LTC1698IGN datasheet(15/24 Pages) LINER

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LTC1698

1698f

If the application generates a bigger current sense voltage,

a potential divider can be easily obtained by adding a

resistor across C2. With this additional resistor, the volt-

age sensed by the current comparator becomes:

DIV

RSENSE

+ (• )

•

An RC network formed by RCILM and CCILM between ICOMP

and VOUT can be used to stabilize the current limit loop.

Connecting the compensation network to VOUT minimizes

output overshoot during start-up or short-circuit recov-

ery. The RCILM and CCILM zero should be chosen to be well

within the closed-loop crossover frequency. This pin can

be left floating if current loop compensation is not re-

quired. The forward converter secondary current limit func-

tion can be disabled by shorting ISNSandISNSGNDtoground.

Auxiliary 3.3V Logic Power Supply

An internal P-channel LDO (low dropout regulator) pro-

duces the 3.3V auxiliary supply that can power external

devices or drive the MARGIN pin. This supply can source

up to 10mA of current and the current limit is provided

internally. The pin requires at least a 0.1

µF bypass

capacitor.

MOSFET Selection

Two logic-level N-channel power MOSFETs (Q3 and Q4 in

Figure 1) are required for most LTC1698 circuits. They are

selected based primarily on the on-resistance and body

diode considerations. The required MOSFET RDS(ON)should

be determined based on input and output voltage, allow-

able power dissipation and maximum required output

current.

The average inductor (L1) current is equal to the output

load current. This current is always flowing through either

Q3 or Q4 with the power dissipation split up according to

the duty cycle:

DC Q

OUT

()

•

()

–

•









where NP/NS is the turns ratio of the transformer T1.

The RDS(ON) required for a given conduction loss can now

be calculated by rearranging the relation P = I2R.

DC Q

IDC Q

DC Q

IDC Q

MAX Q

MAX

DS ON Q

MAX Q

MAX

MAX Q

MAX

DS ON Q

MAX Q

MAX

()

(

)

()

(

)

()

••

(

)

•(

)

••

(

)

•(

)

⇒=

where IMAX is the maximum load current and PMAX is the

allowable conduction loss.

In a typical 2-transistor forward converter circuit, the duty

cycle is less than 50% to prevent the transformer core

from saturating. This results in the duty cycle of Q4 being

greater than that of Q3. Q4 will dissipate more power due

to the higher duty cycle. A lower RDS(ON) MOSFET can be

used for Q4. This will slow down the turn-on time of Q4

since a lower RDS(ON) MOSFET will have a larger gate

capacitance.

The next consideration for the MOSFET is the characteris-

tic of the body diode. The body diodes conduct during the

power-up phase, when the LTC1698 VDD supply is ramp-

ing up and the time-out circuit is adapting to the SYNC

input frequency. The CG and FG signals terminate prema-

turely and the inductor current flows through the body

diodes. The body diodes must be able to take the compa-

rable amount of current as the MOSFETs. Most power

MOSFETs have the same current rating for the body diode

and the MOSFET itself.

The LTC1698 CG and FG MOSFET drivers will dissipate

power. This will increase with higher switching frequency,

higher VDD or larger MOSFETs. To calculate the driver

dissipation, the total gate charge Qg is used. This param-

eter is found on the MOSFET manufacturers data sheet.

The power dissipated in each LTC1698 MOSFET driver is:

PDRIVER = Qg • VDD • fSW

where fSW is the switching frequency of the converter.

APPLICATIO S I FOR ATIO

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