LTC4088 LINER [Linear Technology], LTC4088 Datasheet - Page 21

• Manufacturer Part Number: LTC4088
• Description: High Effi ciency Battery Charger/USB Power Manager
• Manufacturer: LINER [Linear Technology]

Battery Charger Stability Considerations
The LTC4088’s battery charger contains both a con-
stant-voltage and a constant-current control loop. The
constant-voltage loop is stable without any compensation
when a battery is connected with low impedance leads.
Excessive lead length, however, may add enough series
inductance to require a bypass capacitor of at least 1µF
from BAT to GND.
High value, low ESR multilayer ceramic chip capacitors
reduce the constant-voltage loop phase margin, possibly
resulting in instability. Ceramic capacitors up to 22µF may
be used in parallel with a battery, but larger ceramics should
be decoupled with 0.2Ω to 1Ω of series resistance.
Furthermore, a 4.7µF capacitor in series with a 0.2Ω to 1Ω
resistor from BAT to GND is required to prevent oscillation
when the battery is disconnected.
APPLICATIO S I FOR ATIO
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In constant-current mode, the PROG pin is in the feed-
back loop rather than the battery voltage. Because of the
additional pole created by any PROG pin capacitance,
capacitance on this pin must be kept to a minimum. With
no additional capacitance on the PROG pin, the charger
is stable with program resistor values as high as 25k.
However, additional capacitance on this node reduces the
maximum allowed program resistor. The pole frequency at
the PROG pin should be kept above 100kHz. Therefore, if
the PROG pin has a parasitic capacitance, C
lowing equation should be used to calculate the maximum
resistance value for R
R
PROG
≤
2
π •
100
kHz C
PROG
1
•
:
PROG
LTC4088
PROG
, the fol-
21
4088f