LM5010AMH datasheet(11/17 Pages) NSC

11 / 17 page

Current Limit (Continued)

mode. The load current (I

OCL) is equal to the current limit

threshold plus half the ripple current (

∆I/2). The ripple ampli-

tude (

∆I) is calculated from:

(9)

The current limit threshold can be increased by connecting

an external resistor (R

CL) between SGND and ISEN. RCL

typically is less than 1

Ω, and the calculation of its value is

explained in the Applications Information section. If the cur-

rent limit threshold is increased by adding R

CL, the maximum

continuous load current should not exceed 1.5A, and the

peak current out of the SW pin should not exceed 2A.

N - Channel Buck Switch and

Driver

The LM5010A integrates an N-Channel buck switch and

associated floating high voltage gate driver. The peak cur-

rent through the buck switch should not exceed 2A, and the

load current should not exceed 1.5A. The gate driver circuit

is powered by the external bootstrap capacitor between BST

and SW (C4), which is recharged each off-time from V

through the internal high voltage diode. The minimum off-

time, nominally 260 ns, ensures sufficient time during each

cycle to recharge the bootstrap capacitor. A 0.022 µF ce-

ramic capacitor is recommended for C4.

Soft-start

The soft-start feature allows the regulator to gradually reach

a steady state operating point, thereby reducing startup

stresses and current surges. At turn-on, while V

CC is below

the under-voltage threshold (t1 in Figure 1), the SS pin is

internally grounded, and V

OUT is held at 0V. When VCC

exceeds the under-voltage threshold (UVLO) an internal

11.5 µA current source charges the external capacitor (C6)

at the SS pin to 2.5V (t2 in Figure 1). The increasing SS

voltage at the non-inverting input of the regulation compara-

tor gradually increases the output voltage from zero to the

desired value. The soft-start feature keeps the load inductor

current from reaching the current limit threshold during

start-up, thereby reducing inrush currents.

An internal switch grounds the SS pin if V

CC is below the

under-voltage lock-out threshold, if a thermal shutdown oc-

curs, or if the circuit is shutdown using the RON/SD pin.

Thermal Shutdown

The LM5010A should be operated below the Maximum Op-

erating Junction Temperature rating. If the junction tempera-

ture increases during a fault or abnormal operating condi-

tion, the internal Thermal Shutdown circuit activates typically

at 175˚C. The Thermal Shutdown circuit reduces power

dissipation by disabling the buck switch and the on-timer,

and grounding the SS pin. This feature helps prevent cata-

strophic failures from accidental device overheating. When

the junction temperature reduces below approximately

155˚C (20˚C typical hysteresis), the SS pin is released and

normal operation resumes.

Applications Information

EXTERNAL COMPONENTS

The procedure for calculating the external components is

illustrated with a design example. Referring to the Block

Diagram, the circuit is to be configured for the following

specifications:

•

OUT =5V

•

IN =6Vto60V

•

S = 175 kHz

•

Minimum load current = 200 mA

•

Maximum load current = 1.0A

•

Softstart time=5ms.

R1 and R2: These resistors set the output voltage, and their

ratio is calculated from:

R1/R2 = (V

OUT/2.5V) - 1

(10)

R1/R2 calculates to 1.0. The resistors should be chosen

from standard value resistors in the range of 1.0 k

Ω -10kΩ.

A value of 1.0 k

Ω will be used for R1 and for R2.

ON,FS: RON can be chosen using Equation 7 to set the

nominal frequency, or from Equation 6 if the on-time at a

particular V

IN is important. A higher frequency generally

means a smaller inductor and capacitors (value, size and

cost), but higher switching losses. A lower frequency means

20153820

FIGURE 4. Inductor Current - Current Limit Operation

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