MP2671 – CHARGE SYSTEM PROTECTION CIRCUIT
MP2671 Rev. 0.9 www.MonolithicPower.com 10
2/23/2009 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2009 MPS. All Rights Reserved.
APPLICATION INFORMATION
For safe and effective charging, some strict
requirements have to be satisfied during
charging Li-Ion batteries such as high precise
power source for charging (4.2V±50mV), the
accuracy should be higher than 1%. For highly
used capacity, the voltage of the battery should
be charged to the value (4.2V) as possible as
could. Otherwise, the performance and the life
of the battery suffers overcharge. Additionally,
the pre-charge for depleted batteries, charging
voltage, charging current, as well as the
temperature detection and protection, are
required for linear battery chargers. The output
of any MP2602 family chargers has a typical I-V
curve and provides overcharge, input over
voltage, over temperature protection. The
function of the MP2671 is to add a redundant
protection layer such that, under any fault
condition, the charging system output does not
exceed the I-V limits that the battery required.
Additionally, MP2671 provides full protection for
these chargers whose protection function is not
so complete especially those without input
surge voltage sustain. MP2671 guarantees the
safety of the charge system with its perfect 4
protection functions: OVP, BOVP, OCP and
OTP
The MP2671 is a simple device that requires
few external components, in addition to the
MP2602 charger circuit as shown in the Typical
Application Circuit. The selection of MP2671’s
external components is shown as follow, and
the selection of the current limit resistor R
ILIM
has been introduced in the Over-current
Protection section.
C
IN
and C
OUT
Selection
The input capacitor (C
IN
) is for decoupling.
Higher value of C
IN
reduces the voltage drop or
the over shoot during transients. The AC
adapter is inserted live (hot insertion) and
sudden step down of the current in the power
FET will cause the input voltage overshoot.
During an input OVP, the FET is turned off in
less than 1µs and can lead to significant over
shoot. Higher capacitance of C
IN
reduces this
type of over shoot. However, the over shoot
caused by a hot insertion is not very dependent
on the decoupling capacitance value. Usually,
the input decoupling capacitor is recommended
to use a dielectric ceramic capacitor with a
value between 1µF to 4.7µF.
The output of the MP2671 and the input of the
charging circuit typically share one decoupling
capacitor. The selection of that capacitor is
mainly determined by the requirement of the
charging circuit. When using the MP2602 family
chargers, a 1µF to 4.7uF ceramic capacitor is
recommended.
R
VB
Selection
R
VB
limits the current from the VB pin to the
battery terminal in case the MP2671 fails. The
recommended value is between 200k to 1M.
With 200k resistance, during the failure
operation, assuming the VB pin voltage is 30V
and the battery voltage is 4.2V. The worst case
the current flowing from the VB pin to the
charger output is,
(30V - 4.2V)/200k = 130µA,
Such small current can be easily absorbed by
the bias current of other components.
Increasing the R
VB
value reduces the worst
case current, but at the same time increases
the error for the 4.4V battery OVP threshold.
As the typical VB pin leakage current is 20nA,
the error of the battery OVP threshold can be
calculated as 4.4V+20nAxR
VB
. With the 200k
resistor, the worst-case additional error is 4mV
and with a 1M resistor, the worst-case
additional error is 20mV.
.
