LT8705
APPLICATIONS INFORMATION
Switch M3: Switch M3 operates in the boost and buck-
Gate Resistors: In some cases it can be beneficial to add
1Ω to 10Ω of resistance between some of the NMOS gate
pins and their respective gate driver pins on the LT8705
(i.e., TG1, BG1, TG2, BG2). Due to parasitic inductance
and capacitance, ringing can occur on SW1 or SW2 when
low capacitance MOSFETs are turned on/off too quickly.
The ringing can be of greatest concern when operating
the MOSFETs or the LT8705 near the rated voltage limits.
Additional gate resistance slows the switching speed,
minimizing the ringing.
boost regions as a control switch. Similar to the M1
2
switch, the power dissipation comes from I R power and
switchingpower.Themaximumpowerdissipationiswhen
V
is the lowest and V
is the highest. The following
IN
OUT
expression approximates the power dissipation in the M3
switch under those conditions:
2
P
= P + P
≅
M3
I R
SWITCHING
V
– V • V
(
)
IN
OUT
OUT
2
•I
•R
•ρ
τ
DS(ON)
OUT
2
V
Excessive gate resistance can have two negative side
effects on performance:
IN
t
RF2
2
1. Slowing the switch transition times can also increase
powerdissipationintheswitch.Thisisdescribedabove
in the Switch M1 and Switch M3 sections.
+ V
•I
• f •
W
OUT
OUT
V
IN
where the total power is 0 in the buck region.
2. Capacitive coupling from the SW1 or SW2 pin to the
switch gate node can turn it on when it’s supposed to
beoff,thusincreasingpowerdissipation.Withtoomuch
gate resistance, this would most commonly happen to
the M2 switch when SW1 is rising.
t
is the average of the SW2 pin rise and fall times
RF2
and, similar to t , is typically 20ns to 40ns or can be
RF1
estimated using:
R
0.8
GATE
tRF2 ≅ VOUT •CRSS • 2+
Careful board evaluation should be performed when
optimizing the gate resistance values. SW1 and SW2 pin
ringing can be affected by the inductor current levels,
therefore board evaluation should include measurements
at a wide range of load currents. When performing PCB
measurements of the SW1 and SW2 pins, be sure to use a
very short ground post from the PCB ground to the scope
probe ground sleeve in order to minimize false inductive
voltages readings.
As with the M1 switch, the switching power (P
often dominates. Look for MOSFETs with lower C
consideroperatingatalowerfrequencytominimizepower
loss and increase efficiency.
)
SWITCHING
or
RSS
Switch M4: In most cases the switching power dissipa-
2
tion in the M4 switch is quite small and I R power losses
2
dominate. I R power is greatest in the boost region where
the switch operates as the synchronous rectifier. Lower
V and higher V
increases the inductor current for a
IN
OUT
C and C
Selection
IN
OUT
given I , leading to the highest power consumption.
OUT
Input and output capacitance is necessary to suppress
voltage ripple caused by discontinuous current moving in
and out of the regulator. A parallel combination of capaci-
tors is typically used to achieve high capacitance and low
ESR (equivalent series resistance). Dry tantalum, special
The M4 switch power consumption in the boost region
can be approximated as:
VOUT
P
≅
•IOUT2 •ρτ •RDS(ON)
W
(M4,BOOST)
V
IN
8705p
27
For more information www.linear.com/8705
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