LTC1707 datasheet(10/16 Pages) LINER

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LTC1707

D1 in Figure 5 reduces the start delay but allows CSS to

ramp up slowly providing the soft-start function. This

diode can be deleted if soft-start is not needed.

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, two main sources usually account for most of the

losses in LTC1707 circuits: VIN quiescent current and I2R

losses. The VIN quiescent current loss dominates the

efficiency loss at very low load currents whereas the I2R

loss dominates the efficiency loss at medium to high load

currents. In a typical efficiency plot, the efficiency curve at

very low load currents can be misleading since the actual

power lost is of no consequence as illustrated in Figure 6.

1. The VIN quiescent current is due to two components: the

DC bias current as given in the electrical characteristics

and the internal main switch and synchronous switch

gate charge currents. The gate charge current results

from switching the gate capacitance of the internal power

MOSFET switches. Each time the gate is switched from

high to low or from low to high, a packet of charge dQ

moves from VIN to ground. The resulting dQ/dt is the

current out of VINthat is typically larger than the DC bias

current. In continuous mode, IGATECHG=f(QT+QB)where

QT and QB are the gate charges of the internal top and

bottom switches. Both the DC bias and gate charge losses

are proportional to VIN and thus their effects will be more

pronounced at higher supply voltages.

2. I2R losses are calculated from the resistances of the

internal switches RSW and external inductor RL. In

continuous mode the average output current flowing

through inductor L is “chopped” between the main

switch and the synchronous switch. Thus, the series

resistance looking into SW pin from L is a function of

both top and bottom MOSFET RDS(ON) and the duty

cycle (DC) as follows:

RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)

The RDS(ON) for both the top and bottom MOSFETs can

be obtained from the Typical Performance Characteris-

tics curves. Thus, to obtain I2R losses, simply add RSW

to RL and multiply by the square of the average output

current.

Other losses including CIN and COUT ESR dissipative losses,

MOSFET switching losses and inductor core and copper

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 (

∆ILOAD • ESR), where ESR is the effective series

resistance of COUT. ∆ILOAD also begins to charge or dis-

charge 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 that would indicate a stability

problem. The internal compensation provides adequate

compensation for most applications. But if additional

compensation is required, the ITH pin can be used for

external compensation as shown in Figure 7 (the 47pF

capacitor, CC2, is typically needed for noise decoupling).

LOAD CURRENT (mA)

0.001

0.01

0.1

100

1000

1707 F06

VOUT = 1.5V

VOUT = 3.3V

VOUT = 5V

VIN = 6V

Figure 6. Power Lost vs Load Current

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