LTM8045 datasheet(34/54 Pages) AD

34 / 54 page

LTM4655

Rev. 0

For more information www.analog.com

body diode that creates a diode-drop of reverse polarity

(positive voltage) on VOUTn–, as shown in Figure 49. The

voltage excursion is highest when RUN

n toggles high

because that is the instant when INTVCCn powers-up, with

a corresponding increase in ISVINn/ISVOUTn–/ILn current

flow. With higher current flow, the forward voltage drop

(VF) of MBn’s body diode—and thus, the positive voltage

excursion on VOUTn– is higher.

If this transient voltage excursion is unwelcome for the

load or polarized output capacitors, minimize it with a

low VF Schottky diode that straddles VOUTn– and VOUTn+

(see Figure 48 circuit and Figure 49 performance).

Additionally, the voltage excursion can be empirically

reduced by increasing output capacitance.

Lastly: in applications where it is anticipated that VINn

may be rapidly applied (e.g., <10μs) and CINOUTn is

used, the resulting capacitor-divider network formed by

CINOUTn and CINLn||CINHn may transiently drag VOUTn–

positive. It is recommended to apply a low VF Schottky

diode from VOUTn– to VOUTn+ in such applications. The

reverse mechanism applies, as well: in applications where

it is anticipated that VINn may be rapidly discharged and

CINOUTn is used, the resulting capacitor-divider network

formed by CINOUTn and CINLn||CINHn may transiently drag

VOUTn– excessively negative. It is recommended to strad-

dle VOUTn– and VOUTn+ with a TVS diode, if output voltage

excursions during VINn-discharge are anticipated.

APPLICATIONS INFORMATION

Frequency Adjustment, Negative-VOUT– Operation

The default switching frequency (fSWn) of channel n of the

LTM4655 is 400kHz. This is suitable for mainly low-VIN or

low-VOUT– applications (VINn < 5V or |VOUTn–| < 5V). For

a practical design, the LTM4655’s inductor ripple current

(∆nPK–PK) is suggested to be less than ~2APK–PK. From

Equation 20, it follows that fSW should be chosen such

that Equation 26).

fSWn =

Ln • ∆InPK-PK •

VINn

–

VOUTn –

⎛

⎝⎜

⎞

⎠⎟

(26)

In some cases, the value of fSWn yielded by Equation 26

violates the supported minimum on-time of the LTM4655

(see Equation 21). If this occurs, choose fSWn instead

according to Equation 12.

The primary consequence of using a lower switching fre-

quency than that dictated by Equation 26 is that the output

current capability of the LTM4655 is reduced, according

to Equation 23.

To configure the channel

n of the LTM4655 for a higher

switching frequency than 400kHz default, apply a resistor,

RfSETn, between the fSETn pin and SVOUTn–. RfSETn is given

(in MΩ) by Equation 13.

The relationship of RfSETn to programmed fSWn is shown

in Figure 1.

See Table 1 and Table 12 for Recommended fSWn and

associated RfSETn values for various combinations of VINn

and VOUTn–.

Table 1. Recommended Channel

n Switching Frequency (fSWn) and RfSETn for Common Combinations of VINn and VOUTn–,

Negative-VOUTn– Operation

VOUTn– (V)

–0.5

–3.3

–5

–8

–12

–15

–20

–24

3.6

400kHz,

No RfSETn

400kHz,

No RfSETn

400kHz,

No RfSETn

400kHz,

No RfSETn

400kHz,

No RfSETn

400kHz,

No RfSETn

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 CLKINn with a 200kHz

Clock, No RfSETn

450kHz,

2.2MΩ

600kHz,

499kΩ

800kHz,

249kΩ

1.1MHz,

143kΩ

1.2MHz,

124kΩ

N/A

Not Recommended Due to

On-Time Criteria Violation

500kHz,

1MΩ

N/A Due to SOA Criteria Violation

LTM8045 Datasheet(PDF) 34 Page - Analog Devices

LTM8045 Datasheet(HTML) 34 Page - Analog Devices

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 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

Datasheet Download

Link URL