LTC4417_15 Datasheet, PDF (14/32 Pages) Linear Technology – Prioritized PowerPat Controller

LTC4417
Applications Information
UV1 pin is pulled below 1V. The LTC4417 then discon-
nects V1 and connects the next highest valid priority to
V_OUT. When selecting the external N-channel MOSFET,
be sure to account for drain leakage current when setting
UV and OV thresholds by adjusting the resistive divider to
consume more current.
Selecting External P-Channel MOSFETS
The LTC4417 drives external back-to-back P-channel
MOSFETs to conduct or block load current between an
input supply and load. When selecting external P-channel
MOSFETs, the key parameters to consider are on-resistance
(R_DS(ON)), absolute maximum rated drain to source break-
down voltage (BV_DSS(MAX)), threshold voltage (V_GS(TH)),
power dissipation, and safe operating area (SOA).
To determine the required R_DS(ON) use Equation (8), where
V_DROP is the maximum desired voltage drop across the
two series MOSFETs at full load current, I_L(MAX), for the
application. External P-channel MOSFET devices may be
paralleled to further decrease resistance and decrease
power dissipation of each paralleled MOSFET.
R_DS(ON)
≤
2
V_DROP
• I_L(MAX)
(8)
The clamped gate drive output is 4.5V (minimum) from
the common source connection. Select logic level or lower
threshold external MOSFETs to ensure adequate overdrive.
For applications with input supplies lower than the clamp
voltage, choose external MOSFET with thresholds suf-
ficiently lower than the input supply voltage to guarantee
full enhancement.
It is imperative that external P-channel MOSFET devices
never exceed their BV_DSS(MAX) rating in the application.
Select devices with BV_DSS(MAX) ratings higher than seen
in the application. Switching inductive supply inputs with
low value input and/or output capacitances may require
additional precautions; see Transient Supply Protection
section in this data sheet for more information.
In normal operation, the external P-channel MOSFET de-
vices are either fully on, dissipating relatively low power,
or off, dissipating no power. However, during slew-rate
controlled start-up, significant power is dissipated in the
external P-channel MOSFETs. The external P-channel
MOSFETs dissipate the maximum amount of power during
the initial slew-rate limited turn on, where the full input
voltage is applied across the MOSFET while it sources
current. Power dissipation immediately starts to decrease
as the output voltage rises, decreasing the voltage drop
across the MOSFETs.
A conservative approach for determining if a particular
device is capable of supporting soft-start, is to ensure its
maximum instantaneous power, at the start of the output
slewing, is within the manufacturer’s SOA curve. First
determine the duration of soft-start using Equation (9)
and find the inrush current into the load capacitor using
Equation (10).
t_STARTUP
=
V_IN
5_[V/ ms_]
(9)
I_MAXCAP = C_L • 5000[V/s]
(10)
Using V_IN and I_MAXCAP, the power dissipated by the external
MOSFETs during start-up, P_SS, is defined by Equation (11).
If the LTC4417 soft-starts with a live I_L, the extra load cur-
rent needs to be added to I_MAXCAP, and P_SS is calculated
by Equation (12).
P_SS = V_IN • I_MAXCAP
(11)
P_SS = V_IN • (I_MAXCAP + I_L)
(12)
Check to ensure P_SS with a t_STARTUP single pulse duration
lies within the safe operating area (SOA) of the chosen
MOSFET. Ensure the resistive dividers can sink the drain-
source leakage current at the maximum operating tem-
perature. Refer to manufacturer’s data sheet for maximum
drain to source leakage currents, I_DSS.
A list of suggested P-channel MOSFETs is shown in
Table 1. Use procedures outlined in this section and the
SOA curves in the chosen MOSFET manufacturer’s data
sheet to verify suitability for the application.
4417f
14