LT7101 datasheet(29/38 Pages) AD

29 / 38 page

LT7101

Rev. 0

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APPLICATIONS INFORMATION

When observing the response of VOUT to a load step, the

initial output voltage step may not be within the bandwidth

of the feedback loop. As a result, the standard second

order overshoot/DC ratio cannot be used to estimate

phase margin. The output voltage settling behavior is

related to the stability of the closed-loop system and will

demonstrate the actual overall supply performance. For

a detailed explanation of optimizing the compensation

components, including a review of control loop theory,

refer to Analog Devices Application Note 76.

In some applications, severe transients can be caused

by switching in loads with large (>1μF) supply bypass

capacitors. The discharged input capacitors are effectively

put in parallel with COUT, causing a rapid drop in VOUT.

No regulator can deliver enough current to prevent this

output droop if the switch connecting the load has low

resistance and is driven quickly. The solution is to limit

the turn-on speed of the load switch driver. A Hot Swap™

controller is designed specifically for this purpose and

usually incorporates current limit, short-circuit protection

and soft-start functions.

Average Output Current Limit and Monitor

The LT7101 contains a fast and accurate average cur-

rent limit that can be externally controlled and monitored.

This fast current loop is useful in applications such as the

charging of batteries and capacitors or current program-

ming in LEDs and laser diodes. The average output current

limit is set using the ICTRL pin. The voltage on the ICTRL

pin sets the average output current limit according to:

ILIM(AVG) =

VICTRL – 0.4

0.811

This allows for the average current limit to be set anywhere

between 0A and 1.11A by adjusting the ICTRL voltage from

0.4V to 1.3V. If the ICTRL voltage is less than 0.4V, it will

be internally limited to 0.4V, so that the average output

current cannot be set to a negative value. Likewise, the

ICTRL voltage is internally limited to 1.3V if the ICTRL pin

is floated or tied to a voltage greater than 1.3V.

An internal 20μA pull-up on this pin allows a single resis-

tor to SGND to be used to set the voltage. To program

a particular fixed average output current limit ILIM(AVG),

chose a resistor according to:

RICTRL =

0.811•ILIM(AVG)+ 0.4

20µA

Since the LT7101 uses average current mode control with

a high speed inner current loop, there are no stability

concerns when operating in constant current mode. In

addition, the LT7101 automatically optimizes the cur-

rent loop based on switching frequency and operating

condition. The unity-gain bandwidth of the average cur-

rent loop is maintained at approximately 1/3 of the switch-

ing frequency. This enables the LT7101 to respond to

changes in the ITH pin voltage on a nearly cycle-by-cycle

basis. This is orders of magnitude faster than competing

solutions, where a slow, average current loop is placed

outside of the voltage regulation loop.

When operating in constant current mode with a low volt-

age on ICTRL, the inductor current will become discontinu-

ous. In this situation, the LT7101 average current loop

maintains good output current programming accuracy

down to no load.

The average output current can be monitored at the IMON

pin. This pin generates a voltage that represents a filtered

version (fC = 10kHz) of the internally sensed inductor cur-

rent. The DC voltage on IMON normally varies between

0.4V and 1.3V, corresponding to an average output cur-

rent between 0A and 1.11A according to:

VIMON = 0.811 • IOUT(AVG) + 0.4

The IMON voltage may momentarily be less than 0.4V or

greater than 1.3V, but eventually is limited to these levels

by the average current loop. During SLEEP, this pin is held

at 0.4V. To ensure stability of the internal IMON buffer,

place a 2k or higher resistor in series with any capacitive

load that is greater than 100pF.

LT7101 Datasheet(PDF) 29 Page - Analog Devices

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