Current Limit

The LM5009 contains an intelligent current limit OFF timer. If

the current in the Buck switch exceeds 0.31A the present

cycle is immediately terminated, and a non-resetable OFF

timer is initiated. The length of off-time is controlled by an

external resistor (R

maximum off-time is required, and the time is preset to 35µs.

This condition occurs when the output is shorted, and during

the initial part of start-up. This amount of time ensures safe

short circuit operation up to the maximum input voltage of

95V. In cases of overload where the FB voltage is above

zero volts (not a short circuit) the current limit off-time will be

less than 35µs. Reducing the off-time during less severe

overloads reduces the amount of foldback, recovery time,

and the start-up time. The off-time is calculated from the

following equation:

T

(3)

The current limit sensing circuit is blanked for the first 50-

70ns of each on-time so it is not falsely tripped by the current

surge which occurs at turn-on. The current surge is required

by the re-circulating diode (D1) for its turn-off recovery.

N - Channel Buck Switch and

Driver

The LM5009 integrates an N-Channel buck switch and as-

sociated floating high voltage gate driver. The gate driver

circuit works in conjunction with an external bootstrap ca-

pacitor and an internal high voltage diode. A 0.01µF ceramic

capacitor (C4) connected between the BST pin and SW pin

provides the voltage to the driver during the on-time.

During each off-time, the SW pin is at approximately -1V, and

the bootstrap capacitor charges from Vcc through the inter-

nal diode. The minimum OFF timer ensures a minimum time

each cycle to recharge the bootstrap capacitor.

An external re-circulating diode (D1) carries the inductor

current after the internal buck switch turns off. This diode

should be of the Ultra-fast or Schottky type to minimize

turn-on losses and current over-shoot.

Thermal Protection

The LM5009 should be operated so the junction temperature

does not exceed 125˚C during normal operation. An internal

Thermal Shutdown circuit is provided to protect the LM5009

in the event of a higher than normal junction temperature.

When activated, typically at 165˚C, the controller is forced

into a low power reset state, disabling the buck switch. This

feature prevents catastrophic failures from accidental device

overheating. When the junction temperature reduces below

140˚C (typical hysteresis = 25˚C), the buck switch is en-

abled, and normal operation is resumed.

Applications Information

SELECTION OF EXTERNAL COMPONENTS

A guide for determining the component values will be illus-

trated with a design example. Refer to Figure 1. The follow-

ing steps will configure the LM5009 for:

•

Input voltage range (Vin): 12V to 90V

•

Output voltage (V

•

Load current (for continuous conduction mode): 100mA

to 150mA

R1 and R2: From Figure 1,V

and since V

Standard values of 3.01 k

Ω (R1) and 1.00 kΩ (R2) are

chosen. Other values could be used as long as the 3:1 ratio

is maintained. The selected values, however, provide a small

amount of output loading (2.5 mA) in the event the main load

is disconnected. This allows the circuit to maintain regulation

until the main load is reconnected.

F

quency, the choice of frequency is generally a compromise

since it affects the size of L1 and C2, and the switching

losses. The maximum allowed frequency, based on a mini-

mum on-time of 250 ns, is calculated from:

F

For this exercise, Fmax = 444 kHz. From equation 1, R

ON

calculates to 180 k

Ω. A standard value 237 kΩ resistor will

be used to allow for tolerances in equation 1, resulting in a

nominal frequency of 337 kHz.

L1: The main parameter affected by the inductor is the

output current ripple amplitude. The choice of inductor value

therefore depends on both the minimum and maximum load

currents, keeping in mind that the maximum ripple current

occurs at maximum Vin.

a) Minimum load current: To maintain continuous conduc-

tion at minimum Io (100 mA), the ripple amplitude (I

20165807

FIGURE 4. Shutdown Implementation

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