LT3844EFE-PBF(RevB) Просмотр технического описания (PDF) - Linear Technology

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LT3844EFE-PBF
(Rev.:RevB)

High Voltage, Current Mode Switching Regulator Controller with Programmable Operating Frequency

Linear Technology 

LT3844

APPLICATIONS INFORMATION

Therefore, the peak inductor current is 100mV/RSENSE.

The maximum output load current, IOUT(MAX), is the peak

inductor current minus half the peak-to-peak ripple cur-

rent, ΔIL.

Allowing adequate margin for ripple current and exter-

nal component tolerances, RSENSE can be calculated as

follows:

RSENSE

=

70mV

IOUT(MAX)

Typical values for RSENSE are in the range of 0.005Ω to

0.05Ω.

Operating Frequency

The choice of operating frequency and inductor value is

a trade off between efficiency and component size. Low

frequency operation improves efficiency by reducing

MOSFET switching losses and gate charge losses. However,

lower frequency operation requires more inductance for a

given amount of ripple current, resulting in a larger induc-

tor size and higher cost. If the ripple current is allowed

to increase, larger output capacitors may be required to

maintain the same output ripple. For converters with high

step-down VIN-to-VOUT ratios, another consideration is

the minimum on-time of the LT3844 (see the Minimum

On-time Considerations section). A final consideration

for operating frequency is that in noise-sensitive com-

munications systems, it is often desirable to keep the

switching noise out of a sensitive frequency band. The

LT3844 uses a constant frequency architecture that can

be programmed over a 100kHz to 500kHz range with a

single resistor from the fSET pin to ground, as shown in

Figure 1. The nominal voltage on the fSET pin is 1V and

the current that flows from this pin is used to charge an

internal oscillator capacitor. The value of RSET for a given

operating frequency can be chosen from Figure 4 or from

the following equation:

RSET(kΩ) = 8.4 • 104 • fSW(–1.31)

Table 1 lists typical resistor values for common operating

frequencies:

Table 1. Recommended 1% Standard Values

RSET

191kΩ

fSW

100kHz

118kΩ

150kHz

80.6kΩ

200kHz

63.4kΩ

250kHz

49.9kΩ

300kHz

40.2kΩ

350kHz

33.2kΩ

400kHz

27.4kΩ

450kHz

23.2kΩ

500kHz

200

180

160

140

120

100

80

60

40

20

0

100 200 300 400 500 600

FREQUENCY (kHz)

3844 G19

Figure 1. Timing Resistor (RSET) Value

Step-Down Converter: Inductor Selection

The critical parameters for selection of an inductor are

minimum inductance value, volt-second product, satura-

tion current and/or RMS current.

For a given ΔI, The minimum inductance value is calculated

as follows:

L

≥

VOUT

•

VIN(MAX) – VOUT

fSW • VIN(MAX) • ΔIL

fSW is the switch frequency.

3844fb

11

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