Linear Technology LTC1760, Manual

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LTC1760
39
1760fc
For more information www.linear.com/LTC1760
For thermistors that are COLD-RANGE:
I
SAFETYX
= 4/64 • V
VCC2
/(RXB + R
THX
)
For thermistors that are IDEAL-RANGE:
I
SAFETYX
= 4/64 • V
VCC2
/(RXB + R
THX
) + 2/64 • V
VCC2
/
(R1A +R
THX
)
For thermistors that are HOT-RANGE:
I
SAFETYX
= 4/64 • V
VCC2
/(RXB + R
THX
) + 4/64 • V
VCC2
/
(R1A +R
THX
)
R
THX
is the impedance of the battery’s thermistor to
ground.
RXB
= 54.9k
RXA = 1.13k
Sample calculation of I
SAFETYX
with V
VCC2
= 5.2V
Thermistor Impedance
R
THX
(W) Thermistor Range I
SAFETYX
(µA)
400 OVER_RANGE 1.05
3.3k IDEAL_RANGE 42.2
400 UNDER_RANGE 218
I
VLIMIT
= V
VCC2
/(R
VLIMIT
+ R
LIM_PU
).
I
ILIMIT
= V
VCC2
/(R
ILIMIT
+ R
LIM_PU
).
R
LIM_PU
is the typical pull-up impedance at V
LIMIT
and I
LIMIT.
R
LIM_PU
= 34k.
R
VLIMIT
is the value of the resistance from V
LIMIT
to
GND.
R
ILIMIT
is the value of the resistance from I
LIMIT
to
GND.
I
SMB
is the current used for communicating with the
SMBus Host and depends on the amount of bus traffic.
I
SMB_BATX
is the current used for communicating
with Battery1 or Battery2.
I
SMB_BATX
= 350µA • 0.0155 = 5.425µA.
I
SMBALERT
is defined in “Electrical Characteristics.”
Sample calculation of I
DCIN_CHG
with two Li-Ion batteries
(R
THX
= 400), R
VLIMIT
= R
ILIMIT
= 30k, V
CC2
= 5.2V, and
no SMBus Host communication:
APPLICATIONS INFORMATION
I
DCIN_CHG
= I
CH1
+ I
VCC2_AC1
+ I
SAFETY1
+ I
SAFETY2
+
I
VLIM
+ I
ILIM
+ I
SMB
+ I
SMB_BAT1
+ I
SMB_BAT2
+
I
SMBALERT
= 1.3mA + 700µA + 218µA + 218µA +81µA + 81µA
+ 0µA + 5.4µA + 5.4µA + 0µA = 2.62mA
The total operating current through BAT1 and BAT2 when
AC is not present (I
BAT_NOAC
) is given by:
I
BAT_NOAC
= I
BAT
+ I
VCC2_AC0
+ I
SAFETY1
+ I
SAFETY2
+
I
SMB
+ I
SMB_BAT1_AC0
+ I
SMB_BAT2_AC0
+ I
SMBALERT
where:
I
BAT
is defined in “Electrical Characteristics.”
I
VCC2_AC0
is defined in “Electrical Characteristics.”
I
SAFETYX
is the current used to test the battery
thermistor connected to SAFETY1 or SAFETY2.
I
SAFETYX
= 2/64 • V
VCC2
/(RXB + R
THX
).
R
THX
is the impedance of the battery’s thermistor to
ground.
RXB
= 54.9k.
Sample calculation of I
SAFETY
with V
VCC2
= 5.2V
Thermistor Impedance
R
THX
(Ω) Thermistor Range I
SAFETYX
(µA)
400 UNDER_RANGE 2.9
I
SMB_BATX_ACO
is the current used for communicat-
ing with Battery1 or Battery2 when AC in not present.
I
SMB_BATX_AC0
= 350µA • 0.00687 = 2.404µA.
I
SMB
is the current used for communicating with the
SMBus Host and depends on the amount of bus
traffic.
Sample calculation with two Li-Ion batteries (R
THX
=
400), V
CC2
= 5.2V, and no SMBus Host communication:
I
BAT_NOAC
– I
BAT
+ I
VCC2_AC0
+ I
SAFETY1
+ I
SAFETY2
+
I
SMB
+ I
SMB_BAT1_AC0
+ I
SMB_BAT2_AC0
+ I
SMBALERT
= 175µA + 80µA + 2.9µA + 2.9µA + 0µA + 2.4µA +
2.4µA + 0µA = 265µA