Datasheet

LTC3809-1
18
38091fc
The auxiliary output voltage V
AUX
is normally set, as shown
in Figure 6, by the turns ratio N of the transformer:
V
AUX
= (N + 1) • V
OUT
APPLICATIONS INFORMATION
In a hard short (V
OUT
= 0V), the top P-channel MOSFET
is turned off and kept off until the short-circuit condition
is cleared. In this case, there is no current path from
input supply (V
IN
) to either V
OUT
or GND, which prevents
excessive MOSFET and inductor heating.
Figure 6. Auxiliary Output Loop Connection
V
AUX
V
OUT
C
OUT
V
IN
LTC3809-1
+
+
R6
R5
1μF
L1
1:N
MODE
BG
SW
TG
38091 F06
However, if the controller goes into pulse-skipping operation
and halts switching due to a light primary load current, then
V
AUX
will droop. An external resistor divider from V
AUX
to
the MODE sets a minimum voltage V
AUX(MIN)
:
VV
R
R
AUX MIN()
.•=+
⎛
⎝
⎜
⎞
⎠
⎟
04 1
6
5
If V
AUX
drops below this value, the MODE voltage forces
temporary continuous switching operation until V
AUX
is
again above its minimum.
Fault Condition: Short-Circuit and Current Limit
If the LTC3809-1’s load current exceeds the short-circuit
current limit (I
SC
), which is set by the short-circuit sense
threshold (ΔV
SC
) and the on resistance (R
DS(ON)
) of
bottom N-channel MOSFET, the top P-channel MOSFET
is turned off and will not be turned on at the next clock
cycle unless the load current decreases below I
SC
. In this
case, the controller’s switching frequency is decreased
and the output is regulated by short-circuit (current limit)
protection.
Figure 7. Line Regulation of V
REF
and Maximum Sense Voltage
INPUT VOLTAGE (V)
75
NORMALIZED VOLTAGE OR CURRENT (%)
85
95
105
80
90
100
2.2 2.4 2.6 2.8
38091 F07
3.02.12.0 2.3 2.5 2.7 2.9
V
REF
MAXIMUM
SENSE VOLTAGE
Low Supply Voltage
Although the LTC3809-1 can function down to below 2.4V,
the maximum allowable output current is reduced as V
IN
decreases below 3V. Figure 7 shows the amount of change
as the supply is reduced down to 2.4V. Also shown is the
effect on V
REF
.
Minimum On-Time Considerations
Minimum on-time, t
ON(MIN)
is the smallest amount of time
that the LTC3809-1 is capable of turning the top P-channel
MOSFET on. It is determined by internal timing delays and
the gate charge required to turn on the top MOSFET. Low
duty cycle and high frequency applications may approach
the minimum on-time limit and care should be taken to
ensure that:
t
V
fV
ON MIN
OUT
OSC IN
()
•
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