2s

10

ON

LM5017

SNVS783I – JANUARY 2012 – REVISED OCTOBER 2015

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Feature Description (continued)

At high input voltages, the power dissipated in the high voltage regulator is significant and can limit the overall

voltage is driven externally by an alternate voltage source between 8.55 V and 13 V, the internal regulator is

disabled. This reduces the power dissipation in the IC.

7.3.3 Regulation Comparator

The feedback voltage at FB is compared to an internal 1.225 V reference. In normal operation, when the output

voltage is in regulation, an on-time period is initiated when the voltage at FB falls below 1.225 V. The high side

switch will stay on for the on-time, causing the FB voltage to rise above 1.225 V. After the on-time period, the

high side switch will stay off until the FB voltage again falls below 1.225 V. During start-up, the FB voltage will be

below 1.225 V at the end of each on-time, causing the high side switch to turn on immediately after the minimum

forced off-time of 144 ns. The high side switch can be turned off before the on-time is over if the peak current in

the inductor reaches the current limit threshold.

7.3.4 Overvoltage Comparator

The feedback voltage at FB is compared to an internal 1.62 V reference. If the voltage at FB rises above 1.62 V

the on-time pulse is immediately terminated. This condition can occur if the input voltage and/or the output load

changes suddenly. The high side switch will not turn on again until the voltage at FB falls below 1.225 V.

7.3.5 On-Time Generator

the LM5017 can be calculated using Equation 3.

(3)

7.3.6 Current Limit

The LM5017 device contains an intelligent current limit off-timer. If the current in the buck switch exceeds 1.02 A,

the present cycle is immediately terminated, and a non-resetable off-timer is initiated. The length of the off-time is

time is set to 16

μs. This condition occurs when the output is shorted and during the initial part of start-up. This

In cases of overload where the FB voltage is above zero volts (not a short circuit) the current limit off-time is

reduced. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery time, and

start-up time. The off-time is calculated from Equation 4.

(4)

The current limit protection feature is peak limited. The maximum average output current will be less than the

peak.

7.3.7 N-Channel Buck Switch and Driver

The LM5017 device integrates an N-Channel Buck switch and associated floating high voltage gate driver. The

gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A

0.01 uF ceramic capacitor connected between the BST pin and the SW pin provides the voltage to the driver

during the on-time. During each off-time, the SW pin is at approximately 0 V, and the bootstrap capacitor charges

recharge the bootstrap capacitor.

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