LTC3737_15 Datasheet, PDF(19/24 Page) Linear Technology – Dual 2-Phase, No RSENSE, DC/DC Controller with Output Tracking

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LTC3737_15 Datasheet, PDF (19/24 Pages) Linear Technology – Dual 2-Phase, No RSENSE, DC/DC Controller with Output Tracking

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LTC3737

APPLICATIO S I FOR ATIO

If the duty cycle falls below what can be accommodated by

the minimum on-time, the LTC3737 will begin to skip

cycles. The output voltage will continue to be regulated,

but the ripple current and ripple voltage will increase.

Efficiency Considerations

The efficiency of a switching regulator is equal to the

output power divided by the input power times 100%. It is

often useful to analyze individual losses to determine what

is limiting efficiency and which change would produce the

most improvement. Efficiency can be expressed as:

Efficiency = 100% - (L1 + L2 + L3 + â¦)

where L1, L2, etc. are the individual losses as a percentage

of input power.

Although all dissipative elements in the circuit produce

losses, five main sources usually account for most of the

losses in LTC3737 circuits: 1) LTC3737 DC bias current,

2) MOSFET gate charge current, 3) I2R losses, 4) voltage

drop of the output diode and 5) transition losses.

1) The VIN (pin) current is the DC supply current, given in

the electrical characteristics, that excludes MOSFET

driver currents. VIN current results in a small loss that

increases with VIN.

2) MOSFET gate charge current results from switching the

gate capacitance of the power MOSFET. Each time a

MOSFET gate is switched from low to high to low again,

a packet of charge dQ moves from PVIN to ground. The

resulting dQ/dt is a current out of PVIN, which is

typically much larger than the DC supply current. In

continuous mode, IGATECHG = f â¢ QP.

3) I2R losses are calculated from the DC resistances of the

MOSFET, inductor and sense resistor. In continuous

mode, the average output current flows through L but

is âchoppedâ between the P-channel MOSFET and the

output diode. The MOSFET RDS(ON) multiplied by duty

cycle can be summed with the resistance of L to obtain

I2R losses.

4) The output diode is a major source of power loss at high

currents and is worse at high input voltages. The diode

loss is calculated by multiplying the forward voltage

times the load current times the diode duty cycle.

5) Transition losses apply to the external MOSFET and

increase with higher operating frequencies and input

voltages. Transition losses can be estimated from:

Transition Loss = 2(VIN)2 â¢ IO(MAX) â¢ CRSS(f)

Other losses, including CIN and COUT ESR dissipative

losses and inductor core losses, generally account for less

than 2% total additional loss.

Checking Transient Response

The regulator loop response can be checked by looking at

the load transient response. Switching regulators take

several cycles to respond to a step in load current. When

a load step occurs, VOUT immediately shifts by an amount

equal to (âILOAD)(ESR), where ESR is the effective series

resistance of COUT. âILOAD also begins to charge or dis-

charge COUT, which generates a feedback error signal. The

regulator loop then returns VOUT to its steady-state value.

During this recovery time, VOUT can be monitored for over-

shoot or ringing. OPTI-LOOPÂ® compensation allows the

transient response to be optimized over a wide range of

output capacitance and ESR values.

The ITH series RC-CC filter (see Functional Diagram) sets

the dominant pole-zero loop compensation. The ITH exter-

nal components shown in the Figure 1 circuit will provide

an adequate starting point for most applications. The

values can be modified slightly (from 0.2 to 5 times their

suggested values) to optimize transient response once the

final PC layout is done and the particular output capacitor

type and value have been determined. The output capaci-

tors need to be decided upon because the various types

and values determine the loop feedback factor gain and

phase. An output current pulse of 20% to 100% of full load

current having a rise time of 1Âµs to 10Âµs will produce

output voltage and ITH pin waveforms that will give a sense

of the overall loop stability. The gain of the loop will be

increased by increasing RC, and the bandwidth of the loop

will be increased by decreasing CC. The output voltage

settling behavior is related to the stability of the closed-

loop system and will demonstrate the actual overall supply

performance. For a detailed explanation of optimizing the

compensation components, including a review of control

loop theory, refer to Application Note 76.

OPTI-LOOP is a registered trademark of Linear Technology Corporation.

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