LT1943 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT1943

High Current Quad Output Regulator for TFT LCD Panels

Linear Technology 

LT1943

U

OPERATIO

must have low impedance at the switching frequency to do

this effectively and it must have an adequate ripple current

rating. The input capacitor RMS current can be calculated

from the step-down output voltage and current, and the

input voltage:

CINRMS = IOUT •

VOUT(VIN – V0UT) < IOUT

VIN

2

and is largest when VIN = 2 VOUT (50% duty cycle). The

ripple current contribution from the other channels will be

minimal. Considering that the maximum load current from

switcher 1 is ~2.8A, RMS ripple current will always be less

than 1.4A.

The high frequency of the LT1943 reduces the energy

storage requirements of the input capacitor, so that the

capacitance required is less than 10µF. The combination

of small size and low impedance (low equivalent series

resistance or ESR) of ceramic capacitors makes them the

preferred choice. The low ESR results in very low voltage

ripple. Ceramic capacitors can handle larger magnitudes

of ripple current than other capacitor types of the same

value. Use X5R and X7R types.

An alternative to a high value ceramic capacitor is a lower

value along with a larger electrolytic capacitor, for ex-

ample a 1µF ceramic capacitor in parallel with a low ESR

tantalum capacitor. For the electrolytic capacitor, a value

larger than 10µF will be required to meet the ESR and

ripple current requirements. Because the input capacitor

is likely to see high surge currents when the input source

is applied, only consider a tantalum capacitor if it has the

appropriate surge current rating. The manufacturer may

also recommend operation below the rated voltage of the

capacitor. Be sure to place the 1µF ceramic as close as

possible to the VIN and GND pins on the IC for optimal

noise immunity.

A final caution is in order regarding the use of ceramic

capacitors at the input. A ceramic input capacitor can

combine with stray inductance to form a resonant tank

circuit. If power is applied quickly (for example by plug-

ging the circuit into a live power source), this tank can ring,

doubling the input voltage and damaging the LT1943. The

solution is to either clamp the input voltage or dampen the

tank circuit by adding a lossy capacitor (an electrolytic) in

parallel with the ceramic capacitor. For details, see Appli-

cation Note 88.

Soft-Start and Shutdown

The RUN/SS (Run/Soft-Start) pin is used to place the

switching regulators and the internal bias circuits in shut-

down mode. It also provides a soft-start function, along

with SS-234. If the RUN/SS is pulled to ground, the

LT1943 enters its shutdown mode with all regulators off

and quiescent current reduced to ~35µA. An internal

1.7µA current source pulls up on the RUN/SS and SS-234

pins. If the RUN/SS pin reaches ~0.8V, the internal bias

circuits start and the quiescent currents increase to their

nominal levels.

If a capacitor is tied from the RUN/SS or SS-234 pins to

ground, then the internal pull-up current will generate a

voltage ramp on these pins. This voltage clamps the VC

pin, limiting the peak switch current and therefore input

current during start-up. The RUN/SS pin clamps VC1, and

the SS-234 pin clamps the VC2, VC3, and VC4 pins. A good

value for the soft-start capacitors is COUT/10,000, where

COUT is the value of the largest output capacitor.

To shut down SW2, SW3, and SW4, pull the SS-234 pin to

ground with an open drain or collector.

If the shutdown and soft-start features are not used, leave

the RUN/SS and SS-234 pins floating.

VON Pin Considerations

The VON pin is the delayed output for switching regulator

3. When the CT pin reaches 1.1V, the output disconnect

PNP turns on, connecting VON to E3. The VON pin is current

limited, and will protect the LT1943 and input source from

a shorted output. However, if the VON pin is charged to a

high output voltage, and then shorted to ground through

a long wire, unpredictable results can occur. The resonant

tank circuit created by the inductance of the long wire and

the capacitance at the VON pin can ring the VON pin several

volts below ground. This can lead to large and potentially

damaging currents internal to the LT1943. If the VON

output may be shorted after being fully charged, there

1943fa

16

Share Link: