LTC1772B datasheet(11/16 Pages) LINER

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LTC1772B

1772bfa

APPLICATIONS INFORMATION

current, 2) MOSFET gate charge current, 3) I2R losses

and 4) voltage drop of the output diode.

1. The VIN current is the DC supply current, given in the

electrical characteristics, that excludes MOSFET driver

and control currents. VIN current results in a small loss

which increases with VIN.

2. MOSFET gate charge current results from switching

the gate capacitance of the power MOSFET. 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 which is typically

much larger than the DC supply current. In continuous

mode, IGATECHG = f(Qp).

3. I2R losses are predicted from the DC resistances of

the MOSFET, inductor and current shunt. In continu-

ous mode the average output current ﬂows through L

but is “chopped” between the P-channel MOSFET (in

series with RSENSE) and the output diode. The MOSFET

RDS(ON) plus RSENSE multiplied by duty cycle can be

summed with the resistances of L and RSENSE to obtain

I2R losses.

4. The output diode is a major source of power loss at

high currents and gets worse at high input voltages.

The diode loss is calculated by multiplying the forward

voltage times the diode duty cycle multiplied by the

load current. For example, assuming a duty cycle of

50% with a Schottky diode forward voltage drop of

0.4V, the loss increases from 0.5% to 8% as the load

current increases from 0.5A to 2A.

5. Transition losses apply to the external MOSFET and

increase at higher operating frequencies and input

voltages. Transition losses can be estimated from:

Transition Loss = 2(VIN)2IO(MAX)CRSS(f)

Other losses including CINandCOUTESRdissipativelosses,

and inductor core losses, generally account for less than

2% total additional loss.

Foldback Current Limiting

As described in the Output Diode Selection, the worst-case

dissipation occurs with a short-circuited output when the

diode conducts the current limit value almost continuously.

To prevent excessive heating in the diode, foldback current

limiting can be added to reduce the current in proportion

to the severity of the fault.

Foldback current limiting is implemented by adding diodes

DFB1 and DFB2 between the output and the ITH/RUN pin as

shown in Figure 6. In a hard short (VOUT = 0V), the current

will be reduced to approximately 50% of the maximum

output current.

Figure 6. Foldback Current Limiting

VFB

ITH/RUN

VOUT

LTC1772B

1772 F05

DFB1

DFB2

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