Datasheet

FB
SW
L1
C2
V
OUT2
R3
LM5008
BST
V
CC
C4
D2
D1
R1
R2
C3
FB
SW
L1
C2
R1
R2
V
OUT2
R3
LM5008
LM5008
SNVS280G –APRIL 2004–REVISED MARCH 2013 www.ti.com
Figure 10. Low Ripple Output Configuration
High Voltage Start-Up Regulator
The LM5008 contains an internal high voltage startup regulator. The input pin (V
IN
) can be connected directly to
the line voltages up to 95 Volts, with transient capability to 100 volts. The regulator is internally current limited to
9.5mA at V
CC
. Upon power up, the regulator sources current into the external capacitor at V
CC
(C3). When the
voltage on the V
CC
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 V
CC
pin. Setting the auxiliary voltage to 8.0 -14V will shut off the
internal regulator, reducing internal power dissipation. See Figure 11. The current required into the V
CC
pin is
shown in Figure 3.
Figure 11. 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 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 of 300 ns. Bias current at the FB pin is nominally 100 nA.
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