LTC3703 datasheet(16/32 Pages) LINER

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LTC3703

3703f

Since

∆IL increases with input voltage, the output ripple is

highest at maximum input voltage. ESR also has a signifi-

cant effect on the load transient response. Fast load

transitions at the output will appear as voltage across the

ESR of COUT until the feedback loop in the LTC3703 can

change the inductor current to match the new load current

value. Typically, once the ESR requirement is satisfied the

capacitance is adequate for filtering and has the required

RMS current rating.

Manufacturers such as Nichicon, United Chemicon and

Sanyo should be considered for high performance

throughhole capacitors. The OS-CON (organic semicon-

ductor dielectric) capacitor available from Sanyo has the

lowest product of ESR and size of any aluminum electro-

lytic at a somewhat higher price. An additional ceramic

capacitor in parallel with OS-CON capacitors is recom-

mended to reduce the effect of their lead inductance.

In surface mount applications, multiple capacitors placed

in parallel may be required to meet the ESR, RMS current

handling and load step requirements. Dry tantalum, spe-

cial polymer and aluminum electrolytic capacitors are

available in surface mount packages. Special polymer

capacitors offer very low ESR but have lower capacitance

density than other types. Tantalum capacitors have the

highest capacitance density but it is important to only use

types that have been surge tested for use in switching

power supplies. Several excellent surge-tested choices

are the AVX TPS and TPSV or the KEMET T510 series.

Aluminum electrolytic capacitors have significantly higher

ESR, but can be used in cost-driven applications providing

that consideration is given to ripple current ratings and

long term reliability. Other capacitor types include

Panasonic SP and Sanyo POSCAPs.

Output Voltage

The LTC3703 output voltage is set by a resistor divider

according to the following formula:

OUT =+









The external resistor divider is connected to the output as

shown in the Functional Diagram, allowing remote voltage

sensing. The resultant feedback signal is compared with

the internal precision 800mV voltage reference by the

error amplifier. The internal reference has a guaranteed

tolerance of

±1%. Tolerance of the feedback resistors will

add additional error to the output voltage. 0.1% to 1%

resistors are recommended.

MOSFET Driver Supplies (DRVCC and BOOST)

The LTC3703 drivers are supplied from the DRVCC and

BOOST pins (see Figure 3), which have an absolute

maximum voltage of 15V. If the main supply voltage, VIN,

is higher than 15V a separate supply with a voltage

between 9V and 15V must be used to power the drivers. If

a separate supply is not available, one can easily be

generated from the main supply using one of the circuits

shown in Figure 10. If the output voltage is between 10V

and 15V, the output can be used to directly power the

drivers as shown in Figure 10a. If the output is below 10V,

Figure 10b shows an easy way to boost the supply voltage

to a sufficient level. This boost circuit uses the LT1613 in

a ThinSOT

TM package and a chip inductor for minimal extra

area (<0.2 in2). Two other possible schemes are an extra

winding on the inductor (Figure 10c) or a capacitive

charge pump (Figure 10d). All the circuits shown in

Figure 10 require a start-up circuit (Q1, D1 and R1) to

provide driver power at initial start-up or following a short-

circuit. The resistor R1 must be sized so that it supplies

sufficient base current and zener bias current at the lowest

expected value of VIN. When using an existing supply, the

supply must be capable of supplying the required gate

driver current which can be estimated from:

IDRVCC = (f)(QG(TOP) + QG(BOTTOM))

This equation for IDRVCC is also useful for properly sizing

the circuit components shown in Figure 10.

An external bootstrap capacitor, CB, connected to the

BOOST pin supplies the gate drive voltage for the topside

MOSFETs. Capacitor CB is charged through external

diode, DB, from the DRVCC supply when SW is low. When

the top side MOSFET is turned on, the driver places the CB

voltage across the gate-source of the top MOSFET. The

switch node voltage, SW, rises to VIN and the BOOST pin

follows. With the topside MOSFET on, the boost voltage

APPLICATIO S I FOR ATIO

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