LTC1624CS datasheet(20/28 Pages) LINER

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LTC1624

Positive-to-negative converters have high ripple current in

both the input and output capacitors. For long capacitor

lifetime, the RMS value of this current must be less than

the high frequency ripple rating of the capacitor.

The following formula gives an approximate value for RMS

ripple current. This formula assumes continuous mode

and low current ripple. Small inductors will give somewhat

higher ripple current, especially in discontinuous mode.

For the exact formulas refer to Application Note 44, pages

28 to 30. The input and output capacitor ripple current

(occurring at VIN(MIN)) is:

Capacitor

ff I

OUT

IRMS =

()( )

ff = Fudge factor (1.2 to 2.0)

The output peak-to-peak ripple voltage is:

VOUT(P-P) = RESR (ID(MAX))

The input capacitor can also see a very high surge current

when a battery is suddenly connected, and solid tantalum

capacitors can fail under this condition. Be sure to specify

surge tested capacitors.

Positive-to-Negative Converter: Duty Cycle

Limitations

The minimum on-time of 450ns sets a limit on how high

of input-to-output ratio can be tolerated while not skipping

cycles. This only impacts designs when very low output

voltages (

VOUT< 2.5V) are needed. The maximum input

voltage is:

VIN(MAX) < 10.1VOUT + 5V

For DC > 9%

VIN(MAX)< 36V –VOUTFor absolute maximum ratings

Positive-to-Negative Converter: Shutdown

Considerations

Since the ground pin on the LTC1624 is referenced to

–VOUT, additional circuitry is needed to put the LTC1624

into shutdown. Shutdown is enabled by pulling the

ITH/RUN pin below 0.8V relative to the LTC1624 ground

pin. With the LTC1624 ground pin referenced to – VOUT,

the nonimal range on the ITH/RUN pin is – VOUT (in

shutdown) to (– VOUT + 2.4V)(at Max IOUT). Referring to

Figure 15, M2, M3 and R3 provide a level shift from typical

TTL levels to the LTC1624 operating as positive-to-nega-

tive converter. MOSFET M3 supplies gate drive to M2

during shutdown, while M2 pulls the ITH/RUN pin voltage to

–VOUT, shutting down the LTC1624.

Step-Down Converters: PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC1624. These items are also illustrated graphically in

the layout diagram of Figure 9. Check the following in your

layout:

1. Are the signal and power grounds segregated? The

LTC1624 ground (Pin 4) must return to the (–) plate

of COUT.

2. Does the VFB (Pin 3) connect directly to the feedback

resistors? The resistive divider R1, R2 must be con-

nected between the (+) plate of COUT and signal ground.

The 100pF capacitor should be as close as possible to

the LTC1624.

3. Does the VIN lead connect to the input voltage at the

same point as RSENSE and are the SENSE– and VIN leads

routed together with minimum PC trace spacing? The

filter capacitor between VIN and SENSE

– should be as

close as possible to the LTC1624.

4. Does the (+) plate of CIN connect to RSENSE as closely

as possible? This capacitor provides the AC current to

the MOSFET(s). Also, does CIN connect as close as

possible to the VIN and ground pin of the LTC1624?

This capacitor also supplies the energy required to

recharge the bootstrap capacitor. Adequate input

decoupling is critical for proper operation.

5. Keep the switch node SW away from sensitive small-

signal nodes. Ideally, M1, L1 and D1 should be con-

nected as closely as possible at the switch node.

APPLICATIONS INFORMATION

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