LTC1960 Datasheet, PDF (20/24 Pages) Linear Technology – Dual Battery Charger/Selector with SPI Interface

LTC1960
APPLICATIONS INFORMATION
10µs and the internal IDAC resistor, R_SET, is 18.77k, the
ripple voltage can be approximated by:
∆V_ISET
=
V_REF • T_{∆ ∑}
R_SET • C7
Then the equation to extract C7 is:
C7 = ^{VREF • T∆ ∑}
∆V_ISET •R_SET
= 0.8/0.01/18.77k(10µs) ≅ 0.043µF
In order to prevent overshoot during start-up transients
the time constant associated with C7 must be shorter than
the time constant of C5 at the I_TH pin. If C7 is increased to
improve ripple rejection, then C5 should be increased
proportionally and charger response time to average cur-
rent variation will degrade.
Capacitor C_B1 and C_B2 are used to filter the VDAC delta-
sigma modulation frequency components to a level which
is essentially DC. C_B2 is the primary filter capacitor and
CB1 is used to provide a zero in the response to cancel the
pole associated with C_B2. Acceptable voltage ripple at V_SET
is about 10mV_P-P. Since the period of the delta-sigma
switch closure, T_∆Σ, is about 11µs and the internal VDAC
resistor, R_VSET, is 7.2kΩ, the ripple voltage can be ap-
proximated by:
( ) ∆VVSET = VREF • T∆ ∑
R_VSET C_B1 || C_B2
Then the equation to extract C_B1 || C_B2 is:
C_B1 || C_B2
=
V_REF • T_{∆ ∑}
R_VSET ∆V_VSET
C_B2 should be 10× to 20× C_B1 to divide the ripple voltage
present at the charger output. Therefore C_B1 = 0.01µF and
C_B2 = 0.1µF are good starting values. In order to prevent
overshoot during start-up transients the time constant
associated with C_B2 must be shorter than the time constant
of C5 at the I_TH pin. If C_B2 is increased to improve ripple
rejection, then C5 should be increased proportionally and
charger response time to voltage variation will degrade.
20
Input and Output Capacitors
In the 4A Lithium Battery Charger (Typical Application
section), the input capacitor (C_IN) is assumed to absorb all
input switching ripple current in the converter, so it must
have adequate ripple current rating. Worst-case RMS
ripple current will be equal to one half of output charging
current. Actual capacitance value is not critical. Solid
tantalum low ESR capacitors have high ripple current
rating in a relatively small surface mount package, but
caution must be used when tantalum capacitors are used
for input or output bypass. High input surge currents can
be created when the adapter is hot-plugged to the charger
or when a battery is connected to the charger. Solid
tantalum capacitors have a known failure mechanism
when subjected to very high turn-on surge currents. Only
Kemet T495 series of “Surge Robust” low ESR tantalums
are rated for high surge conditions such as battery to
ground.
The relatively high ESR of an aluminum electrolytic for
C15, located at the AC adapter input terminal, is helpful in
reducing ringing during the hot-plug event.
Highest possible voltage rating on the capacitor will mini-
mize problems. Consult with the manufacturer before use.
Alternatives include new high capacity ceramic (at least
20µF) from Tokin, United Chemi-Con/Marcon, et al. Other
alternative capacitors include OSCON capacitors from
Sanyo.
The output capacitor (C_OUT) is also assumed to absorb
output switching current ripple. The general formula for
capacitor current is:
( ) _{IRMS =}
0.29
(V_BAT)
1
–
V_BAT
V_DCIN
(L1)(f)
For example:
V_DCIN = 19V, V_BAT = 12.6V, L1 = 10µH, and
f = 300kHz, I_RMS = 0.41A.
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
1960fa