LT1339 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1339' PDF : 20 Pages View PDF
LT1339
APPLICATIONS INFORMATION
(20V)(0.01Ω)(1.5 −1)
( )( ) LMIN ≥ 0.084 100000 = 11.9µH
Since L = 5µH is less than LMIN, additional slope compen-
sation is necessary. The total slope compensation
required is:
( ) ( ) SX
≥
20V
5µH
1.5 −1
=
2
106
Amp/s
Subtracting the internally generated slope compensation
and solving for the required effective resistance at SL/ADJ
yields:
( ) ( ( ))( )( )( ) REQ ≤
2500
2 106 RSENSE
fO
−
0.084
fO
= 21.5k
Setting the resistor divider reference voltage at 2V assures
that the additional compensation waveform will be
enabled at 75% duty cycle. As shown in Figure 7a, using
2
5VREF
RSL1
45k
LT1339
4 SL/ADJ
RSL2
30k
1339 • F07a
Figure 7a. External Slope Compensation Resistors
2.5V
2V
0.8V
DC = 0.75
(0.084 + 0.139)(fO)
RSENSE
(0.084)(fO)
RSENSE
1339 • F07b
Figure 7b. Slope Compensation Waveforms
RSL1 = 45k and RSL2 = 30k sets the desired reference
voltage and has a REQ of 18k, which meets both design
requirements. Figure 7b shows the slope compensation
effective waveforms both with and without the SL/ADJ
external resistors.
Power MOSFET and Catch Diode Selection
External N-channel MOSFET switches are used with the
LT1339. The positive gate-source drive voltage of the
LT1339 for both switches is roughly equivalent to the
12VIN supply voltage, so standard threshold MOSFETs
can be used.
Selection criteria for the power MOSFETs include the “ON”
resistance (RDS(ON)), reverse transfer capacitance (CRSS),
maximum drain-source voltage (VDSS) and maximum
output current.
The power FETs selected must have a maximum operating
VDSS exceeding the maximum VIN. VGS voltage maximum
must exceed the 12VIN supply voltage.
Once voltage requirements have been determined, RDS(ON)
can be selected based on allowable power dissipation and
required output current.
In an LT1339 buck converter, the average inductor current
is equal to the DC load current. The average currents
through the main and synchronous switches are:
IMAIN = (ILOAD)(DC)
ISYNC = (ILOAD)(1 – DC)
The RDS(ON) required for a given conduction loss can be
calculated using the relation:
PLOSS = (ISWITCH)2(RDS(ON))
In high voltage applications (VIN > 20V), the topside switch
is required to slew very large voltages. As VIN increases,
transition losses increase through a square relation, until
it becomes the dominant power loss term in the main
switch. This transition loss takes the form:
PTR ≈ (k)(VIN)2(IMAX)(CRSS)(fO)
where k is a constant inversely related to the gate drive
current, approximated by k = 2 in LT1339 applications.
14
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