ltc1709-7 Linear Technology Corporation, ltc1709-7 Datasheet - Page 21

• Manufacturer Part Number: ltc1709-7
• Description: 2-phase, 5-bit Vid, Current Mode, High Efficiency, Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
The secondary output voltage V
shown in Figure 5a by the turns ratio N of the transformer:
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current,
then V
V
where R5 and R6 are shown in the Functional Diagram.
If V
temporary continuous switching operation until V
again above its minimum.
In order to prevent erratic operation if no external connec-
tions are made to the FCB pin, the FCB pin has a 0.18 A
internal current source pulling the pin high. Include this
current when choosing resistor values R5 and R6.
The following table summarizes the possible states avail-
able on the FCB pin:
Table 2
FCB Pin
0.85V < V
Feedback Resistors
>4.8V
Voltage Positioning
Voltage positioning can be used to minimize peak-to-peak
output voltage excursion under worst-case transient load-
ing conditions. The open-loop DC gain of the control loop
is reduced depending upon the maximum load step speci-
fications. Voltage positioning can easily be added to the
LTC1709-7 by loading the I
having a Thevenin equivalent voltage source equal to the
0V to 0.75V
SEC
V
V
SEC
SEC
SEC MIN
to the FCB pin sets a minimum voltage V
SEC
FCB
(
drops below this level, the FCB voltage forces
(N + 1) V
< 4.3V
will droop. An external resistive divider from
)
. 0 8
V
U
OUT
1
R
R
U
6
5
Condition
Forced Continuous (Current Reversal
Allowed—Burst Inhibited)
Minimum Peak Current Induces
Burst Mode Operation
No Current Reversal Allowed
Regulating a Secondary Winding
Burst Mode Operation Disabled
Constant Frequency Mode Enabled
No Current Reversal Allowed
No Minimum Peak Current
TH
pin with a resistive divider
SEC
W
is normally set as
SEC(MIN)
U
SEC
:
is
midpoint operating voltage of the error amplifier, or 1.2V
(see Figure 8).
The resistive load reduces the DC loop gain while main-
taining the linear control range of the error amplifier. The
worst-case peak-to-peak output voltage deviation due to
transient loading can theoretically be reduced to half or
alternatively the amount of output capacitance can be
reduced for a particular application. A complete explana-
tion is included in Design Solutions 10 or the LTC1736
data sheet. (See www.linear-tech.com)
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can be
expressed as:
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1709-7 circuits: 1) I
MOSFET transition losses, 3) INTV
and 4) LTC1709-7 V
differential amplifier output).
1) I
fuse (if used), MOSFET, inductor, current sense resistor,
and input and output capacitor ESR. In continuous mode
the average output current flows through L and R
but is “chopped” between the topside MOSFET and the
Figure 8. Active Voltage Positioning Applied to the LTC1709-7
%Efficiency = 100% – (L1 + L2 + L3 + ...)
2
R losses are predicted from the DC resistances of the
INTV
CC
R
R
T2
T1
IN
R
current (including loading on the
C
C
C
I
TH
LTC1709-7
2
R losses, 2) Topside
CC
LTC1709-7
17097 F08
regulator current
21
SENSE
,