LTC4009-2 datasheet(21/28 Pages) LINER

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4009fb

LTC4009

LTC4009-1/LTC4009-2

The LTC4009 limits maximum instantaneous peak inductor

current during every PWM cycle. To avoid unstable switch

waveforms, the ripple current must satisfy:

ΔI

SENSE

MAX

⎛

⎝⎜

⎞

⎠⎟

150

•–

so choose:

CLP

PWM

SENSE

MAX

0 125

150

⎛

⎝⎜

⎞

⎠⎟

.•

•–

A reasonable starting point for setting ripple current

ΔIL = 0.4 • IMAX. The voltage compliance of internal

LTC4009 circuits also imposes limits on ripple current.

Select RIN (in Figure 1) to avoid average current errors in

high ripple designs. The following equation can be used

for guidance:

μA

SENSE

••

ΔΔ

≤≤

RIN should not be less than 2.37k or more than 6.04k. Val-

ues of RIN greater than 3.01k may cause some reduction in

programmed current accuracy. Use these equations and

guidelines, as represented in Table 6, to help select the cor-

rect inductor value. This table was developed to maintain

maximum

ΔIL near 0.6 • IMAX with fPWM at 550kHz and

VBAT=0.5•VCLP(thepointofmaximumΔIL),assumingthat

inductor value could also vary by 25% at IMAX.

Table 6. Minimum Typical Inductor Values

VCLP

L1 (Typ)

IMAX

RSENSE

RIN

RPROG

<10V

≥10μH

100m

3.01k

26.7k

10V to 20V

≥20μH

100m

3.01k

26.7k

>20V

≥28μH

100m

3.01k

26.7k

<10V

≥5.1μH

50m

3.01k

26.7k

10V to 20V

≥10μH

50m

3.01k

26.7k

>20V

≥14μH

50m

3.01k

26.7k

To guarantee that a chosen inductor is optimized in any

given application, use the design equations provided and

perform bench evaluation in the target application, par-

ticularly at duty cycles below 20% or above 80% where

PWM frequency can be much less than the nominal value

of 550kHz.

TGATE BOOST Supply

Use the external components shown in Figure 11 to de-

velop a bootstrapped BOOST supply for the TGATE FET

driver. A good set of equations governing selection of the

two capacitors is:

220

•

,•

APPLICATIONS INFORMATION

Figure 11. TGATE Boost Supply

BOOST

INTVDD

LTC4009

4009 F11

2μF

0.1μF

RSENSE

1N4148

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