LTC1960CUHF-TRPBF View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1960CUHF-TRPBF' PDF : 28 Pages View PDF
LTC1960
APPLICATIONS INFORMATION
VSET/ISET Capacitors
Capacitor C7 is used to filter the delta-sigma modulation
frequency components to a level which is essentially DC.
Acceptable voltage ripple at ISET is about 10mVP-P . Since
the period of the delta-sigma switch closure, T∆∑, is about
10µs and the internal IDAC resistor, RSET, is 18.77k, the
ripple voltage can be approximated by:
∆ VIS E T
=
VREF • T∆ ∑
RSET • C7
Then the equation to extract C7 is:
C7 = VREF • T∆ ∑
∆VISET • RSET
= 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 ITH 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 CB1 and CB2 are used to filter the VDAC delta-
sigma modulation frequency components to a level which
is essentially DC. CB2 is the primary filter capacitor and
CB1 is used to provide a zero in the response to cancel
the pole associated with CB2. Acceptable voltage ripple
at VSET is about 10mVP-P . Since the period of the delta-
sigma switch closure, T∆∑, is about 11µs and the internal
VDAC resistor, RVSET , is 7.2kΩ, the ripple voltage can be
approximated by:
( ) ∆VVSET
=
VREF • T∆ ∑
RVSET CB1 || CB2
Then the equation to extract CB1 || CB2 is:
CB1 || CB2
=
VREF • T∆ ∑
RVSET ∆VVSET
CB2 should be 10× to 20× CB1 to divide the ripple voltage
present at the charger output. Therefore CB1 = 0.01µF and
CB2 = 0.1µF are good starting values. In order to prevent
22
overshoot during start-up transients the time constant as-
sociated with CB2 must be shorter than the time constant
of C5 at the ITH pin. If CB2 is increased to improve ripple
rejection, then C5 should be increased proportionally and
charger response time to voltage variation will degrade.
Input and Output Capacitors
In the 4A Lithium Battery Charger (Typical Application
section), the input capacitor (CIN) 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 a 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 capaci-
tors 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
minimize 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 (COUT) is also assumed to absorb
output switching current ripple. The general formula for
capacitor current is:
IRMS
=
0.29(VBAT ) 1−
(L1)(f)
VBAT
VDCIN
For example:
VDCIN = 19V, VBAT = 12.6V, L1 = 10µH, and f = 300kHz,
IRMS = 0.41A.
1960fb
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