LTC3621 datasheet(11/20 Pages) AD

11 / 20 page

LTC3621/LTC3621-2

Rev. D

For more information www.analog.com

OPERATION

selected, the part will transition in and out of sleep mode

depending on the output load current. This significantly

reduces the quiescent current, thus prolonging the use

of the input supply.

VIN Overvoltage Protection

In order to protect the internal power MOSFET devices

against transient voltage spikes, the LTC3621 constantly

monitors the VIN pin for an overvoltage condition. When

VIN rises above 19V, the regulator suspends operation by

shutting off both power MOSFETs. Once VIN drops below

18.7V, the regulator immediately resumes normal opera-

tion. The regulator executes its soft-start function when

exiting an overvoltage condition.

Output Voltage Programming

For non-fixed output voltage parts, the output voltage is

set by external resistive divider according to the following

equation:

VOUT =0.6V • 1+









The resistive divider allows the FB pin to sense a fraction

of the output voltage as shown in Figure 1.

IRMS ≅ IOUT(MAX)

VOUT

VIN

VOUT

– 1

This formula has a maximum at VIN = 2VOUT, where:

IRMS ≅

IOUT

This simple worst-case condition is commonly used for

design because even significant deviations do not offer

much relief. Note that ripple current ratings from capacitor

manufacturers are often based on only 2000 hours of life

which makes it advisable to further derate the capacitor,

or choose a capacitor rated at a higher temperature than

required.Severalcapacitorsmayalsobeparalleledtomeet

size or height requirements in the design. For low input

voltage applications, sufficient bulk input capacitance is

needed to minimize transient effects during output load

changes.

Output Capacitor (COUT) Selection

The selection of COUT is determined by the effective series

resistance(ESR)thatisrequiredtominimizevoltageripple

and load step transients as well as the amount of bulk

capacitance that is necessary to ensure that the control

loop is stable. Loop stability can be checked by viewing

Low Supply Operation

The LTC3621 incorporates an undervoltage lockout circuit

which shuts down the part when the input voltage drops

below 2.7V. As the input voltage rises slightly above the

undervoltage threshold, the switcher will begin its basic

operation. However, the RDS(ON) of the top and bottom

switch will be slightly higher than that specified in the

electrical characteristics due to lack of gate drive. Refer

to graph of RDS(ON) versus VIN for more details.

Soft-Start

TheLTC3621hasaninternal800µssoft-startramp.During

start-up soft-start operation, the switcher will operate in

pulse-skipping mode.

APPLICATIONS INFORMATION

VOUT

3621 F01

CFF

LTC3621

SGND

Figure 1. Setting the Output Voltage

Input Capacitor (CIN) Selection

The input capacitance, CIN, is needed to filter the square

wave current at the drain of the top power MOSFET. To

prevent large voltage transients from occurring, a low

ESR input capacitor sized for the maximum RMS current

should be used. The maximum RMS current is given by:

LTC3621 Folha de dados(PDF) 11 Page - Analog Devices

LTC3621 Folha de dados(HTML) 11 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Folha de dados Download

Ligação URL