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LTC3633

3633f

APPLICATIONS INFORMATION

The ratio of this divider should be the same as that of the

slave channel’s feedback divider shown in Figure 5a. In

this tracking mode, VOUT1 must be set higher than VOUT2.

To implement the ratiometric tracking, the feedback pin of

the master channel should connect to the TRACKSS pin of

the slave channel (as in Figure 5b). By selecting different

resistors, the LTC3633 can achieve different modes of

tracking including the two in Figure 4.

Upon start-up, the regulator defaults to Burst Mode opera-

tion until the output exceeds 80% of its ﬁnal value (VFB >

0.48V). Once the output reaches this voltage, the operating

mode of the regulator switches to the mode selected by

the MODE/SYNC pin as described above. During normal

operation, if the output drops below 10% of its ﬁnal value

(as it may when tracking down, for instance), the regula-

tor will automatically switch to Burst Mode operation to

prevent inductor saturation and improve TRACKSS pin

accuracy.

Output Power Good

The PGOOD output of the LTC3633 is driven by a 15Ω

(typical) open-drain pull-down device. This device will be

turned off once the output voltage is within 5% (typical) of

the target regulation point, allowing the voltage at PGOOD

to rise via an external pull-up resistor. If the output voltage

exits an 8% (typical) regulation window around the target

regulation point, the open-drain output will pull down with

15Ω output resistance to ground, thus dropping the PGOOD

pin voltage. This behavior is described in Figure 6.

Figure 6. PGOOD Pin Behavior

PGOOD

VOLTAGE

OUTPUT VOLTAGE

NOMINAL OUTPUT

–5%

3633 F06

–8%

VOUT2

(5a) Coincident Tracking Setup

VFB1

TRACKSS2

VFB2

VOUT1

VOUT2

3633 F05

(5b) Ratiometric Tracking Setup

VFB1

TRACKSS2

VFB2

VOUT1

Figure 5. Setup for Coincident and Ratiometric Tracking

A ﬁlter time of 40μs (typical) acts to prevent unwanted

PGOOD output changes during VOUT transient events.

As a result, the output voltage must be within the target

regulation window of 5% for 40μs before the PGOOD pin

pulls high. Conversely, the output voltage must exit the

8% regulation window for 40μs before the PGOOD pin

pulls to ground.

Efﬁciency Considerations

The percent efﬁciency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

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