LTC1760 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1760' PDF : 44 Pages View PDF
LTC1760
APPLICATIONS INFORMATION
CA1. Remember the maximum ΔIL occurs at the maximum
input voltage. In practice 10μH is the lowest value recom-
mended 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 VCC volt-
age. This voltage is typically 5.2V. Consequently, logic-level
threshold MOSFETs must be used. Pay close attention to
the BVDSS specification for the MOSFETs as well; many of
the logic level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance RDS(ON), reverse transfer capacitance CRSS,
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:
Main Switch Duty Cycle = VOUT/VIN
Synchronous Switch Duty Cycle = (VIN – VOUT)/VIN
The MOSFET power dissipations at maximum output
current are given by:
PMAIN = VOUT/VIN(IMAX)2(1 + δΔΤ)RDS(ON) + k(VIN)2
(IMAX)(CRSS)(f)
PSYNC = (VIN – VOUT)/VIN(IMAX)2(1 + δΔΤ) RDS(ON)
Where δΔΤ is the temperature dependency of RDS(ON)
and k is a constant inversely related to the gate drive
current. Both MOSFETs have I2R losses while the topside
N-channel equation includes an additional term for transi-
tion losses, which are highest at high input voltages. For
VIN < 20V the high current efficiency generally improves
with larger MOSFETs, while for VIN > 20V the transition
losses rapidly increase to the point that the use of a higher
RDS(ON) device with lower CRSS actually provides higher
efficiency. The synchronous MOSFET losses are great-
est at high input voltage or during a short-circuit when
the duty cycle in this switch is nearly 100%. The term
(1 + δΔΤ) is generally given for a MOSFET in the form of a
normalized RDS(ON) vs Temperature curve, but δ = 0.005/°C
can be used as an approximation for low voltage MOSFETs.
CRSS is usually specified in the MOSFET characteristics. The
constant k = 1.7 can be used to estimate the contributions
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 top-
side 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 (IDCIN_CHG) is given by:
IDCIN_CHG = ICH1 + IVCC2_AC1 + ISAFETY1 + ISAFETY2 +
IVLIM + IILIM + ISMB + ISMB_BAT1 + ISMB_BAT2 + ISMBALERT
where:
ICH1 is defined in “Electrical Characteristics.”
IVCC2_AC1 is defined in “Electrical Characteristics.”
ISAFETYX is the current used to test the battery thermistor
connected to SAFETY1 OR SAFETY2.
For thermistors that are OVER-RANGE:
ISAFETYX = 2/64 • VVCC2/(RXB + RTHX)
For thermistors that are COLD-RANGE:
ISAFETYX = 4/64 • VVCC2/(RXB + RTHX)
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