LTC1709 datasheet(14/28 Pages) LINER

14 / 28 page

LTC1709

of the bottom MOSFET from turning on, storing charge

during the dead-time, and requiring a reverse recovery

period which would reduce efficiency. A 1A to 3A Schottky

(depending on output current) diode is generally a good

compromise for both regions of operation due to the

relatively small average current. Larger diodes result in

additional transition losses due to their larger junction

capacitance.

CIN and COUT Selection

In continuous mode, the source current of each top

N-channel MOSFET is a square wave of duty cycle VOUT/

VIN. A low ESR input capacitor sized for the maximum

RMS current must be used. The details of a closed form

equation can be found in Application Note 77. Figure 4

shows the input capacitor ripple current for a 2-phase

configuration with the output voltage fixed and input

voltage varied. The input ripple current is normalized

against the DC output current. The graph can be used in

place of tedious calculations. The minimum input ripple

current can be achieved when the input voltage is twice the

output voltage

In the graph of Figure 4, the 2-phase local maximum input

RMS capacitor currents are reached when:

OUT

−

where k = 1, 2.

These worst-case conditions are commonly used for

design because even significant deviations do not offer

much relief. Note that capacitor manufacturer’s ripple

current ratings are often based on only 2000 hours of life.

This makes it advisable to further derate the capacitor, or

to choose a capacitor rated at a higher temperature than

required. Several capacitors may also be paralleled to

meet size or height requirements in the design. Always

consult the capacitor manufacturer if there is any

question.

It is important to note that the efficiency loss is propor-

tional to the input RMS current

squared and therefore a

2-phase implementation results in 75% less power loss

when compared to a single phase design. Battery/input

protection fuse resistance (if used), PC board trace and

connector resistance losses are also reduced by the re-

duction of the input ripple current in a 2-phase system. The

required amount of input capacitance is further reduced by

the factor, 2, due to the effective increase in the frequency

of the current pulses.

The selection of COUT is driven by the required effective

series resistance (ESR). Typically once the ESR require-

ment has been met, the RMS current rating generally far

exceeds the IRIPPLE(P-P) requirements. The steady state

output ripple (

∆VOUT) is determined by:

∆∆

ESR

OUT

RIPPLE

OUT

≈+









Where f = operating frequency of each stage, COUT =

output capacitance and

∆IRIPPLE = combined inductor

ripple currents.

The output ripple varies with input voltage since

∆IL is a

function of input voltage. The output ripple will be less than

50mV at max VIN with ∆IL = 0.4IOUT(MAX)/2 assuming:

COUT required ESR < 4(RSENSE) and

COUT > 1/(16f)(RSENSE)

The emergence of very low ESR capacitors in small,

surface mount packages makes very physically small

implementations possible. The ability to externally com-

pensate the switching regulator loop using the ITHpin(OPTI-

LOOP compensation) allows a much wider selection of

APPLICATIO S I FOR ATIO

DUTY FACTOR (VOUT/VIN)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.6

0.5

0.4

0.3

0.2

0.1

1709 F04

2-PHASE

1-PHASE

Figure 4. Normalized RMS Input Ripple Current vs

Duty Factor for 1 and 2 Output Stages

LTC1709 Datasheet(PDF) 14 Page - Linear Technology

LTC1709 Datasheet(HTML) 14 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL