LTC4008EGN-1#PBF Linear Technology, LTC4008EGN-1#PBF Datasheet - Page 15

• Manufacturer Part Number: LTC4008EGN-1#PBF
• Description: IC BATT CHARGER CTRLR 4A 20SSOP
• Manufacturer: Linear Technology

Specifications of LTC4008EGN-1#PBF

• Function: Charge Management
• Battery Type: Multi-Chemistry
• Voltage - Supply: 6 V ~ 28 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 20-SSOP (0.200", 5.30mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS INFORMATION
The relatively high ESR of an aluminum electrolytic for
C1, located at the AC adapter input terminal, is helpful
in reducing ringing during the hot-plug event. Refer to
Application Note 88 for more information.
Highest possible voltage rating on the capacitor will
minimize problems. Consult with the manufacturer before
use. Alternatives include high capacity ceramic (at least
20μF) from Tokin, United Chemi-Con/Marcon, et al. Other
alternative capacitors include OS-CON capacitors from
Sanyo.
The output capacitor (C3) is also assumed to absorb
output switching current ripple. The general formula for
capacitor current is:
For example:
EMI considerations usually make it desirable to minimize
ripple current in the battery leads, and beads or inductors
may be added to increase battery impedance at the 300kHz
switching frequency. Switching ripple current splits between
the battery and the output capacitor depending on the ESR
of the output capacitor and the battery impedance. If the
ESR of C3 is 0.2Ω and the battery impedance is raised to
4Ω with a bead or inductor, only 5% of the current ripple
will fl ow in the battery.
Inductor Selection
Higher operating frequencies allow the use of smaller
inductor and capacitor values. A higher frequency gener-
ally results in lower effi ciency because of MOSFET gate
charge losses. In addition, the effect of inductor value
on ripple current and low current operation must also be
considered. The inductor ripple current ΔI
higher frequency and increases with higher V
V
f = 300kHz, I
I
RMS
DCIN
=
= 19V, V
0 29
.
(
RMS
V
BAT
BAT
( )( )
= 0.41A.
L f
= 12.6V, L1 = 10μH, and
1
)
⎛
⎜
⎝
1
–
V
V
DCIN
BAT
⎞
⎟
⎠
L
decreases with
IN
.
Accepting larger values of ΔI
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ΔI
ΔI
CA1. Remember the maximum ΔI
mum input voltage. In practice 10μH is the lowest value
recommended for use.
Lower charger currents generally call for larger inductor
values. Use Table 4 as a guide for selecting the correct
inductor value for your application.
Table 4
Charger Switching Power MOSFET
and Diode Selection
Two external power MOSFETs must be selected for use
with the charger: a P-channel MOSFET for the top (main)
switch and an N-channel MOSFET for the bottom (syn-
chronous) switch.
The peak-to-peak gate drive levels are set internally. This
voltage is typically 6V. Consequently, logic-level threshold
MOSFETs must be used. Pay close attention to the BV
specifi cation for the MOSFETs as well; many of the logic
level MOSFETs are limited to 30V or less.
MAXIMUM AVERAGE
L
Δ =
exceed 0.6(I
I
CURRENT (A)
L
( )( )
1
1
2
2
3
3
4
4
f L
1
V
MAX
OUT
) due to limits imposed by I
⎛
⎜
⎝
1–
VOLTAGE (V)
INPUT
L
V
≤20
>20
≤20
>20
≤20
>20
≤20
>20
V
OUT
= 0.4(I
IN
L
⎞
⎟
⎠
allows the use of low
MAX
L
occurs at the maxi-
MINIMUM INDUCTOR
). In no case should
LTC4008
VALUE (μH)
40 ±20%
56 ±20%
20 ±20%
30 ±20%
15 ±20%
20 ±20%
10 ±20%
15 ±20%
REV
15
4008fb
and
DSS