LTC3608 datasheet(14/24 Pages) LINER

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LTC3608

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Since ΔIL increases with input voltage, the output ripple

is highest at maximum input voltage. Typically, once the

ESR requirement is satisﬁed, the capacitance is adequate

for ﬁltering and has the necessary RMS current rating.

Multiple capacitors placed in parallel may be needed to

meet the ESR and RMS current handling requirements.

Dry tantalum, special polymer, aluminum electrolytic and

ceramic capacitors are all available in surface mount pack-

ages. 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. Aluminum electrolytic

capacitors have signiﬁcantly higher ESR, but can be used

in cost-sensitive applications providing that consideration

is given to ripple current ratings and long term reliability.

Ceramic capacitors have excellent low ESR characteris-

tics but can have a high voltage coefﬁcient and audible

piezoelectric effects. The high Q of ceramic capacitors with

trace inductance can also lead to signiﬁcant ringing. When

used as input capacitors, care must be taken to ensure

that ringing from inrush currents and switching does not

pose an overvoltage hazard to the power switches and

controller. To dampen input voltage transients, add a small

5μF to 50μF aluminum electrolytic capacitor with an ESR in

the range of 0.5Ω to 2Ω. High performance through-hole

capacitors may also be used, but an additional ceramic

capacitor in parallel is recommended to reduce the effect

of their lead inductance.

Top MOSFET Driver Supply (CB, DB)

An external bootstrap capacitor CBconnectedtotheBOOST

pin supplies the gate drive voltage for the topside MOSFET.

This capacitor is charged through diode DB from INTVCC

when the switch node is low. When the top MOSFET turns

on, the switch node rises to VIN and the BOOST pin rises

to approximately VIN + INTVCC. The boost capacitor needs

to store about 100 times the gate charge required by the

top MOSFET. In most applications an 0.1μF to 0.47μF, X5R

or X7R dielectric capacitor is adequate.

Discontinuous Mode Operation and FCB Pin

The FCB pin determines whether the bottom MOSFET

remains on when current reverses in the inductor. Tying

this pin above its 0.6V threshold enables discontinuous

operation where the bottom MOSFET turns off when in-

ductor current reverses. The load current at which current

reverses and discontinuous operation begins depends on

the amplitude of the inductor ripple current and will vary

with changes in VIN. Tying the FCB pin below the 0.6V

threshold forces continuous synchronous operation, al-

lowing current to reverse at light loads and maintaining

high frequency operation.

In addition to providing a logic input to force continuous

operation, the FCB pin provides a means to maintain a

ﬂyback winding output when the primary is operating

in discontinuous mode. The secondary output VOUT2 is

normally set as shown in Figure 4 by the turns ratio N

of the transformer. However, if the controller goes into

discontinuous mode and halts switching due to a light

primary load current, then VOUT2 will droop. An external

resistor divider from VOUT2 to the FCB pin sets a minimum

voltage VOUT2(MIN) below which continuous operation is

forced until VOUT2 has risen above its minimum:

VOUT2(MIN) = 0.6V 1+

Fault Conditions: Current Limit and Foldback

The LTC3608 has a current mode controller which inher-

ently limits the cycle-by-cycle inductor current not only

in steady state operation but also in transient. To further

limit current in the event of a short circuit to ground, the

LTC3608 includes foldback current limiting. If the output

falls by more than 25%, then the maximum sense voltage is

progressively lowered to about one sixth of its full value.

INTVCC Regulator and EXTVCC Connection

An internal P-channel low dropout regulator produces the

5V supply that powers the drivers and internal circuitry

APPLICATIONS INFORMATION

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