LTC3703 datasheet(12/32 Pages) LINER

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LTC3703

3703f

The basic LTC3703 application circuit is shown in

Figure 1. External component selection is determined by

the input voltage and load requirements as explained in the

following sections. After the operating frequency is se-

lected, RSET and L can be chosen. The operating frequency

and the inductor are chosen for a desired amount of ripple

current and also to optimize efficiency and component

size. Next, the power MOSFETs and D1 are selected based

on voltage, load and efficiency requirements. CIN is se-

lected for its ability to handle the large RMS currents in the

converter and COUT is chosen with low enough ESR to

meet the output voltage ripple and transient specifica-

tions. Finally, the loop compensation components are

chosen to meet the desired transient specifications.

Operating Frequency

The choice of operating frequency and inductor value is a

trade off between efficiency and component size. Low

frequency operation improves efficiency by reducing

MOSFET switching losses and gate charge losses. How-

ever, lower frequency operation requires more induc-

tance for a given amount of ripple current, resulting in a

larger inductor size and higher cost. If the ripple current

is allowed to increase, larger output capacitors may be

required to maintain the same output ripple. For convert-

ers with high step-down VIN to VOUT ratios, another

consideration is the minimum on-time of the LTC3703

(see the Minimum On-time Considerations section). A

final consideration for operating frequency is that in

APPLICATIO S I FOR ATIO

OPERATIO

noise-sensitive communications systems, it is often de-

sirable to keep the switching noise out of a sensitive

frequency band.

The LTC3703 uses a constant frequency architecture that

can be programmed over a 100kHz to 600kHz range with

a single resistor from the fSET pin to ground, as shown in

Figure 1. The nominal voltage on the fSET pin is 1.2V, and

the current that flows from this pin is used to charge and

discharge an internal oscillator capacitor. The value of

RSET for a given operating frequency can be chosen from

Figure 7 or from the following equation:

f kHz

SET ()

() –

Ω=

7100

Buck or Boost Mode Operation

The LTC3703 has the capability of operating both as a

step-down (buck) and step-up (boost) controller. In boost

mode, output voltages as high as 80V can be tightly

regulated. With the INV pin grounded, the LTC3703 oper-

ates in buck mode with TG driving the main (top side)

switch and BG driving the synchronous (bottom side)

switch. If the INV pin is pulled above 2V, the LTC3703

operates in boost mode with BG driving the main (bottom

side) switch and TG driving the synchronous (top side)

switch. Internal circuit operation is very similar regardless

of the operating mode with the following exceptions: In

boost mode, Pulse Skip Mode operation is always dis-

abled regardless of the level of the MODE/SYNC pin and

the line feedforward compensation is also disabled. The

overcurrent circuitry continues to monitor the load current

by looking at the drain voltage of the main (bottom side)

MOSFET. In boost mode, however, the peak MOSFET

current does not equal the load current but instead

ID = ILOAD/(1 – D). This factor needs to be taken into

account when programming the IMAX voltage.

FREQUENCY (kHz)

1000

3703 F07

100

200

1000

800

600

400

Figure 7. Timing Resistor (RSET) Value

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