LTC4267CDHC datasheet(22/32 Pages) LINEAR

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LTC4267

4267fc

APPLICATIO S I FOR ATIO

Programmable Slope Compensation

The LTC4267 switching regulator injects a ramping current

through its SENSE pin into an external slope compensation

resistor (RSL in Figure 11). This current ramp starts at

zero after the NGATE pin has been high for the LTC4267’s

minimum duty cycle of 6%. The current rises linearly to-

wards a peak of 5µA at the maximum duty cycle of 80%,

shutting off once the NGATE pin goes low. A series resis-

tor (RSL) connecting the SENSE pin to the current sense

resistor (RSENSE) develops a ramping voltage drop. From

the perspective of the LTC4267 SENSE pin, this ramping

voltage adds to the voltage across the sense resistor,

effectively reducing the current comparator threshold in

proportion to duty cycle. This stabilizes the control loop

against subharmonic oscillation. The amount of reduction

in the current comparator threshold (∆VSENSE) can be

calculated using the following equation:

∆VSENSE = 5µA • RSL • [(Duty Cycle – 6%)/74%]

Note: The LTC4267 enforces 6% < Duty Cycle < 80%.

Designs not needing slope compensation may replace RSL

with a short-circuit.

Applications Employing a Third Transformer Winding

A standard operating topology may employ a third

winding on the transformer’s primary side that provides

power to the LTC4267 switching regulator via its PVCC pin

(Figure 11). However, this arrangement is not inherently

self-starting. Start-up is usually implemented by the use of

an external “trickle-charge” resistor (RSTART) in conjunc-

tion with the internal wide hysteresis undervoltage lockout

circuit that monitors the PVCC pin voltage.

RSTART is connected to VPORTP and supplies a current,

typically 100µA, to charge CPVCC. After some time, the

voltage on CPVCC reaches the PVCC turn-on threshold. The

LTC4267 switching regulator then turns on abruptly and

draws its normal supply current. The NGATE pin begins

switching and the external MOSFET (Q1) begins to deliver

power. The voltage on CPVCC begins to decline as the

switching regulator draws its normal supply current, which

exceeds the delivery from RSTART.Aftersometime,typically

tens of milliseconds, the output voltage approaches the

desired value. By this time, the third transformer winding

is providing virtually all the supply current required by the

LTC4267 switching regulator.

One potential design pitfall is under-sizing the value of

capacitor CPVCC. In this case, the normal supply current

drawn through PVCC will discharge CPVCC rapidly before the

third winding drive becomes effective. Depending on the

particular situation, this may result in either several off-on

cycles before proper operation is reached or permanent

relaxation oscillation at the PVCC node.

Resistor RSTART should be selected to yield a worst-case

minimum charging current greater that the maximum rated

LTC4267 start-up current to ensure there is enough current

to charge CPVCC to the PVCC turn-on threshold. RSTART

should also be selected large enough to yield a worst-case

maximum charging current less than the minimum-rated

PVCC supply current, so that in operation, most of the

PVCC current is delivered through the third winding. This

results in the highest possible efﬁciency.

Capacitor CPVCCshouldthenbemadelargeenoughtoavoid

the relaxation oscillation behavior described previously.

This is difﬁcult to determine theoretically as it depends on

the particulars of the secondary circuit and load behavior.

Empirical testing is recommended.

The third transformer winding should be designed so

that its output voltage, after accounting for the forward

diode voltage drop, exceeds the maximum PVCC turn-off

threshold. Also, the third winding’s nominal output voltage

should be at least 0.5V below the minimum rated PVCC

clamp voltage to avoid running up against the LTC4267

shunt regulator, needlessly wasting power.

PVCC Shunt Regulator

In applications including a third transformer winding,

the internal PVCC shunt regulator serves to protect the

LTC4267 switching regulator from overvoltage transients

as the third winding is powering up.

If a third transformer winding is undesirable or unavail-

able, the shunt regulator allows the LTC4267 switching

regulator to be powered through a single dropping resistor

from VPORTP as shown in Figure 12. This simplicity comes

at the expense of reduced efﬁciency due to static power

dissipation in the RSTART dropping resistor.

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