LTM8049 datasheet(16/34 Pages) LINER

16 / 34 page

LTM4651

4651f

For more information www.linear.com/LTM4651

VIN may be rapidly discharged and CINOUT is used, the

resulting capacitor-divider network formed by CINOUT and

CINL||CINHmaytransientlydragVOUT–excessivelynegative.

It is recommended to straddle VOUT–andPGNDwithaTVS

diode, if output voltage excursions during VIN-discharge

are anticipated.

Frequency Adjustment

The default switching frequency (fSW) of the LTM4651 is

400kHz. This is suitable for mainly low-VIN or low-VOUT–

applications (VIN < 5V or |VOUT–| < 5V). For a practical

design, the LTM4651’s inductor ripple current (∆PK-PK) is

suggested to be less than ~2APK-PK. From EQ. 2, it follows

that fSW should be chosen such that:

L • ∆IPK-PK •

VIN

–

VOUT–

















(12)

In some cases, the value of fSW yielded by EQ. 12 violates

the supported minimum on time of the LTM4651 (see

EQ. 3). If this occurs, choose fSW instead according to:

fSW <

TON(MIN)

(13)

The primary consequence of using a lower switching

frequency than that dictated by EQ. 12 is that the output

current capability of the LTM4651 is reduced, according

to EQ. 5.

ToconfiguretheLTM4651forahigherswitchingfrequency

than 400kHz default, apply a resistor, RfSET, between the

fSET pin and SVOUT–. RfSET is given (in MΩ) by:

RfSET(MΩ)=

10pF •[fSW(MHz)–0.4(MHz)]

(14)

The relationship of RfSET to programmed fSW is shown

in Figure 2.

See Table 1 and Table 8 for Recommended fSW and as-

sociated RfSET values for various combinations of VIN

and VOUT–.

APPLICATIONS INFORMATION

Figure 2. Relationship Between RfSET and Target fSW

VOUT– (V)

–0.5

–3.3

–5

–8

–12

–15

–20

–24

3.6

400kHz,

No RfSET

400kHz,

No RfSET

400kHz,

No RfSET

400kHz,

No RfSET

400kHz,

No RfSET

400kHz,

No RfSET

425kHz,

4.3MΩ

450kHz,

2.2MΩ

450kHz,

2.2MΩ

475kHz,

1.3MΩ

500kHz,

1MΩ

525kHz,

806kΩ

550kHz,

665kΩ

550kHz,

665kΩ

700kHz,

332kΩ

825kHz,

237kΩ

875kHz,

210kΩ

900kHz,

200kΩ

1MHz,

165kΩ

Drive CLKIN

with a 200kHz

Clock, No RfSET

450kHz,

2.2MΩ

600kHz,

499kΩ

800kHz,

249kΩ

1.1MHz,

143kΩ

1.2MHz,

124kΩ

1.4MHz,

100kΩ

1.5MHz,

90.9kΩ

Not

Recommended

Due to On-

Time Criteria

Violation

500kHz,

1MΩ

850kHz,

221kΩ

1.2MHz,

124kΩ

1.4MHz,

100kΩ

1.6MHz,

82.5kΩ

N/A

900kHz,

200kΩ

N/A Due to SOA Criteria Violation

Table 1. Recommended Switching Frequency (fSW) and RfSET Values for Common Combinations of VIN and VOUT–

RfSET NOT USED

RfSET (kΩ)

100

10k

0.1

4651 F02

LTM8049 Datasheet(PDF) 16 Page - Linear Technology

LTM8049 Datasheet(HTML) 16 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 33 34

Datasheet Download

Link URL