Datasheet

21
4009fd
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:
∆I
mV
R
I
L
SENSE
MAX
<






2
150
• –
so choose:
L
V
f
mV
R
I
CLP
PWM
SENSE
MAX
1
0 125
150
>






. •
• –
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 I
µA
R
R I
µA
SENSE L
IN
SENSE L
• •∆ ∆
50 20
≤ ≤
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
R
IN
R
PROG
<10V ≥10µH 1A
100mΩ
3.01k 26.7k
10V to 20V ≥20µH 1A
100mΩ
3.01k 26.7k
>20V ≥28µH 1A
100mΩ
3.01k 26.7k
<10V ≥5.1µH 2A
50mΩ
3.01k 26.7k
10V to 20V ≥10µH 2A
50mΩ
3.01k 26.7k
>20V ≥14µH 2A
50mΩ
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:
C
Q
V
C C
G
1
20
4 5
2 20 1= =
•
.
, •
applicaTions inForMaTion
Figure 11. TGATE Boost Supply
20
17
BOOST
INTV
DD
18
SW
LTC4009
4009 F11
C2
2µF
C1
0.1µF
L1
TO
R
SENSE
D1
1N4148