Datasheet

LM5007
SNVS252F –SEPTEMBER 2003–REVISED MARCH 2013 www.ti.com
Applications utilizing low resistance inductors and/or a low voltage drop rectifier may require special evaluation at
high line, short circuit conditions. In this special case the preset 17µs (FB = 0V) off time may be insufficient to
balance the inductor volt*time product. Additional inductor resistance, output resistance or a larger voltage drop
rectifier may be necessary to balance the inductor cycle volt*time product and limit the short circuit current.
N-Channel Buck Switch and Driver
The LM5007 integrates an N-Channel Buck switch and associated floating high voltage gate driver. This gate
driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. The
bootstrap capacitor is charged by V
CC
through the internal high voltage diode. A 0.01uF ceramic capacitor
connected between the BST pin and SW pin is recommended.
During each cycle when the Buck switch turns off, the SW pin is approximately 0V. When the SW pin voltage is
low, the bootstrap capacitor will be charged from Vcc through the internal diode. The minimum off timer, set to
300ns, ensures that there will be a minimum interval every cycle to recharge the bootstrap capacitor.
An external re-circulating diode from the SW pin to ground is necessary to carry the inductor current after the
internal Buck switch turns off. This external diode must be of the Ultra-fast or Schottky type to reduce turn-on
losses and current over-shoot. The reverse voltage rating of the re-circulating diode must be greater than the
maximum line input voltage.
Thermal Protection
Internal Thermal Shutdown circuitry is provided to protect the integrated circuit in the event the maximum junction
temperature is exceeded. When thermal protection is activated, typically at 165 degrees Celsius, the controller is
forced into a low power reset state, disabling the output driver. This feature is provided to prevent catastrophic
failures from accidental device overheating.
Minimum Load Current
A minimum load current of 1 mA is required to maintain proper operation. If the load current falls below that level,
the bootstrap capacitor may discharge during the long off-time, and the circuit will either shutdown, or cycle on
and off at a low frequency. If the load current is expected to drop below 1 mA in the application, the feedback
resistors should be chosen low enough in value so they provide the minimum required current at nominal Vout.
Figure 5. Operational Waveforms: Figure 6. Operational Waveforms:
LM5007 Operation: LM5007 Operation:
V
OUT
= 10V, V
IN
= 20V, I
OUT
= 250mA V
OUT
= 10V, V
IN
= 75V, I
OUT
= 250mA
CH1: Switch Node, CH2: V
OUT
(AC), CH1: Switch Node, CH2: V
OUT
(AC),
CH4: Inductor Current CH4: Inductor Current
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