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LTC3890-1 数据表(PDF) 21 Page - Linear Technology

部件名 LTC3890-1
功能描述  60V Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller
Download  38 Pages
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制造商  LINER [Linear Technology]
网页  http://www.linear.com
标志 LINER - Linear Technology

LTC3890-1 数据表(HTML) 21 Page - Linear Technology

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LTC3899
21
3899fa
For more information www.linear.com/LTC3899
APPLICATIONS INFORMATION
Ferrite designs have very low core loss and are preferred
for high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates hard, which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Power MOSFET and Schottky Diode
(Optional) Selection
Two external power MOSFETs must be selected for each
controller in the LTC3899: one N-channel MOSFET for the
topswitch(mainswitchforthebucks,synchronousforthe
boost), and one N-channel MOSFET for the bottom switch
(main switch for the boost, synchronous for the bucks).
The peak-to-peak drive levels are set by the DRVCC volt-
age. This voltage can range from 5V to 10V depending on
configurationoftheDRVSETpin.Therefore,bothlogic-level
and standard-level threshold MOSFETs can be used in
most applications depending on the programmed DRVCC
voltage. Pay close attention to the BVDSS specification for
the MOSFETs as well.
The LTC3899’s unique ability to adjust the gate drive level
between 5V to 10V (OPTI-DRIVE) allows an application
circuit to be precisely optimized for efficiency. When
adjusting the gate drive level, the final arbiter is the total
input current for the regulator. If a change is made and
the input current decreases, then the efficiency has im-
proved. If there is no change in input current, then there
is no change in efficiency.
Selection criteria for the power MOSFETs include the
on-resistance RDS(ON), Miller capacitance CMILLER, input
voltage and maximum output current. Miller capacitance,
CMILLER, can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data
sheet. CMILLER is equal to the increase in gate charge
along the horizontal axis while the curve is approximately
flat divided by the specified change in VDS. This result is
then multiplied by the ratio of the application applied VDS
to the gate charge curve specified VDS. When the IC is
operating in continuous mode the duty cycles for the top
and bottom MOSFETs are given by:
Buck Main Switch Duty Cycle =
VOUT
VIN
Buck Sync Switch Duty Cycle =
VIN − VOUT
VIN
Boost Main Switch Duty Cycle =
VOUT − VIN
VOUT
Boost Sync Switch Duty Cycle =
VIN
VOUT
The MOSFET power dissipations at maximum output
current are given by:
PMAIN_BUCK =
VOUT
VIN
IOUT(MAX)
(
)2 1+δ
(
)RDS(ON)+
(VIN)2
IOUT(MAX)
2

 (RDR)(CMILLER)•
1
VDRVCC − VTHMIN
+
1
VTHMIN
(f)
PSYNC_BUCK =
VIN − VOUT
VIN
IOUT(MAX)
(
)2 1+δ
(
)RDS(ON)
PMAIN_BOOST =
VOUT − VIN
(
)VOUT
VIN2
IOUT(MAX)
(
)2
1+δ
(
)RDS(ON)+
VOUT3
VIN


IOUT(MAX)
2

 •
RDR
(
) CMILLER
(
)
1
VDRVCC − VTHMIN
+
1
VTHMIN
(f)
PSYNC_BOOST =
VIN
VOUT
IOUT(MAX)
(
)2 1+δ
(
)RDS(ON)
where δ is the temperature dependency of RDS(ON) and
RDR (approximately 2Ω) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. VTHMIN is the
typical MOSFET minimum threshold voltage.


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