LTC4110 Datasheet, PDF (43/52 Pages) Linear Technology – Battery Backup System Manager

LTC4110
APPLICATIONS INFORMATION
The V_DS ratings of the MOSFETs need to be higher than
these values.
The MOSFET current ratings for the primary side must be
higher than I_PRI, which is I_PRI(CHG) or I_PRI(CAL) for charge
and Calibration mode respectively. See Equations 1 and 2.
MOSFET current ratings for the secondary side must be
higher than I_PRI/N. Since both MOSFETs must perform
both roles, the minimum current rating of the MOSFETs
should be greater than the higher of these values.
MOSFET power dissipation is a function of the RMS cur-
rent flowing through the MOSFET.
Charge Mode:
I_PRI(FETCHG) =
I_{CHG •} V_BAT • (V_BAT + N • V_DCIN)
E
V_DCIN
I_SEC(FETCHG) = I_CHG •
V_BAT + N • V_DCIN
N • V_DCIN
Calibration Mode:
I_PRI(FETCAL) = I_CAL •
V_BAT + N • V_DCIN
N • V_DCIN
I_SEC(FETCAL) =
I_CAL • E •
V_BAT • (V_BAT + N • V_DCIN)
V_DCIN
Where I_PRI(FETCHG) is the same FET as I_SEC(FETCAL) and
I_PRI(FETCAL) is the same FET as I_SEC(FETCHG).
Using the equation below, plug in the higher current from
above into I_FET to find each FET’s power dissipation for
the given mode.
P_FET = I_FET2 • R_DS(ON)
The R_DS(ON) value of the MOSFET depends on V_GS. Conser-
vatively you can use the R_DS(ON) value with a V_GS rating of
4.5V. If you are using a dual-MOSFET package, determine
whether charge mode or calibration mode results is the
highest overall power dissipation and use that as the rating
for the dual MOSFET.
The MOSFET should be specified for fast or PWM switching.
The MOSFET that meets all the above specifications but
has the lowest Q_G and/or Q_GD is often the best choice.
PowerPath MOSFET SELECTION
Important parameters for the selection of PowerPath
MOSFETS are the maximum drain-source voltage V_DS(MAX),
threshold voltage V_GS(VT), on-resistance R_DS(ON) and
Q_GATE.
The maximum allowable drain-source voltage, V_DS(MAX),
must be high enough to withstand the maximum drain-
source voltage seen in the application.
The gates of these MOSFETs are driven by the INID (Input
Ideal Diode) and BATID (Battery Ideal Diode) pins. The
gate turn-on voltage, V_GS, is set by the smaller of the
PowerPath supply voltage or the internal clamping volt-
age V_GON. For the MOSFET driven from the INID pin its
PowerPath supply voltage is the higher of the DCIN pin
or DCOUT pin voltage. For the MOSFETs driven from the
BATID pin, their PowerPath supply voltage is the higher
of the DCOUT pin or BAT pin voltage. Logic-level V_GS(VT)
MOSFET is commonly used, but if a low supply voltage
limits the gate voltage a sub-logic-level threshold MOSFET
should be considered.
As a general rule, select a MOSFET with a low enough
R_DS(ON) to obtain the desired V_DS while operating at full
current load and an achievable V_GS. The MOSFET normally
operates in the linear region and acts like a voltage con-
trolled resistor. If the MOSFET is grossly undersized then it
can enter the saturation region and a large V_DS may result.
However, the drain-source diode of the MOSFET, if forward
biased will limit V_DS. A large V_DS combined with the load
current could result in excessively high MOSFET power
dissipation. Keep in mind that the LTC4110 will regulate
the forward voltage drop across the MOSFETs at 20mV
(V_FR) if R_DS(ON) is low enough. The required R_DS(ON) can
be calculated by dividing 0.02V by the load current in amps.
Achieving forward regulation will minimize power loss and
heat dissipation, but it is not a necessity. If a forward volt-
age drop of more than 20mV is acceptable then a smaller
MOSFET can be used, but must be sized compatible with the
higher power dissipation. Care should be taken to ensure
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