LTC3809-1_15 Datasheet, PDF(17/24 Page) Linear Technology – No RSENSE, Low Input Voltage, Synchronous DC/DC Controller with Output Tracking

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LTC3809-1_15 Datasheet, PDF (17/24 Pages) Linear Technology – No RSENSE, Low Input Voltage, Synchronous DC/DC Controller with Output Tracking

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APPLICATIONS INFORMATION

VOUT

LTC3809-1

VOUT

TIME

(5b) Coincident Tracking

TIME

38091 F05b,c

(5c) Ratiometric Tracking

Figure 5b and 5c. Two Different Modes of Output Voltage Tracking

For coincident tracking (VOUT = VX during start-up),

RTA = RA, RTB = RB

VX should always be greater than VOUT when using the

tracking function of TRACK/SS pin.

The internal current source (1Î¼A), which is for external

soft-start, will cause a tracking error at VOUT. For example,

if a 59k resistor is chosen for RTA, the RTA current will be

about 10Î¼A (600mV/59k). In this case, the 1Î¼A internal

current source will cause about 10% (1Î¼A/10Î¼A â¢ 100%)

tracking error, which is about 60mV (600mV â¢ 10%)

referred to VFB. This is acceptable for most applications.

If a better tracking accuracy is required, the value of RTA

should be reduced.

Table 1 summarizes the different states in which the

TRACK/SS can be used.

Table 1. The States of the TRACK/SS Pin

TRACK/SS Pin

FREQUENCY

Capacitor CSS

VIN

External Soft-Start

Internal Soft-Start

Resistor Divider

VOUT Tracking an External Voltage VX

Auxiliary Winding Control Using the MODE Pin

The MODE pin can be used as an auxiliary feedback to

provide a means of regulating a ï¬yback winding output.

When this pin drops below its ground-referenced 0.4V

threshold, continuous mode operation is forced.

During continuous mode, current ï¬ows continuously in

the transformer primary side. The auxiliary winding draws

current only when the bottom synchronous N-channel

MOSFET is on. When primary load currents are low and/

or the VIN/VOUT ratio is close to unity, the synchronous

MOSFET may not be on for a sufï¬cient amount of time to

transfer power from the output capacitor to the auxiliary

load. Forced continuous operation will support an auxiliary

winding as long as there is a sufï¬cient synchronous

MOSFET duty factor. The MODE input pin removes

the requirement that power must be drawn from the

transformer primary side in order to extract power from

the auxiliary winding. With the loop in continuous mode,

the auxiliary output may nominally be loaded without

regard to the primary output load.

38091fc

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