LTC1773EMS データシートの表示（PDF） - Linear Technology

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LTC1773EMS

Synchronous Step-Down DC/DC Controller

Linear Technology 

LTC1773

APPLICATIONS INFORMATION

The ITH pin OPTI-LOOP compensation components can be

optimized to provide stable, high performance transient

response regardless of the output capacitors selected.

Manufacturers such as Nichicon, United Chemicon and

Sanyo should be considered for high performance through-

hole capacitors. The OS-CON semiconductor dielectric

capacitor available from Sanyo has the lowest ESR/size

ratio of any aluminum electrolytic at a somewhat higher

price. Once the ESR requirement for COUT has been met,

the RMS current rating generally far exceeds the

IRIPPLE(P-P) requirement.

In surface mount applications multiple capacitors may

have to be paralleled to meet the ESR or RMS current

handling requirements of the application. Aluminum elec-

trolytic and dry tantalum capacitors are both available in

surface mount configurations. In the case of tantalum, it is

critical that the capacitors are surge tested for use in

switching power supplies. An excellent choice is the AVX

TPS series of surface mount tantalum, available in case

heights ranging from 2mm to 4mm. Other capacitor types

include Sanyo OS-CON and POSCAP, Nichicon PL series,

Panisonic SP series and Sprague 593D and 595D series.

Consult the manufacturer for other specific recommenda-

tions.

Output Voltage Programming

The output voltage is set by a resistive divider according

to the following formula:

VOUT

=

⎛

0.8V⎝⎜1+

R2⎞

R1⎠⎟

(2)

The external resistive divider is connected to the output as

shown in Figure 3, allowing remote voltage sensing.

0.8V ≤ VOUT ≤ 8.5V

R2

VFB

LTC1773

R1

GND

1773 F03

Figure 3. Setting the LTC1773 Output Voltage

10

Run/Soft-Start Function

The RUN/SS pin is a dual purpose pin that provides the

soft-start function and a means to shut down the LTC1773.

Soft-start reduces surge currents from VIN by gradually

increasing the internal current limit. Power supply se-

quencing can also be accomplished using this pin.

An internal 1.5µA current source charges up an external

capacitor CSS. When the voltage on RUN/SS reaches 0.7V

the LTC1773 begins operating. As the voltage on RUN/SS

continues to ramp from 0.7V to 1.8V, the ITH clamp is also

ramped at a proportionally linear rate. Depending on the

external RSENSE used, the peak inductor current, and thus

the internal current limit, rises with the RUN/SS voltage.

The output current thus ramps up slowly, charging the

output capacitor. If RUN/SS has been pulled all the way to

ground, there will be a delay before the current starts

increasing and is given by:

( ) t DELAY

=

0.7 C SS

1.5µA

=

0.47s / µF

C SS

Pulling the RUN/SS pin below 0.4V puts the LTC1773 into

a low quiescent current shutdown mode (IQ < 10µA). This

pin can be driven directly from logic as shown in Figure 4.

Diode D1 in Figure 4 reduces the start delay but allows CSS

to ramp up slowly providing the soft-start function. This

diode can be deleted if soft-start is not needed.

3.3V OR 5V

RUN/SS

D1

CSS

RUN/SS

CSS

1773 F04

Figure 4. RUN/SS Pin Interfacing

Auxiliary Winding Control Using SYNC/FCB Pin

The SYNC/FCB pin can be used as a secondary feedback

to provide a means of regulating a flyback winding output.

When this pin drops below its ground referenced 0.8V

threshold, continuous mode operation is forced. In con-

tinuous mode, the P-channel main and N-channel syn-

chronous switches are switched continuously regardless

of the load on the main output.

1773fb

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