Current Limit

The LM5008 contains an intelligent current limit OFF timer. If

the current in the Buck switch exceeds 0.5A 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

When FB = 0V, a 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 fold-

back, 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 LM5008 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 0V, and

the bootstrap capacitor charges from Vcc through the inter-

nal diode. The minimum OFF timer, set to 300ns, ensures a

minimum time each cycle to recharge the bootstrap capaci-

tor.

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

current after the internal Buck switch turns off. This diode

must be of the Ultra-fast or Schottky type to minimize turn-on

losses and current over-shoot.

Thermal Protection

The LM5008 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 LM5008

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 and

the V

from accidental device overheating. When the junction tem-

perature reduces below 140˚C (typical hysteresis = 25˚C),

the Vcc regulator is enabled, and normal operation is re-

sumed.

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 LM5008 for:

•

Input voltage range (Vin): 12V to 95V

•

Output voltage (V

•

Load current (for continuous conduction mode): 100 mA

to 300 mA

•

Maximum ripple at V

voltage

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

for the LM5008 is 50kHz to 600 kHz. Unless the application

requires a specific frequency, the choice of frequency is

generally a compromise since it affects the size of L1 and

C2, and the switching losses. The maximum allowed fre-

quency, based on a minimum on-time of 400 ns, is calcu-

lated from:

F

For this exercise, Fmax = 263kHz. From equation 1, R

ON

calculates to 304 k

Ω. A standard value 357 kΩ resistor will

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

frequency of 224kHz.

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

be less than 200 mA p-p so the lower peak of the waveform

does not reach zero. L1 is calculated using the following

equation:

At Vin = 95V, L1(min) calculates to 200 µH. The next larger

standard value (220 µH) is chosen and with this value I

OR

calculates to 181 mA p-p at Vin = 95V, and 34 mA p-p at Vin

= 12V.

b) Maximum load current: At a load current of 300 mA, the

peak of the ripple waveform must not reach the minimum

guaranteed value of the LM5008’s current limit threshold

(410 mA). Therefore the ripple amplitude must be less than

220 mA p-p, which is already satisfied in the above calcula-

tion. With L1 = 220 µH, at maximum Vin and Io, the peak of

the ripple will be 391 mA. While L1 must carry this peak

current without saturating or exceeding its temperature rat-

ing, it also must be capable of carrying the maximum guar-

anteed value of the LM5008’s current limit threshold (610

mA) without saturating, since the current limit is reached

during startup.

The DC resistance of the inductor should be as low as

possible. For example, if the inductor’s DCR is one ohm, the

power dissipated at maximum load current is 0.09W. While

small, it is not insignificant compared to the load power of

3W.

C3: The capacitor on the V

filtering and stability, but its primary purpose is to prevent

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