LTC3548EDD-2(RevA) Просмотр технического описания (PDF) - Linear Technology

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LTC3548EDD-2
(Rev.:RevA)

Dual Synchronous, Fixed/Adjustable Output, 2.25MHz Step-Down DC/DC Regulator

Linear Technology 

LTC3548-2

APPLICATIO S I FOR ATIO

Checking Transient Response

The regulator loop response can be checked by looking at

the load transient response. Switching regulators take

several cycles to respond to a step in load current. When

a load step occurs, VOUT immediately shifts by an amount

equal to ∆ILOAD • ESR, where ESR is the effective series

resistance of COUT. ∆ILOAD also begins to charge or

discharge COUT, generating a feedback error signal used

by the regulator to return VOUT to its steady-state value.

During this recovery time, VOUT can be monitored for

overshoot or ringing that would indicate a stability

problem.

The initial output voltage step may not be within the

bandwidth of the feedback loop, so the standard second-

order overshoot/DC ratio cannot be used to determine

phase margin. In addition, a feed-forward capacitor, CF,

can be added to improve the high frequency response, as

shown in Figure 2. Capacitor CF provides phase lead by

creating a high frequency zero with R2, which improves

the 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 Applica-

tion Note 76.

In some applications, a more severe transient can be

caused by switching loads with large (>1µF) load input

capacitors. The discharged load input capacitors are ef-

fectively 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

resistance and is driven quickly. The solution is to limit the

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

controller is designed specifically for this purpose and

usually incorporates current limiting, short-circuit protec-

tion 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, 4 main sources usually account for most of the

losses in LTC3548-2 circuits: 1)VIN quiescent 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 results

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 continu-

ous 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 flows through

inductor L, but is “chopped” between the internal top and

bottom switches. Thus, the series resistance looking into

the SW pin is a function of both top and bottom MOSFET

RDS(ON) and the duty cycle (D) as follows:

RSW = (RDS(ON)TOP)(D) + (RDS(ON)BOT)(1 – D)

The RDS(ON) for both the top and bottom MOSFETs can be

obtained from the Typical Performance Characteristics

curves. Thus, to obtain I2R losses:

I2R losses = IOUT2(RSW + RL)

Hot Swap is a trademark of Linear Technology Corporation.

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