11

LM5007

www.ti.com

SNVS252H – SEPTEMBER 2003 – REVISED NOVEMBER 2018

Product Folder Links: LM5007

Submit Documentation Feedback

Copyright © 2003–2018, Texas Instruments Incorporated

Feature Description (continued)

Select the current limit off-time such that it is less than the MOSFET off-time during normal steady-state

switching operation. Applications utilizing low-resistance inductors and/or a low-voltage-drop freewheeling power

diodes may require special evaluation at high line, short-circuited conditions. In this special case the preset 17-µs

off-time (VFB = 0 V) may be insufficient to provide inductor volt-seconds balance. Additional inductor resistance,

output resistance or a larger voltage drop diode may be necessary to balance inductor volt-seconds and limit the

short-circuit current.

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

bootstrap capacitor is charged by VCC through the internal high voltage diode. A 0.01-µF ceramic capacitor

connected between BST and SW is recommended.

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

low, the bootstrap capacitor is charged from VCC through the internal bootstrap diode. The minimum off-timer, set

to 300 ns, ensures that there is a minimum interval every switching cycle to recharge the bootstrap capacitor.

An external recirculating diode from the SW to RTN is necessary to carry the inductor current after the internal

buck switch turns off. This external diode must be an ultra-fast switching or Schottky type to reduce turn-on

losses and switch current overshoot. The reverse voltage rating of the recirculating 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°C, the converter 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, choose the

feedback resistors with sufficiently low value to provide the minimum required load current at nominal VOUT.

7.3.9 Ripple Configuration

The LM5007 uses an adaptive constant on-time (COT) control in which the conduction time of the buck MOSFET

is terminated by an on-timer and the off-time is terminated by the feedback voltage (VFB) falling below the

reference voltage (VREF). Therefore, for stable operation, the feedback voltage must decrease monotonically and

in phase with the inductor current during the off-time interval. Furthermore, this change in feedback voltage (VFB)

during the off-time must be larger than any noise 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

node (VOUT) for stable operation. If this condition is not satisfied, unstable switching behavior is observed in COT

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