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4009fd

LTC4009

LTC4009-1/LTC4009-2

TheLTC4009limitsmaximuminstantaneouspeakinductor

current during every PWM cycle. To avoid unstable switch

waveforms, the ripple current must satisfy:

∆I

SENSE

MAX









150

•

–

so choose:

CLP

PWM

SENSE

MAX

0 125

150









•

–

For C-grade parts, a reasonable starting point for setting

ripple current is ΔIL = 0.4 • IMAX. For I-grade parts, use ΔIL

= 0.2 • IMAX only if the IC will actually be used to charge

batteries over the wider I-grade temperature range. The

voltage compliance of internal LTC4009 circuits also im-

poses 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

•

∆

≤

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

ues of RINgreaterthan3.01kmaycausesomereductionin

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 for C-grade

partstomaintainmaximumΔILnear0.6•IMAXwithfPWMat

550kHz and VBAT = 0.5 • VCLP (the point of maximum ΔIL),

assuming that inductor value could also vary by 25% at

IMAX. For I-grade parts, reduce maximum ΔIL to less than

0.4 • IMAX, but only if the IC will actually be used to charge

batteries over the wider I-grade temperature range. In that

case, a good starting point can be found by multiplying

the inductor values shown in Table 6 by a factor of 1.6 and

roundinguptotheneareststandardvalue.

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:

4 5

2 20 1

•

applicaTions inForMaTion

Figure 11. TGATE Boost Supply

BOOST

INTVDD

LTC4009

4009 F11

2µF

0.1µF

RSENSE

1N4148

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