LTC3608(RevC) Просмотр технического описания (PDF) - Linear Technology

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LTC3608
(Rev.:RevC)

18V, 8A Monolithic Synchronous Step-Down DC/DC Converter

Linear Technology 

LTC3608

Applications Information

INTVCC

3.3V OR 5V

D1

VIN

RUN/SS

RSS*

CSS

(5a)

RSS*

D2* RUN/SS

2N7002

CSS

3608 F05

*OPTIONAL TO OVERRIDE

OVERCURRENT LATCHOFF

(5b)

Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated

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. Although all dissipative

elements in the circuit produce losses, four main sources

account for most of the losses in LTC3608 circuits:

1. DC I2R losses. These arise from the resistance of the

internal resistance of the MOSFETs, inductor and PC

board traces and cause the efficiency to drop at high

output currents. In continuous mode the average output

current flows through L, but is chopped between the top

and bottom MOSFETs. The DC I2R loss for one MOSFET

can simply be determined by [RDS(ON) + RL] • IO.

2. Transition loss. This loss arises from the brief amount

of time the top MOSFET spends in the saturated re-

gion during switch node transitions. It depends upon

the input voltage, load current, driver strength and

MOSFET capacitance, among other factors. The loss

is significant at input voltages above 20V and can be

estimated from:

Transition Loss ≅ (1.7A–1) VIN2 IOUT CRSS f

3. INTVCC current. This is the sum of the MOSFET driver

and control currents. This loss can be reduced by sup-

plying INTVCC current through the EXTVCC pin from a

high efficiency source, such as an output derived boost

network or alternate supply if available.

4. CIN loss. The input capacitor has the difficult job of

filtering the large RMS input current to the regulator. It

must have a very low ESR to minimize the AC I2R loss

and sufficient capacitance to prevent the RMS current

from causing additional upstream losses in fuses or

batteries.

Other losses, including COUT ESR loss, Schottky diode D1

conduction loss during dead time and inductor core loss

generally account for less than 2% additional loss.

When making adjustments to improve efficiency, the input

current is the best indicator of changes in efficiency. If you

make a change and the input current decreases, then the

efficiency has increased. If there is no change in input

current, then there is no change in efficiency.

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 ΔILOAD (ESR), where ESR is the effective series

resistance of COUT. ΔILOAD also begins to charge or dis-

charge 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 ITH

pin external components shown in Figure 6 will provide

adequate compensation for most applications. For a

detailed explanation of switching control loop theory see

Application Note 76.

3608fc

17

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