LTC3873_15 Datasheet, PDF (10/16 Pages) Linear Technology – Frequency Current Mode Boost/Flyback/SEPIC DC/DC Controller

LTC3873
APPLICATIONS INFORMATION
The circuit in Figure 6 shows a third way to power the
LTC3873. An external series pre-regulator consisting of
series pass transistor Q1, zener diode D1 and bias resis-
tor R_B brings V_CC to at least 7.6V nominal, well above
the maximum rated V_CC turn-off threshold of 4V. Resistor
R_START momentarily charges the V_CC node up to the V_CC
turn-on threshold, enabling the LTC3873.
V_IN
R_B
D1
8.2V
Q1 RSTART ^LTC3873
C_VCC
0.1μF
V_CC
GND
3873 F06
Figure 6
Slope Compensation
The LTC3873 has built-in internal slope compensation to
stabilize the control loop against sub-harmonic oscillation.
It also provides the ability to externally increase slope
compensation by injecting a ramping current out of its SW
pin into an external slope compensation resistor (R_SL in
Figure 5). This current ramp starts at zero right after the
NGATE pin has been set high. The current rises linearly
towards a peak of 20μA at the maximum duty cycle of
80%, shutting off once the NGATE pin goes low. A series
resistor (R_SL) connecting the SW pin to the current sense
resistor (R_SENSE) thus develops a ramping voltage drop.
From the perspective of the SW pin, this ramping voltage
adds to the voltage across the sense resistor, effectively
reducing the current comparator threshold in proportion
to duty cycle. The amount of reduction in the current
comparator threshold (ΔV_SENSE) can be calculated using
the following equation:
ΔV_SENSE
=
Duty
Cycle
80%
–
6% _20μA
• R_SLOPE
Note the external programmable slope compensation is
only needed when the internal slope compensation is not
sufficient. In most applications R_SL can be shorted. For the
LTC3873, when the R_DS(ON) sensing technique is used, the
ringing on the SW pin disrupts the tiny slope compensa-
tion current out of the pin. It is not recommended to add
external slope compensation in this case.
Output Voltage Programming
The output voltage is set by a resistor divider according
to the following formula:
V_O
=
1.2V
•
⎛⎝⎜₁₊
R2⎞
R1⎠⎟
The external resistor divider is connected to the output
as shown in Figure 5, allowing remote voltage sensing.
Choose resistance values for R1 and R2 to be as large as
possible in order to minimize any efficiency loss due to
the static current drawn from V_OUT, but just small enough
so that when V_OUT is in regulation, the error caused by
the nonzero input current to the V_FB pin is less than 1%.
A good rule of thumb is to choose R1 to be 24k or less.
Transformer Design Considerations
Transformer specification and design is perhaps the
most critical part of applying the LTC3873 successfully.
In addition to the usual list of caveats dealing with high
frequency power transformer design, the following should
prove useful.
Turns Ratios
Due to the use of the external feedback resistor divider
ratio to set output voltage, the user has relative freedom
in selecting a transformer turns ratio to suit a given ap-
plication. Simple ratios of small integers, e.g., 1:1, 2:1, 3:2,
etc. can be employed which yield more freedom in setting
total turns and mutual inductance. Simple integer turns
ratios also facilitate the use of “off-the-shelf” configurable
transformers such as the Coiltronics VERSA-PAC series
in applications with high input-to-output voltage ratios.
For example, if a 6-winding VERSA-PAC is used with three
windings in series on the primary and three windings in
parallel on the secondary, a 3:1 turns ratio will be achieved.
Turns ratio can be chosen on the basis of desired duty
cycle. However, remember that the input supply voltage
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