ltc1760 Linear Technology Corporation, ltc1760 Datasheet - Page 37

• Manufacturer Part Number: ltc1760
• Description: Dual Smart Battery System Manager
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Calculating IC Power Dissipation
The power dissipation of the LTC1760 is dependent upon
the gate charge of Q
Application). The gate charge is determined from the
manufacturer’s data sheet and is dependent upon both the
gate voltage swing and the drain voltage swing of the FET.
where:
Example:
V
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 10mV
the period of the delta-sigma switch closure, T
10µs and the internal IDAC resistor, R
ripple voltage can be approximated by:
Then the equation to extract C7 is:
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
improve ripple rejection, then C5 should be increased
proportionally and charger response time to average cur-
rent variation will degrade.
Capacitors C
sigma modulation frequency components to a level which
SET
P
I
I
previous section.
V
Q
I
P
C
∆
DCIN_CHG
DCIN_CHG
SAFETY2
D
VCC
D
BG
/I
7 =
V
= (V
= 190mW
ISET
SET
= 0.8/0.01/18.77k(10µs) ≅ 0.043µF
= 15nC, I
= 5.2V, V
∆
Capacitors
DCIN
V
=
V
= 218µA.
REF
ISET
, I
B1
– V
V
R
SAFETY1
REF
– V
SET
and C
•
VCC
DCIN_CHG
•
T
R
DCIN
∆
VCC
•
SET
U
• 7
T
∑
• (I
C
∆
B2
, I
) • f
∑
= 19V, f
SAFETY1
SAFETY2
are used to filter the VDAC delta-
TG
OSC
U
= 2.62mA, I
and Q
• (Q
TH
OSC
+ I
are defined in the
pin. If C7 is increased to
TG
SAFETY2
BG
W
= 345kHz, Q
+ Q
.(Refer to Typical
SAFETY1
SET
BG
)
, is 18.77k, the
) + V
∆Σ
=
U
DCIN
P-P
TG
, is about
. Since
=
•
is essentially DC. C
CB1 is used to provide a zero in the response to cancel the
pole associated with C
is about 10mV
switch closure, T
resistor, R
proximated by:
Then the equation to extract C
C
present at the charger output. Therefore C
C
overshoot during start-up transients the time constant
associated with C
of C5 at the I
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 (C
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.
B2
B2
∆
C
should be 10× to 20× C
= 0.1µF are good starting values. In order to prevent
V
B
1
VSET
||
C
VSET
B
=
2
TH
R
=
, is 7.2kΩ, the ripple voltage can be ap-
P-P
VSET
pin. If C
R
∆Σ
B2
V
V
VSET
. Since the period of the delta-sigma
REF
REF
must be shorter than the time constant
, is about 11µs and the internal VDAC
B2
(
C
B2
is the primary filter capacitor and
∆
•
B
•
. Acceptable voltage ripple at V
B2
T
1
T
V
∆
∆
VSET
||
is increased to improve ripple
∑
∑
C
B1
B
IN
2
B1
to divide the ripple voltage
)
) is assumed to absorb all
|| C
B2
is:
LTC1760
B1
= 0.01µF and
37
1760f
SET