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LTC4012/

LTC4012-1/LTC4012-2

4012f

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

(Typ)

IMAX

RSENSE

RIN

RPROG

<10V

≥10μH

100mΩ

3.01k

26.7k

10V to 20V

≥20μH

100mΩ

3.01k

26.7k

>20V

≥28μH

100mΩ

3.01k

26.7k

<10V

≥5.1μH

50mΩ

3.01k

26.7k

10V to 20V

≥10μH

50mΩ

3.01k

26.7k

>20V

≥14μH

50mΩ

3.01k

26.7k

<10V

≥3.4μH

33mΩ

3.01k

26.7k

10V to 20V

≥6.8μH

33mΩ

3.01k

26.7k

>20V

≥9.5μH

33mΩ

3.01k

26.7k

<10V

≥2.5μH

25mΩ

3.01k

26.7k

10V to 20V

≥5.1μH

25mΩ

3.01k

26.7k

>20V

≥7.1μH

25mΩ

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

•

,•

where QG is the rated gate charge of the top external NFET

with VGS = 4.5V. The maximum average diode current is

then given by:

ID = QG • 665kHz

To improve efﬁciency by increasing VGS applied to the

top FET, substitute a Schottky diode with low reverse

leakage for D1.

PWM jitter has been observed in some designs operating

at higher VIN/VOUT ratios. This jitter does not substantially

affect DC charge current accuracy. A series resistor with a

value of 5Ω to 20Ω can be inserted between the cathode

of D1 and the BOOST pin to remove this jitter, if present.

A resistor case size of 0603 or larger is recommended to

lower ESL and achieve the best results.

APPLICATIONS INFORMATION

Figure 11. TGATE Boost Supply

BOOST

INTVDD

LTC4012

4012 F11

2μF

0.1μF

RSENSE

1N4148

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