LTC1778 datasheet(14/24 Pages) LINER

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LTC1778/LTC1778-1

1778fb

but an additional ceramic capacitor in parallel is recom-

mended to reduce the effect of their lead inductance.

Top MOSFET Driver Supply (CB, DB)

An external bootstrap capacitor CB connectedtotheBOOST

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 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 0.8V threshold enables discontinuous

operation where the bottom MOSFET turns off when

inductor current reverses. The load current at which

current reverses and discontinuous operation begins de-

pends on the amplitude of the inductor ripple current and

will vary with changes in VIN. Tying the FCB pin below the

0.8V threshold forces continuous synchronous operation,

allowing 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 nor-

mally set as shown in Figure 6 by the turns ratio N of the

transformer. However, if the controller goes into discon-

tinuous 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.

OUT MIN

()

⎛

⎝

⎜

⎞

⎠

⎟

Fault Conditions: Current Limit and Foldback

The maximum inductor current is inherently limited in a

current mode controller by the maximum sense voltage. In

the LTC1778, the maximum sense voltage is controlled by

the voltage on the VRNG pin. With valley current control,

the maximum sense voltage and the sense resistance

determine the maximum allowed inductor valley current.

The corresponding output current limit is:

LIMIT

SNS MAX

DS ON

=+ ∆

()

() ρ

The current limit value should be checked to ensure that

ILIMIT(MIN) > IOUT(MAX). The minimum value of current limit

generally occurs with the largest VIN at the highest ambi-

ent temperature, conditions that cause the largest power

loss in the converter. Note that it is important to check for

self-consistency between the assumed MOSFET junction

temperature and the resulting value of ILIMIT which heats

the MOSFET switches.

Caution should be used when setting the current limit

based upon the RDS(ON) of the MOSFETs. The maximum

current limit is determined by the minimum MOSFET on-

resistance. Data sheets typically specify nominal and

maximum values for RDS(ON), but not a minimum. A

reasonable assumption is that the minimum RDS(ON) lies

the same amount below the typical value as the maximum

lies above it. Consult the MOSFET manufacturer for further

guidelines.

To further limit current in the event of a short circuit to

ground, the LTC1778 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.

APPLICATIO S I FOR ATIO

Figure 6. Secondary Output Loop and EXTVCC Connection

VIN

LTC1778

SGND

FCB

EXTVCC

OPTIONAL

EXTVCC

CONNECTION

5V < VOUT2 < 7V

1778 F06

1:N

PGND

COUT2

1µF

VOUT1

VOUT2

VIN

CIN

1N4148

• +

COUT

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