LM5007

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SNVS252G – SEPTEMBER 2003 – REVISED NOVEMBER 2015

Feature Description (continued)

7.3.6 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

connected between the BST pin and SW pin is recommended.

During each cycle when the Buck switch turns off, the SW pin is approximately 0 V. When the SW pin voltage is

300 ns, 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.

7.3.7 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.

7.3.8 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

7.3.9 Ripple Configuration

LM5007 uses Constant-On-Time (COT) control in which the on-time is terminated by an on-timer and the off-time

operation, the feedback voltage must decrease monotonically, in phase with the inductor current during the off-

component present at the feedback node.

Table 1 shows three different methods for generating appropriate voltage ripple at the feedback node. Type 1

and Type 2 ripple circuits couple the ripple at the output of the converter to the feedback node (FB). The output

voltage ripple has two components:

1. Capacitive ripple caused by the inductor current ripple charging/discharging the output capacitor.

2. Resistive ripple caused by the inductor current ripple flowing through the ESR of the output capacitor.

The capacitive ripple is not in phase with the inductor current. As a result, the capacitive ripple does not

decrease monotonically during the off-time. The resistive ripple is in phase with the inductor current and

decreases monotonically during the off-time. The resistive ripple must exceed the capacitive ripple at the output

converters, with multiple on-time bursts in close succession followed by a long off-time.

voltage ripple, it is ideally suited for applications where low output voltage ripple is required. See AN-1481

Controlling Output Ripple and Achieving ESR Independence in Constant On-Time (COT) Regulator Designs

(SNVA166) for more details for each ripple generation method.

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