LTC4012-3_15 Datasheet, PDF (22/28 Pages) Linear Technology – High Efficiency, Multi-Chemistry Battery Charger with PowerPath Control

LTC4012-3
Applications Information
R_IN should not be less than 2.37k or more than 6.04k. Val-
ues of R_IN greater than 3.01k may cause some reduction in
programmed current accuracy. Use these equations and
guidelines, as represented in Table 5, to help select the cor-
rect inductor value. This table was developed for C-grade
parts to maintain maximum ΔI_L near 0.6 • I_MAX with f_PWM at
550kHz and V_BAT = 0.5 • V_CLP (the point of maximum ΔI_L),
assuming that inductor value could also vary by 25% at
I_MAX. For I-grade parts, reduce maximum ΔI_L to less than
0.4 • I_MAX, 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 5 by a factor of 1.6 and
rounding up to the nearest standard value.
Table 5. Minimum Typical Inductor Values
L1
V_CLP
(Typ)
I_MAX R_SENSE
<10V
≥10µH
1A 100mΩ
10V to 20V ≥20µH
1A 100mΩ
>20V
≥28µH
1A 100mΩ
<10V
≥5.1µH
2A
50mΩ
10V to 20V ≥10µH
2A 50mΩ
>20V
≥14µH
2A 50mΩ
<10V
≥3.4µH 3A 33mΩ
10V to 20V ≥6.8µH 3A 33mΩ
>20V
≥9.5µH 3A 33mΩ
<10V
≥2.5µH 4A 25mΩ
10V to 20V ≥5.1µH
4A
25mΩ
>20V
≥7.1µH
4A
25mΩ
R_IN
3.01k
3.01k
3.01k
3.01k
3.01k
3.01k
3.01k
3.01k
3.01k
3.01k
3.01k
3.01k
R_PROG
26.7k
26.7k
26.7k
26.7k
26.7k
26.7k
26.7k
26.7k
26.7k
26.7k
26.7k
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 develop
a bootstrapped BOOST supply for the TGATE FET driver.
A good set of equations governing selection of the two
capacitors is:
C1=
20 • Q_G
4.5V
,
C2
=
20
•
C1
where Q_G is the rated gate charge of the top external NFET
with V_GS = 4.5V. The maximum average diode current is
then given by:
I_D = Q_G • 665kHz
To improve efficiency by increasing V_GS 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 V_IN/V_OUT 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.
BOOST 20
LTC4012-3
INTV_DD 17
SW 18
4012-3 F11
D1
1N4148
C2
2µF
C1
0.1µF
L1
TO
R_SENSE
Figure 11. TGATE Boost Supply
40123fb
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