LTC3561 Ver la hoja de datos (PDF) - Linear Technology

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LTC3561

1A, 4MHz, Synchronous Step-Down DC/DC Converter

Linear Technology 

LTC3561

APPLICATIO S I FOR ATIO

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 Linear

Technology Application Note 76.

Although a buck regulator is capable of providing the full

output current in dropout, it should be noted that as the

input voltage VIN drops toward VOUT, the load step capa-

bility does decrease due to the decreasing voltage across

the inductor. Applications that require large load step

capability near dropout should use a different topology

such as SEPIC, Zeta or single inductor, positive buck/

boost.

In some applications, a more severe transient can be

caused by switching in loads with large (>1uF) input

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

problem, if the switch connecting the load has low resis-

tance 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 incor-

porates current limiting, short-circuit protection, and soft-

starting.

Efficiency Considerations

The percent efficiency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efficiency and which change would

produce the most improvement. Percent efficiency can be

expressed as:

%Efficiency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percentage

of input power.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC3561 circuits: 1) LTC3561 VIN current,

2) switching losses, 3) I2R losses, 4) other losses.

1) The VIN current is the DC supply current given in the

electrical characteristics which excludes MOSFET driver

and control currents. VIN current results in a small

(<0.1%) loss that increases with VIN, even at no load.

2) The switching current is the sum of the MOSFET driver

and control currents. The MOSFET driver current re-

sults from switching the gate capacitance of the power

MOSFETs. Each time a MOSFET gate is switched from

low to high to low again, a packet of charge dQ moves

from VIN to ground. The resulting dQ/dt is a current out

of VIN that is typically much larger than the DC bias

current. In continuous mode, IGATECHG = fO(QT + QB),

where QT and QB are the gate charges of the internal top

and bottom MOSFET switches. The gate charge losses

are proportional to VIN and thus their effects will be

more pronounced at higher supply voltages.

3) I2R Losses are calculated from the DC resistances of the

internal switches, RSW, and external inductor, RL. In

continuous mode, the average output current flowing

through inductor L is “chopped” between the internal

top and bottom switches. Thus, the series resistance

VIN

2.63V

TO 5.5V

+

C6

CIN

PGND PGND

CITH

R6

SVIN

PVIN

SW

C8

LTC3561

L1

D1

OPTIONAL

CF

PGND

ITH

VFB

RC

SGND PGND SHDN/RT

CC

RT

R1 R2

+

COUT

VOUT

C5

PGND

PGND

3561 F05

Figure 4. LTC3561 General Schematic

3561f

10

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