LTC3733 datasheet(22/32 Pages) LINER

22 / 32 page

LTC3733/LTC3733-1

3733f

APPLICATIO S I FOR ATIO

The loop filter components (CLP, RLP) smooth out the

current pulses from the phase detector and provide a

stable input to the voltage controlled oscillator. The filter

components CLP and RLP determine how fast the loop

acquires lock. Typically RLP =10k and CLP ranges from

0.01

µF to 0.1µF.

Minimum On-Time Considerations

Minimum on-time, tON(MIN), is the smallest time duration

that the IC is capable of turning on the top MOSFET. It is

determined by internal timing delays and the gate charge

of the top MOSFET. Low duty cycle applications may

approach this minimum on-time limit and care should be

taken to ensure that:

ON MIN

OUT

() < ()

If the duty cycle falls below what can be accommodated by

the minimum on-time, the IC will begin to skip every other

cycle, resulting in half-frequency operation. The output

voltage will continue to be regulated, but the ripple current

and ripple voltage will increase.

The minimum on-time for the IC is generally about 120ns.

However, as the peak sense voltage decreases the mini-

mum on-time gradually increases. This is of particular

concern in forced continuous applications with low ripple

current at light loads. If the duty cycle drops below the

minimum on-time limit in this situation, a significant

amount of cycle skipping can occur with correspondingly

larger current and voltage ripple.

If an application can operate close to the minimum on-

time limit, an inductor must be chosen that is low enough

in value to provide sufficient ripple amplitude to meet the

minimum on-time requirement.

As a general rule, keep

the inductor ripple current equal to or greater than 30%

of IOUT(MAX) at VIN(MAX).

Efficiency Considerations

The percent efficiency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efficiency and which change would

produce the most improvement. Percent efficiency can be

expressed as:

%Efficiency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percentage

of input power.

Checking Transient Response

The regulator loop response can be checked by looking at

the load transient response. Switching regulators take

several cycles to respond to a step in DC (resistive) load

current. When a load step occurs, VOUT shifts by an

amount equal to

∆ILOAD • ESR, where ESR is the effective

series resistance of COUT. ∆ILOAD also begins to charge or

discharge COUT, generating the feedback error signal that

forces the regulator to adapt to the current change and

return VOUT to its steady-state value. During this recovery

time, VOUT can be monitored for excessive overshoot or

ringing, which would indicate a stability problem. The

availability of the ITH pin not only allows optimization of

control loop behavior, but also provides a DC coupled

and AC filtered closed-loop response test point. The DC

step, rise time and settling at this test point truly reflects

the closed-loop response. Assuming a predominantly

second order system, phase margin and/or damping

factor can be estimated using the percentage of overshoot

seen at this pin. The bandwidth can also be estimated by

examining the rise time at the pin. The ITH external com-

ponents shown in the Figure 1 circuit will provide an

adequate starting point for most applications.

LTC3733 Datasheet(PDF) 22 Page - Linear Technology

LTC3733 Datasheet(HTML) 22 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL