LTC4101EG View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC4101EG' PDF : 30 Pages View PDF
LTC4101
APPLICATIONS INFORMATION
Table 10. Recommended Inductor Values
Inductance
VIN Range (V)
≤ 7.5
1
16μH ± 20%
IMAX (A)
2
8μH ± 20%
3* and 4
4μH ± 20%
≤ 9.0
20μH ± 20%
10μH ± 20%
5μH ± 20%
≤ 12.0
24μH ± 20%
12μH ± 20%
6μH ± 20%
≤ 15.0
26μH ± 20%
13μH ± 20% 6.5μH ± 20%
≤ 28.0
30μH ± 20%
15μH ± 20% 7.5μH ± 20%
RSENSE
0.1Ω
0.05Ω
0.025Ω
* 3 Amp uses the same RSENSE that 4 amps uses. Thus the inductance
can be the same.
Choose and inductor who’s inductance value is equal to
or greater than the value shown. Values assume:
1. –32% RSS result from –20% inductance tolerance
and a –25% inductance loss at IMAX.
2. Inductor ripple current ratio of 0.51 of IOUT across
RSENSE.
3. VOUT is at 4.2V
Charger Switching Power MOSFET
and Diode Selection
Two external power MOSFETs must be selected for use
with the charger: a P-channel MOSFET for the top (main)
switch and an N-channel MOSFET for the bottom (syn-
chronous) switch.
The peak-to-peak gate drive levels are set internally. This
voltage is typically 6V. 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), total gate capacitance QG, reverse
transfer capacitance CRSS, input voltage and maximum
output current. The charger is 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 + δΔT)RDS(ON)
+ k(VIN)2(IMAX)(CRSS)(fOSC)
PSYNC = (VIN – VOUT)/VIN(IMAX)2(1 + δΔT)RDS(ON)
Where δΔT 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 PMAIN equation
includes an additional term for transition 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 greatest at high input voltage or during
a short circuit when the duty cycle in this switch in nearly
100%. The term (1 + δΔT) 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 = QGD/ΔVDS is usually specified
in the MOSFET characteristics. The constant k = 2 can be
used to estimate the contributions of the two terms in the
main switch dissipation equation.
If the charger is to operate in low dropout mode or with
a high duty cycle less than 50%, then the bottomside
N-Channel efficiency generally improves with a larger
MOSFET. Using asymmetrical MOSFETs may achieve cost
savings or efficiency gains.
Both of the LTC4101 MOSFET drivers are optimized to
take advantage of MOSFETs QG values of less than 22nC
and a TD-off delay specification of around 60ns or less.
Larger FETs may work, but you must qualify them and
monitor LTC4101 temperature rise.
Using excessively large MOSFETs relative to the IMAX
charge current they are working with will actually reduce
efficiency at lighter current levels with very limited gain
at high currents. A good place to start looking for a suit-
able MOSFET in a data sheet is to look for a part with
an ID rating a little over 2 times the IMAX charge current
rating. For the LTC4101, the P-channel FET can typically
be scaled down a bit to take advantage of the lower duty
cycle limits. However make sure you never exceed the PD
rating of the device.
4101fa
23
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