ltc1760 Linear Technology Corporation, ltc1760 Datasheet - Page 35

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

APPLICATIO S I FOR ATIO
CA1. Remember the maximum ∆I
mum input voltage. In practice 10µH is the lowest value
recommended for use.
Charger Switching Power MOSFET and Diode
Selection
Two external power MOSFETs must be selected for use
with the LTC1760 charger: An N-channel MOSFET for the
top (main) switch and an N-channel MOSFET for the
bottom (synchronous) switch.
The peak-to-peak gate drive levels are set by the V
voltage. This voltage is typically 5.2V. Consequently, logic-
level threshold MOSFETs must be used. Pay close atten-
tion to the B
many of the logic level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance R
input voltage and maximum output current. The LTC1760
charger is always operating in continuous mode so the
duty cycles for the top and bottom MOSFETs are given by:
The MOSFET power dissipations at maximum output
current are given by:
Where δ∆Τ is the temperature dependency of R
k is a constant inversely related to the gate drive current.
Both MOSFETs have I
N-channel equation includes an additional term for transi-
tion losses, which are highest at high input voltages. For
V
with larger MOSFETs, while for V
losses rapidly increase to the point that the use of a higher
R
efficiency. The synchronous MOSFET losses are greatest
at high input voltage or during a short-circuit when
the duty cycle in this switch is nearly 100%. The term
(1 + δ∆Τ) is generally given for a MOSFET in the form of a
IN
DS(ON)
Main Switch Duty Cycle = V
Synchronous Switch Duty Cycle = (V
P
(I
P
< 20V the high current efficiency generally improves
MAIN
SYNC
MAX
)(C
device with lower C
= (V
= V
RSS
VDSS
DS(ON)
OUT
IN
)(f)
– V
/V
specification for the MOSFETs as well;
IN
, reverse transfer capacitance C
OUT
U
(I
MAX
)/V
2
U
IN
)
R losses while the topside
2
(I
(1 + δ∆Τ)R
RSS
MAX
OUT
actually provides higher
)
IN
/V
2
L
W
(1 + δ∆Τ) R
IN
> 20V the transition
occurs at the maxi-
IN
DS(ON)
– V
OUT
DS(ON)
U
+ k(V
DS(ON)
)/V
IN
RSS
IN
and
CC
)
2
,
normalized R
can be used as an approximation for low voltage MOSFETs.
C
The constant k = 1.7 can be used to estimate the contribu-
tions of the two terms in the main switch dissipation
equation.
If the LTC1760 charger is to operate in low dropout mode
or with a high duty cycle greater than 85%, then the
topside N-channel efficiency generally improves with a
larger MOSFET. Using asymmetrical MOSFETs may achieve
cost savings or efficiency gains.
The Schottky diode D1, shown in the Typical Application,
conducts during the dead-time between the conduction of
the two power MOSFETs. This prevents the body diode of
the bottom MOSFET from turning on and storing charge
during the dead-time, which could cost as much as 1% in
efficiency. A 1A Schottky is generally a good size for 4A
regulators due to the relatively small average current.
Larger diodes can result in additional transition losses due
to their larger junction capacitance. The diode may be
omitted if the efficiency loss can be tolerated.
Calculating IC Operating Current
This section shows how to use the values supplied in the
Electrical Characteristics table to estimate operating cur-
rent for a given application.
The total IC operating current through DCIN when AC is
present and batteries are charging (I
where:
RSS
I
I
I
I
I
connected to SAFETY1 OR SAFETY2.
For thermistors that are OVER-RANGE:
I
For thermistors that are COLD-RANGE:
I
DCIN_CHG
VLIM
CH1
VCC2_AC1
SAFETYX
SAFETYX
SAFETYX
is usually specified in the MOSFET characteristics.
is defined in “Electrical Characteristics.”
+ I
ILIM
is the current used to test the battery thermistor
= 2/64 • V
= 4/64 • V
DS(ON)
is defined in “Electrical Characteristics.”
= I
+ I
CH1
SMB
vs Temperature curve, but δ = 0.005/°C
+ I
+ I
VCC2
VCC2
VCC2_AC1
SMB_BAT1
/(RXB + R
/(RXB + R
+ I
+ I
SAFETY1
SMB_BAT2
DCIN_CHG
THX
THX
LTC1760
)
)
+ I
) is given by:
SAFETY2
+ I
SMBALERT
35
+
1760f