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LT3844IFE

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

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LT3844

APPLICATIO S I FOR ATIO

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 external

component tolerances, RSENSE can be calculated as

follows:

RSENSE

=

70mV

IOUT(MAX)

sensitive communications 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:

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. How-

ever, lower frequency operation requires more inductance

for a given amount of ripple current, resulting in a larger

inductor 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 convert-

ers 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-

200

180

160

140

120

100

80

60

40

20

0

100 200 300 400 500 600

FREQUENCY (kHz)

3844 G19

RSET(kΩ) = 8.4 • 104 • fSW(–1.31)

The following table lists typical resistor values for com-

mon operating frequencies:

Recommended 1% Standard Values

RSET

191kΩ

118kΩ

80.6kΩ

63.4kΩ

49.9kΩ

40.2kΩ

33.2kΩ

27.4kΩ

23.2kΩ

fSW

100kHz

150kHz

200kHz

250kHz

300kHz

350kHz

400kHz

450kHz

500kHz

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 calcu-

lated as follows:

L

≥

VOUT

•

VIN(MAX) – VOUT

fSW • VIN(MAX) • ΔIL

fSW is the switch frequency.

Figure 1. Timing Resistor (RSET) Value

3844fa

11

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