LTC3823IGN datasheet(15/26 Pages) LINER

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LTC3823

3823fd

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 0.6V 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. To prevent forcing current back

into the main power supply, potentially boosting the input

supply to a dangerous voltage level, forced continuous

mode of operation is disabled when the TRACK/SS volt-

age is 20% below the reference voltage during soft-start

or tracking up. Forced continuous mode of operation is

also disabled when the TRACK/SS voltage is below 0.1V

during tracking down operation. During these two periods,

the PGOOD signal is forced low.

In addition to providing a logic input to forced continu-

ous operation, the FCB pin provides a mean 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 5 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.

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 LTC3823, 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 percentage 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 LTC3823 includes foldback current limiting.

If the output falls by more than 60%, then the maximum

sense voltage is progressively lowered to about one tenth

of its full value.

APPLICATIONS INFORMATION

VIN

LTC3823

SGND

FCB

3823 F05

1:N

PGND

COUT2

1μF

VOUT1

VOUT2

VIN

CIN

1N4148

•

COUT

Figure 5. Secondary Output Loop

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