ltc3858-2 Linear Technology Corporation, ltc3858-2 Datasheet - Page 19

• Manufacturer Part Number: ltc3858-2
• Description: Ltc3858-2 - Low Iq, Dual 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Power MOSFET and Schottky Diode (Optional)
Selection
Two external power MOSFETs must be selected for each
controller in the LTC3858-2: one N-channel MOSFET for
the top (main) switch, and one N-channel MOSFET for the
bottom (synchronous) switch.
The peak-to-peak drive levels are set by the INTV
This voltage is typically 5.2V during start-up (see EXTV
Pin Connection). Consequently, logic-level threshold
MOSFETs must be used in most applications. The only
exception is if low input voltage is expected (V
then, sub-logic level threshold MOSFETs (V
should be used. Pay close attention to the BV
fication 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, R
voltage and maximum output current. Miller capacitance,
C
usually provided on the MOSFET manufacturers’ data
sheet. C
along the horizontal axis while the curve is approximately
flat divided by the specified change in V
then multiplied by the ratio of the application applied V
to the gate charge curve specified V
operating in continuous mode the duty cycles for the top
and bottom MOSFETs are given by:
MILLER
Main Switch Duty Cycle =
Synchronous Switch Duty Cycle =
, can be approximated from the gate charge curve
MILLER
is equal to the increase in gate charge
DS(ON)
, Miller capacitance, C
V
V
OUT
IN
DS
. When the IC is
DS
V
IN
. This result is
MILLER
− V
GS(TH)
V
IN
CC
DSS
OUT
IN
voltage.
< 4V);
speci-
, input
< 3V)
DS
CC
The MOSFET power dissipations at maximum output
current are given by:
where δ is the temperature dependency of R
R
at the MOSFET’s Miller threshold voltage. V
typical MOSFET minimum threshold voltage.
Both MOSFETs have I
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
the high current efficiency generally improves with larger
MOSFETs, while for V
increase to the point that the use of a higher R
with lower C
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
a short-circuit when the synchronous switch is on close
to 100% of the period.
The term (1+ δ) is generally given for a MOSFET in the
form of a normalized R
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
The optional Schottky diodes D3 and D4 shown in Figure 13
conduct during the dead-time between the conduction of
the two power MOSFETs. This prevents the body diode of
the bottom MOSFET from turning on, storing charge during
the dead-time and requiring a reverse recovery period that
could cost as much as 3% in efficiency at high V
to 3A Schottky is generally a good compromise for both
regions of operation due to the relatively small average
current. Larger diodes result in additional transition losses
due to their larger junction capacitance.
DR
P
P
MAIN
SYNC
(approximately 2Ω) is the effective driver resistance
=
=
⎡
⎢
⎣
( )
V
MILLER
V
V
V
V
OUT
IN
IN
INTVCC
IN
– V
V
2
IN
(
I
⎛
⎜
⎝
actually provides higher efficiency. The
MAX
OUT
I
MAX
2
1
– V
IN
2
R losses while the topside N-channel
)
DS(ON)
> 20V the transition losses rapidly
(
2
I
THMIN
⎞
⎟ R
⎠
MAX
(
(
1+ δ
DR
)
2
vs Temperature curve, but
)
)
+
(
(
R
1+ δ
C
V
DS(ON)
MILLER
THMIN
LTC3858-2
1
)
R
DS(ON)
+
⎤
⎥ f
⎦
)
( )
•
DS(ON)
THMIN
DS(ON)
IN
IN
19
device
< 20V
. A 1A
is the
38582f
and