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LTC1143/LTC1143L

LTC1143L-ADJ

APPLICATIONS INFORMATION

The LTC1143L-ADJ outputs can be turned off in one of two

ways: 1) by placing a power MOSFET switch in the VIN line

to the entire regulator or 2) by pulling the VFB pin over

1.4V, which trips comparator V and forces P-DRIVE high

(see Functional Diagram). VFB can be pulled high with a

small current, but any circuitry used to shut down the

LTC1143L-ADJ in this manner must minimize VFB lead

length to prevent noise coupling during normal operation.

In the Figure 6 circuit, taking SHUTDOWN high turns on

PNP QSD that sources a current into VFB. To shut down

properly, RSD must be chosen to pull VFB above 1.4V with

VOUT at 0V and minimum VIN. Note that this technique

depends on the load resistance to prevent VOUT from

floating up due to the current flowing into VFB.

200k

100k

SHUTDOWN

RSD

100pF

50k

VFB

VOUT

VIN

1143 F06

QSD

Figure 6. Local VFB Pull-Up Shuts Down LTC1143L-ADJ

Design Example

As a design example, assume VIN = 12V(nominal), VOUT =

3.3V, IMAX = 2A and ƒ = 200kHz. RSENSE, CT and L can

immediately be calculated:

RSENSE = 100mV/2 = 0.05Ω

tOFF = (1/200kHz)[1 – (3.3/12)] = 3.63µs

CT = 3.63µs/(1.3 × 10

4) = 280pF (use 300pF)

LMIN = 5.1(105 )(0.05Ω)(300pF) 3.3V = 25µH

Assume a dual P-channel power MOSFET is to be

used and dissipation is to be limited to 1W total at

worst-case lowest VIN = 4V. If TA = 50°C and the

thermal resistance of the MOSFET package is 50

°C/W,

then the junction temperature will be 100

°C and

δP = 0.005(100 – 25) = 0.38. The maximum RDS(ON) for

each MOSFET can now be calculated:

P - Ch RDS(ON) =

()

() ( )













33 2

1 38

011

.Ω

Allowing for VIN being slightly below the VGS used to

specify RDS(ON), this requirement can be met by half of

a Siliconix Si4953DY, Fairchild NDS8947 or similar

SO-8 dual P-channel MOSFET.

The most stringent requirement for the Schottky diode is

with VOUT= 0V (i.e. short circuit). Duringacontinuousshort

circuit, the worst-case Schottky diode dissipation rises to:

SC AVG

OUT

() −









()

With the 0.05

Ω sense resistor, ISC(AVG) = 2A will result,

increasing the 0.4V Schottky diode dissipation to 0.8W.

CIN will require an RMS current rating of at least 1A at

temperature and COUT will require an ESR of 0.05Ω for

optimum efficiency.

Troubleshooting Hints

Since efficiency is critical to LTC1143 series applications

it is very important to verify that the circuit is functioning

correctly in both continuous and Burst Mode operation.

The waveform to monitor is the voltage on the timing

capacitor Pins 6 and 14.

In continuous mode (ILOAD > IBURST) the voltage on the CT

pin should be a sawtooth with a 0.9VP-P swing. This

voltage should never dip below 2V as shown in Figure 7a.

When load currents are low (ILOAD < IBURST) Burst Mode

operation occurs. The voltage on the CT pin now falls to

ground for periods of time as shown in Figure 7b.

If Pin 6 or Pin 14 is observed falling to ground at high

output currents, it indicates poor decoupling or improper

grounding. Refer to the Board Layout Checklist.

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