LTC1143CS datasheet(12/20 Pages) LINER

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LTC1143/LTC1143L

LTC1143L-ADJ

APPLICATIONS INFORMATION

During this recovery time VOUT can be monitored for

overshoot or ringing which would indicate a stability

problem. The Pin 15(7) external components shown in the

Figure 1 circuit will prove adequate compensation for

most applications.

A second, more severe transient is caused by switching in

loads with large (>1

µF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in parallel

with COUT causing a rapid drop in VOUT. No regulator can

deliver enough current to prevent this problem if the load

switch resistance is low and it is driven quickly. The only

solution is to limit the rise time of the switch drive so that

the load rise time is limited to approximately 25 • CLOAD.

Thus a 10

µF capacitor would require a 250µs rise time,

limiting the charging current to about 200mA.

Efficiency Considerations

The percent 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. Percent efficiency can be

expressed as:

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

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

of input power. (For high efficiency circuits only small

errors are incurred by expressing losses as a percentage

of output power.)

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC1143 series circuits:

1) DC bias current

2) MOSFET gate charge current

3) I2R losses

4) Voltage drop of the Schottky diode.

1) The DC supply current is the current that flows into

VIN (Pin 13 and Pin 5) less the gate charge current. For

VIN= 10V the DCsupplycurrentforeachsectionis160µA

for no load and increases proportionally with load up to

a constant 1.6mA after the LTC1143 series has entered

continuous mode. Because the DC bias current is

Aluminum electrolytic and dry tantalum capacitors are

both available in surface mount configurations. In the case

of tantalum it is critical that the capacitors are surge tested

for use in switching power supplies. An excellent choice is

the AVX TPS series of surface mount tantalums, available

in case heights ranging from 2mm to 4mm. For example,

if 200

µF/10V is called for in an application requiring 3mm

height, (2) AVX 100

µF/10V(P/NTPSD107M010)couldbe

used. Consult the manufacturer for other specific

recommendations.

At low supply voltages a minimum capacitance at COUT is

needed to prevent an abnormal low frequency operating

mode (see Figure 4). When COUT is made too small the

output ripple at low frequencies will be large enough to trip

the voltage comparator. This causes Burst Mode operation

to be activated when the LTC1143 series would normally

be in continuous operation. The output remains in

regulation at all times.

VIN – VOUT VOLTAGE (V)

1000

800

600

400

200

1143 F04

L = 50

µH

RSENSE = 0.02Ω

L = 25

µH

RSENSE = 0.02Ω

L = 50

µH

RSENSE = 0.05Ω

Figure 4. Minimum Value of COUT

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 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 until the regulator loop adapts to the

current change and returns VOUT to its steady-state value.

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