Datasheet

LT3800
16
3800fc
APPLICATIONS INFORMATION
Power MOSFET Selection
External N-channel MOSFET switches are used with the
LT3800. The positive gate-source drive voltage of the
LT3800 for both switches is roughly equivalent to the V
CC
supply voltage, for use of standard threshold MOSFETs.
Selection criteria for the power MOSFETs include the
“ON” resistance (R
DS(ON)
), total gate charge (Q
G
), reverse
transfer capacitance (C
RSS
), maximum drain-source volt-
age (V
DSS
) and maximum current.
The power FETs selected must have a maximum operating
V
DSS
exceeding the maximum V
IN
. V
GS
voltage maximum
must exceed the V
CC
supply voltage.
Total gate charge (Q
G
) is used to determine the FET gate
drive currents required. Q
G
increases with applied gate
voltage, so the Q
G
for the maximum applied gate voltage
must be used. A graph of Q
G
vs. V
GS
is typically provided
in MOSFET data sheets.
In a confi guration where the LT3800 linear regulator is
providing V
CC
and V
BOOST
currents, the V
CC
8V output
voltage can be used to determine Q
G
. Required drive cur-
rent for a given FET follows the simple relation:
I
GATE
= Q
G(8V)
• f
O
Q
G(8V)
is the total FET gate charge for V
GS
= 8V, and f
0
=
operating frequency. If these currents are externally derived
by backdriving V
CC
, use the backfeed voltage to determine
Q
G
. Be aware, however, that even in a backfeed confi gura-
tion, the drive currents for both boosted and synchronous
FETs are still typically supplied by the LT3800 internal V
CC
regulator during start-up. The LT3800 can start using FETs
with a combined Q
G(8V)
up to 180nC.
Once voltage requirements have been determined, R
DS(ON)
can be selected based on allowable power dissipation and
required output current.
In an LT3800 buck converter, the average inductor cur-
rent is equal to the DC load current. The average cur-
rents through the main (bootstrapped) and synchronous
(ground-referred) switches are:
I
MAIN
= (I
LOAD
)(DC)
I
SYNC
= (I
LOAD
)(1 – DC)
The R
DS(ON)
required for a given conduction loss can be
calculated using the relation:
P
LOSS
= I
SWITCH
2
• R
DS(ON)
In high voltage applications (V
IN
> 20V), the main switch
is required to slew very large voltages. MOSFET transition
losses are proportional to V
IN
2
and can become the domi-
nant power loss term in the main switch. This transition
loss takes the form:
P
TR
≈ (k)(V
IN
)
2
(I
SWITCH
)(C
RSS
)(f
O
)
where k is a constant inversely related to the gate drive
current, approximated by k = 2 in LT3800 applications,
and I
SWITCH
is the converter output current. The power
loss terms for the switches are thus:
P
MAIN
= (DC)(I
SWITCH
)
2
(1 + d)(R
DS(ON)
) +
2(V
IN
)
2
(I
SWITCH
)(C
RSS
)(f
O
)
P
SYNC
= (1 – DC)(I
SWITCH
)
2
(1 + d)(R
DS(ON)
)
The (1 + d) term in the above relations is the temperature
dependency of R
DS(ON)
, typically given in the form of a
normalized R
DS(ON)
vs Temperature curve in a MOSFET
data sheet.
The C
RSS
term is typically smaller for higher voltage FETs,
and it is often advantageous to use a FET with a higher
V
DS
rating to minimize transition losses at the expense of
additional R
DS(ON)
losses.
In some applications, parasitic FET capacitances couple
the negative going switch node transient onto the bottom
gate drive pin of the LT3800, causing a negative voltage
in excess of the Absolute Maximum Rating to be imposed
on that pin. Connection of a catch Schottky diode from
this pin to ground will eliminate this effect. A 1A current
rating is typically suffi cient for the diode.
Input Capacitor Selection
The large currents typical of LT3800 applications require
special consideration for the converter input and output
supply decoupling capacitors. Under normal steady state
buck operation, the source current of the main switch
MOSFET is a square wave of duty cycle V
OUT
/V
IN
. Most
of this current is provided by the input bypass capacitor.