switching regulator

does not begin operation until the interface has completed

charging up the load capacitor. This will ensure that the

switcher load current does not compete with the load

capacitor charging current provided by the PD interface

current limit circuit. Overlooking this consideration may

result in slow power supply ramp up, power-up oscillation,

and possibly thermal shutdown.

The LTC4267 includes a power good signal in the PD inter-

face that can be used to indicate to the switching regulator

that the load capacitor is fully charged and ready to handle

the switcher load. Figure 7 shows two examples of ways

the ⎯P⎯W⎯R⎯G⎯D signal can be used to control the switching

regulator. The ﬁrst example employs an N-channel MOSFET

to drive the ITH/RUN port below the shutdown threshold

(typically 0.28V). The second example drives PVCC below

the PVCCturn-offthreshold.Employingthesecondexample

has the added advantage of adding delay to the switching

regulator start-up beyond the time the power good signal

becomes active. The second example ensures additional

timing margin at start-up without the need for added delay

components. In applications where it is not desirable to

utilize the power good signal, sufﬁcient timing margin can

be achieved with RSTART and CPVCC. RSTART and CPVCC

should be set to a delay of two to three times longer than

the duration needed to charge up C1.

Layout Considerations for the LTC4267

The most critical layout considerations for the LTC4267

are the placement of the supporting external components

associated with the switching regulator. Efﬁciency, stability,

and load transient response can deteriorate without good

layout practices around critical components.

For the LTC4267 switching regulator, the current loop

through C1, T1 primary, Q1, and RSENSE must be given

careful layout attention. (Refer to Figure 11.) Because of

the high switching current circulating in this loop, these

components should be placed in close proximity to each

other. In addition, wide copper traces or copper planes

should be used between these components. If vias are

necessary to complete the connectivity of this loop,

placing multiple vias lined perpendicular to the ﬂow of

current is essential for minimizing parasitic resistance and

reducing current density. Since the switching frequency

and the power levels are substantial, shielding and high

frequency layout techniques should be employed. A low

current, low impedance alternate connection should be

employed between the PGND pins of the LTC4267 and the

PGND side of RSENSE, away from the high current loop.

This Kelvin sensing will ensure an accurate representation

of the sense voltage is measured by the LTC4267.

The placement of the feedback resistors R1 and R2 as

well as the compensation capacitor CC is very important

in the accuracy of the output voltage, the stability of the

main control loop, and the load transient response. In

an isolated design application, R1, R2, and CC should be

placed as close as possible to the error ampliﬁer’s input

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