LM4805 datasheet(14/18 Pages) NSC

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Application Information (Continued)

put ripple. Continuous operation is defined as not allowing

the inductor current to drop to zero during the cycle. It should

be noted that all boost converters shift over to discontinuous

operation as the output load is reduced far enough, but a

larger inductor stays “continuous” over a wider load current

range.

To better understand these trade-offs, a typical application

circuit (5V to 12V boost with a 10µH inductor) will be ana-

lyzed. We will assume:

IN =5V, VOUT = 12V, VDIODE = 0.5V, VSW = 0.5V

Since the frequency is 1.6MHz (nominal), the period is ap-

proximately 0.625µs. The duty cycle will be 62.5%, which

means the ON-time of the switch is 0.390µs. It should be

noted that when the switch is ON, the voltage across the

inductor is approximately 4.5V. Using the equation:

V = L (di/dt)

We can then calculate the di/dt rate of the inductor which is

found to be 0.45 A/µs during the ON-time. Using these facts,

we can then show what the inductor current will look like

during operation:

During the 0.390µs ON-time, the inductor current ramps up

0.176A and ramps down an equal amount during the OFF-

time. This is defined as the inductor “ripple current”. It can

also be seen that if the load current drops to about 33mA,

the inductor current will begin touching the zero axis which

means it will be in discontinuous mode. A similar analysis

can be performed on any boost converter, to make sure the

ripple current is reasonable and continuous operation will be

maintained at the typical load current values.

MAXIMUM SWITCH CURRENT

The maximum FET switch current available before the cur-

rent limiter cuts in is dependent on duty cycle of the appli-

cation. This is illustrated in a graph in the typical perfor-

mance characterization section which shows typical values

of switch current as a function of effective (actual) duty cycle.

CALCULATING OUTPUT CURRENT OF BOOST

CONVERTER (I

AMP)

As shown in Figure 2 which depicts inductor current, the load

current is related to the average inductor current by the

relation:

LOAD =IIND(AVG) x (1 - DC)

(7)

Where "DC" is the duty cycle of the application. The switch

current can be found by:

SW =IIND(AVG) + 1/2 (I

RIPPLE)

(8)

Inductor ripple current is dependent on inductance, duty

cycle, input voltage and frequency:

RIPPLE =DCx(VIN-VSW)/(fxL)

(9)

combining all terms, we can develop an expression which

allows the maximum available load current to be calculated:

LOAD(max) = (1–DC)x(ISW(max)–DC(VIN-VSW))/fL (10)

The equation shown to calculate maximum load current

takes into account the losses in the inductor or turn-OFF

switching losses of the FET and diode.

DESIGN PARAMETERS V

SW AND ISW

The value of the FET "ON" voltage (referred to as V

SW in

equations 7 thru 10) is dependent on load current. A good

approximation can be obtained by multiplying the "ON Re-

sistance" of the FET times the average inductor current.

FET on resistance increases at V

IN values below 5V, since

the internal N-FET has less gate voltage in this input voltage

range (see Typical Performance Characteristics curves).

Above V

IN = 5V, the FET gate voltage is internally clamped

to 5V.

The maximum peak switch current the device can deliver is

dependent on duty cycle. For higher duty cycles, see Typical

Performance Characteristics curves.

INDUCTOR SUPPLIERS

Recommended suppliers of inductors for the LM4805 in-

clude, but are not limited to Taiyo-Yuden, Sumida, Coilcraft,

Panasonic, TDK and Murata. When selecting an inductor,

make certain that the continuous current rating is high

enough to avoid saturation at peak currents. A suitable core

type must be used to minimize core (switching) losses, and

wire power losses must be considered when selecting the

current rating.

PCB LAYOUT GUIDELINES

High frequency boost converters require very careful layout

of components in order to get stable operation and low

noise. All components must be as close as possible to the

LM4805 device. It is recommended that a 4-layer PCB be

used so that internal ground planes are available.

Some additional guidelines to be observed:

1. Keep the path between L1, D1, and Co extremely short.

Parasitic trace inductance in series with D1 and Co will

increase noise and ringing.

2. The feedback components R1, R2 and C

f 1 must be kept

close to the FB pin of U1 to prevent noise injection on the FB

pin trace.

3. If internal ground planes are available (recommended)

use vias to connect directly to ground at pin 2 of U1, as well

as the negative sides of capacitors C

s1 and Co.

20126255

FIGURE 2. 10µH Inductor Current

5V - 12V Boost (LM4805)

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