LTC1436A datasheet(21/28 Pages) LINER

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LTC1436A

LTC1436A-PLL/LTC1437A

APPLICATIONS INFORMATION

Efficiency. For example, in a 20V to 5V application,

10mA of INTVCC current results in approximately 3mA

of VIN current. This reduces the midcurrent loss from

10% or more (if the driver was powered directly from

VIN) to only a few percent.

3. I2R losses are predicted from the DC resistances of the

MOSFET, inductor and current shunt. In continuous

mode the average output current flows through L and

RSENSE, but is “chopped” between the topside main

MOSFET and the synchronous MOSFET. If the two

MOSFETs have approximately the same RDS(ON), then

the resistance of one MOSFET can simply be summed

with the resistances of L and RSENSE to obtain I2R

losses. For example, if each RDS(ON) = 0.05Ω,

RL = 0.15Ω and RSENSE = 0.05Ω, then the total resis-

tance is 0.25

Ω. This results in losses ranging from 3%

to 10% as the output current increases from 0.5A to 2A.

I2R losses cause the efficiency to drop at high output

currents.

4. Transition losses apply only to the topside MOSFET(s),

and only when operating at high input voltages (typi-

cally 20V or greater). Transition losses can be esti-

mated from:

Transition Loss = 2.5(VIN)1.85(IMAX)(CRSS)(f)

Other losses including CIN and COUT ESR dissipative

losses, Schottky conduction losses during dead-time 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 DC (resistive) load

current. When a load step occurs, VOUT immediately shifts

by an amount equal to (

∆ILOAD)(ESR), where ESR is the

effective series resistance of COUT. ∆ILOAD also begins to

charge or discharge COUT which generates a feedback

error signal. The regulator loop then acts to return VOUT to

its steady-state value. During this recovery time VOUT can

be monitored for overshoot or ringing, which would

indicate a stability problem. The ITH external components

shown in the Figure 1 circuit will provide adequate com-

pensation for most applications.

INDUCTOR

1435A F08

Figure 13. Allowable Inductor/RSENSE Layout Orientations

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 the efficiency and which change would produce

the most improvement. Efficiency can be expressed as:

Efficiency = 100% – (L1 + L2 + L3 + ...)

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

of input power.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC1436A/LTC1437A circuits: LTC1436A/

LTC1437A VIN current, INTVCC current, I2R losses and

topside MOSFET transition losses.

1. The VIN current is the DC supply current given in the

Electrical Characteristics table which excludes MOSFET

driver and control currents. VIN current results in a

small (< 1%) loss which increases with VIN.

2. INTVCC current is the sum of the MOSFET driver and

control currents. The MOSFET driver current results

from switching the gate capacitance of the power

MOSFETs. Each time a MOSFET gate is switched from

low to high to low again, a packet of charge dQ moves

from INTVCC to ground. The resulting dQ/dt is a current

out of INTVCC that is typically much larger than the

control circuit current. In continuous mode, IGATECHG =

f(QT + QB), where QT and QB are the gate charges of the

topside and bottom side MOSFETs. It is for this reason

that the Adaptive Power output stage switches to a low

QT MOSFET during low current operation.

By powering EXTVCC from an output-derived source,

the additional VIN current resulting from the driver and

control currents will be scaled by a factor of Duty Cycle/

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