At maximum load current, when the buck switch turns on, the

current into VIN suddenly increases to the lower peak of the

inductor’s ripple current, ramps up to the peak value, then

drop to zero at turn-off. The average current during the

on-time is the load current. For a worst case calculation, C1

must supply this average load current during the maximum

on-time. C1 is calculated from:

where Io is the load current, t

and

∆V is the allowable ripple voltage at V

to help avoid transients and ringing due to long lead induc-

tance at V

recommended, located close to the LM2695 .

C4: The recommended value for C4 is 0.022 µF. A high

quality ceramic capacitor with low ESR is recommended as

C4 supplies a surge current to charge the buck switch gate

at turn-on. A low ESR also helps ensure a complete recharge

during each off-time.

C6: The capacitor at the SS pin determines the softstart

time, i.e. the time for the reference voltage at the regulation

comparator, and the output voltage, to reach their final value.

The time is determined from the following:

PC BOARD LAYOUT

The LM2695 regulation, over-voltage, and current limit com-

parators are very fast, and respond to short duration noise

pulses. Layout considerations are therefore critical for opti-

mum performance. The layout must be as neat and compact

as possible, and all of the components must be as close as

possible to their associated pins. The current loop formed by

D1, L1, C2 and the SGND and ISEN pins should be as small

as possible. The ground connection from C2 to C1 should be

as short and direct as possible.

If it is expected that the internal dissipation of the LM2695

will produce excessive junction temperatures during normal

operation, good use of the PC board’s ground plane can help

considerably to dissipate heat. The exposed pad on the

bottom of the IC package can be soldered to a ground plane,

and that plane should extend out from beneath the IC, and

be connected to ground plane on the board’s other side with

several vias, to help dissipate the heat. The exposed pad is

internally connected to the IC substrate. Additionally the use

of wide PC board traces, where possible, can help conduct

heat away from the IC. Judicious positioning of the PC board

within the end product, along with the use of any available air

flow (forced or natural convection) can help reduce the junc-

tion temperatures.

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