Datasheet

FB
SW
L1
C2
V
OUT1
R3
LM5009
BST
VCC
C4
D2
D1
R1
R2
C3
FB
SW
L1
C2
R1
R2
V
OUT2
R3
LM5009
LM5009
SNVS402G –FEBRUARY 2006–REVISED FEBRUARY 2013 www.ti.com
Figure 9. Low Ripple Output Configuration
High Voltage Start-Up Regulator
The LM5009 contains an internal high voltage startup regulator. The input pin (VIN) can be connected directly to
line voltages up to 95 Volts, with transient capability to 100 volts. The regulator is internally current limited at
9.5mA. Upon power up, the regulator sources current into the external capacitor at VCC (C3). When the voltage
on the VCC pin reaches the under-voltage lockout threshold of 6.3V, the buck switch is enabled.
In applications involving a high value for V
IN
, where power dissipation in the V
CC
regulator is a concern, an
auxiliary voltage can be diode connected to the VCC pin. Setting the voltage between 8V and 14V shuts off the
internal regulator, reducing internal power dissipation. See Figure 10. The current required into the VCC pin is
shown in the Typical Performance Characteristics.
Figure 10. Self-Biased Configuration
Regulation Comparator
The feedback voltage at FB is compared to an internal 2.5V reference. In normal operation (the output voltage is
regulated), an on-time period is initiated when the voltage at FB falls below 2.5V. The buck switch will stay on for
the programmed on-time, causing the FB voltage to rise above 2.5V. After the on-time period, the buck switch
will stay off until the FB voltage again falls below 2.5V. During start-up, the FB voltage will be below 2.5V at the
end of each on-time, resulting in the minimum off-time. Bias current at the FB pin is less than 5 nA over
temperature.
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