LTC4009-2_15 Datasheet, PDF (21/28 Pages) Linear Technology – High Efficiency, Multi-Chemistry Battery Charger

LTC4009
LTC4009-1/LTC4009-2
Applications Information
The LTC4009 limits maximum instantaneous peak inductor
current during every PWM cycle. To avoid unstable switch
waveforms, the ripple current must satisfy:
∆I_L
<
2
•
 _150mV
 ^R_SENSE

– I_{MAX}
so choose:
L1>
0.125 • V_CLP
f_PWM
•
 _150mV
 ^R_SENSE
– I_MAX^{}
For C-grade parts, a reasonable starting point for setting
ripple current is ΔI_L = 0.4 • I_MAX. For I-grade parts, use ΔI_L
= 0.2 • I_MAX 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 R_IN (in Figure 1) to
avoid average current errors in high ripple designs. The
following equation can be used for guidance:
R_SENSE • ∆I_L
50µA
≤ R_IN
≤
R_SENSE • ∆I_L
20µA
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 6, 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 6 by a factor of 1.6 and
rounding up to the nearest standard value.
Table 6. Minimum Typical Inductor Values
V_CLP
L1 (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
R_IN
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
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:
C1= ^{20 • QG} , C2 = 20 • C1
4.5V
BOOST 20
LTC4009
INTV_DD 17
SW ₁₈
4009 F11
D1
1N4148
C2
2µF
C1
0.1µF
L1
TO
R_SENSE
Figure 11. TGATE Boost Supply
4009fd
21