LTC1266CS datasheet(15/20 Pages) LINER

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LTC1266

LTC1266-3.3/LTC1266-5

APPLICATIO S I FOR ATIO

unacceptable levels (see Figure 7). With Burst Mode

operation, the DC supply current represents the lone (and

unavoidable) loss component which continues to become

a higher percentage as output current is reduced. As

expected the I2R losses dominate at high load currents.

Other losses including CIN and COUT ESR dissipative

losses, MOSFET switching losses, Schottky conduction

losses during deadtime and inductor core losses, gener-

ally account for less than 2% total additional loss.

Design Example

As a design example, assume VIN = 5V (nominal),

VOUT = 3.3V, IMAX = 5A and f = 200kHz; RSENSE, CT and L

can immediately be calculated:

RSENSE = 100mV/5 = 0.02Ω

tOFF = (1/200kHz) × [1 – (3.3/5)] = 1.7µs

CT = 1.7µs/(1.3 × 104) = 130pF

LMIN = 5.1 × 105 × 0.02Ω × 130pF × 3.3V = 5µH

Assume that the MOSFET dissipations are to be limited to

PT = PB = 2W.

If TA = 40°C and the thermal resistance of each MOSFET

is 50

°C/W, then the junction temperatures will be 140°C

and

δT = δB = 0.60. The required RDS(ON) for each MOSFET

can now be calculated:

TS RDS(ON) =

5(2)

3.3(5)2 (1.60)

= 0.076

Ω

BS RDS(ON) =

5(2)

1.7(5)2 (1.60)

= 0.147

Ω

The topside FET requirement can be met by an N-channel

Si9410DY which has an RDS(ON) of about 0.04Ω at

VGS = 5V. The bottom-side FET requirement is exceeded

by an Si9410DY. Note that the most stringent requirement

for the bottom-side MOSFET is with VOUT = 0 (i.e., short

circuit). During a continuous short circuit, the worst-case

dissipation rises to:

PB = ISC(AVG)2 × RDS(ON) × (1 + δB)

With the 0.02

Ω sense resistor, ISC(AVG) ≈ 6A will result,

increasing the 0.04

Ωbottom-sideFETdissipationto2.3W.

CIN will require an RMS current rating of at least 2.5A at

temperature and COUT will require an ESR of 0.02Ω for

optimum efficiency.

Now allow VIN to drop to its minimum value. The minimum

VIN can be calculated from the maximum duty cycle and

voltage drop across the topside FET,

VMIN =

DMAX

VOUT + ILOAD × (RDS(ON) + RL + RSENSE)

= 4.0V

At this lower input voltage, the operating frequency de-

creases and the topside FET will be conducting most of the

time, causing the power dissipation to increase.

At dropout,

fMIN =

tON (MAX) + tOFF

= 16kHz

PT = I2LOAD × RDS(ON) × (1 + δT) × DMAX

This last step is necessary to assure that the power

dissipation and junction temperature of the topside FET

are not exceeded.

These last calculations assume that Power VIN is high

enough to keep the topside FET fully turned on at dropout,

as would be the case with the Figure 11circuit. If this isn’t

true (as with the Figure 1 circuit) the RDS(ON) will increase

which in turn increases VMIN and PT.

Adjustable Applications

When an output voltage other than 3.3V or 5V is required,

the LTC1266 adjustable version is used with an external

resistive divider from VOUT to VFB, Pin 10. The regulated

voltage is determined by:

VOUT = 1.25

)

)1+ R2

To prevent stray pickup a 100pF capacitor is suggested

across R1 located close to the LTC1266.

For Figure 1 applications with VOUT below 2V, or when

RSENSE is moved to ground, the current sense comparator

inputs operate near ground. When the current comparator

is operated at less than 2V common mode, the off-time

increases approximately 40%, requiring the use of a

smaller timing capacitor CT.

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