LTC1143CS datasheet(8/20 Pages) LINER

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LTC1143/LTC1143L

LTC1143L-ADJ

OPERATION Refer to Functional Diagram and Figure 1

The circuit now enters sleep mode with the power MOSFET

turned off. In sleep mode a majority of the circuitry is

turned off, dropping the quiescent current from 1.6mA to

160

µA (for one regulator block). The load current is now

being supplied from the output capacitor. When the output

voltage has dropped by the amount of hysteresis in

comparator V, the P-channel MOSFET is again turned on

and the process repeats.

To avoid the operation of the current loop interfering with

Burst Mode operation, a built-in offset (VOS) is incorpo-

rated in the gain stage. This prevents the current compara-

tor threshold from increasing until the output voltage has

dropped below a minimum threshold.

Using constant off-time architecture the operating fre-

quency is a function of the input voltage. To minimize the

frequency variation as dropout is approached, the off-time

controller increases the CT discharge current as VIN drops

below VOUT + 1.5V. In dropout the P-channel MOSFET is

turned on continuously (100% duty cycle), providing

extremely low dropout operation.

For Figure 1 applications with VOUT below 2V, or when

RSENSE is moved to ground, the current sense comparator

inputs operate near ground. When the current comparator

is operated at less than 2V common mode, the off-time

increases approximately 40%, requiring the use of a

smaller timing capacitor CT.

RSENSE Selection for Output Current

RSENSE is chosen based on the required output current.

The LTC1143 series current comparators have a threshold

range that extends from a minimum of 25mV/RSENSE to a

maximum of 150mV/RSENSE. The current comparator

threshold sets the peak of the inductor ripple current,

yielding a maximum output current IMAX equal to the peak

value less half the peak-to-peak ripple current.

For proper

Burst Mode operation, IRIPPLE(P-P) must be less than or

equal to the minimum current comparator threshold.

Since efficiency generally increases with ripple current,

the maximum allowable ripple current is assumed, i.e.,

IRIPPLE(P-P) = 25mV/RSENSE. (See CT and L Selection for

Operating Frequency). Solving for RSENSE and allowing a

margin for variations in the LTC1143 series and external

component values yields:

RSENSE =

100mV

IMAX

A graph for selecting RSENSE versus maximum output

current is given in Figure 2.

The basic LTC1143L-ADJ application circuit is shown in

Figure 1. The LTC1143 and LTC1143L are similar but omit

the external resistive VOUT dividers (see Figures 10 and

13). External component selection is driven by the load

requirement and begins with VOUT and the selection of

RSENSE. Once RSENSE is known, CT and L can be chosen.

Next, the power MOSFET and D1 are selected. Finally, CIN

and COUT are selected and the loop is compensated. Since

the two regulator sections are identical, the process of

component selection is the same for both sections. The

circuit shown in Figure 1 can be configured for operation

up to an input voltage of 16V.

Output Voltage Selection

The LTC1143/LTC1143L output voltages are internally set

to 3.3V and 5V. The LTC1143L-ADJ requires an external

resistive divider from VOUT to VFB on each section as

shown in Figure 1. The regulated LTC1143L-ADJ output

voltages are given by:

OUT

125 1

















To prevent stray pickup, a 100pF capacitor is suggested

across R1 and R3 located close to the LTC1143L-ADJ.

APPLICATIONS INFORMATION

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