LTC3809-1_15 Datasheet, PDF (18/24 Pages) Linear Technology – No RSENSE, Low Input Voltage, Synchronous DC/DC Controller with Output Tracking

LTC3809-1
APPLICATIONS INFORMATION
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
V_IN
+
V_AUX
LTC3809-1
L1
1μF
R6
TG
1:N
V_OUT
MODE _SW
R5
BG
+
C_OUT
38091 F06
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.
105
V_REF
100
95
MAXIMUM
SENSE VOLTAGE
90
85
Figure 6. Auxiliary Output Loop Connection
80
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):
V_AUX(MIN)
=
0.4 V
•
⎛⎝⎜ _{1 +}
R6⎞
R5⎠⎟
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.
75
2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0
INPUT VOLTAGE (V)
38091 F07
Figure 7. Line Regulation of V_REF and 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_ON(MIN)
<
V_OUT
f_OSC • V_IN
38091fc
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