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LM5008EP Datasheet, PDF (9/16 Pages) Texas Instruments – High Voltage (100V) Step Down Switching Regulator
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FIGURE 9. Low Ripple Output Configuration
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High Voltage Start-Up Regulator
The LM5008EP contains an internal high voltage startup reg-
ulator. The input pin (VIN) 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 VCC.
Upon power up, the regulator sources current into the external
capacitor at VCC (C3). When the voltage on the VCC pin reach-
es the under-voltage lockout threshold of 6.3V, the buck
switch is enabled.
In applications involving a high value for VIN, where power
dissipation in the VCC regulator is a concern, an auxiliary volt-
age can be diode connected to the VCC pin. Setting the
auxiliary voltage to 8.0 -14V will shut off the internal regulator,
reducing internal power dissipation. See Figure 10. The cur-
rent required into the VCC pin is shown in Figure 2.
FIGURE 10. Self Biased Configuration
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Regulation Comparator
The feedback voltage at FB is compared to an internal 2.5V
reference. In normal operation (the output voltage is regulat-
ed), 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.
Over-Voltage Comparator
The feedback voltage at FB is compared to an internal 2.875V
reference. If the voltage at FB rises above 2.875V the on-time
pulse is immediately terminated. This condition can occur if
the input voltage, or the output load, change suddenly. The
buck switch will not turn on again until the voltage at FB falls
below 2.5V.
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201876 Version 2 Revision 4 Print Date/Time: 2011/08/30 23:05:33
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