LTC3772B Datasheet, PDF(13/20 Page) Linear Technology – Micropower No RSENSE Constant Frequency Step-Down DC/DC Controller

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LTC3772B Datasheet, PDF (13/20 Pages) Linear Technology – Micropower No RSENSE Constant Frequency Step-Down DC/DC Controller

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LTC3772B

APPLICATIO S I FOR ATIO

For ceramic capacitors, use X7R or X5R types: do not use

Y5V. Manufacturers include AVX, Kemet, Murata, Taiyo

Yuden and TDK.

Setting Output Voltage

The LTC3772B output voltages are each set by an external

feedback resistor divider carefully placed across the out-

put as shown in Figure 3. The regulated output voltage is

determined by:

VOUT

0.8V

â¢ âââ1+

â â

To improve the frequency response, a feed-forward capaci-

tor, CFF, may be used. Great care should be taken to route

the VFB line away from noise sources, such as the inductor

or the SW line.

LTC3772B

VFB

VOUT

CFF

3772B F03

Figure 3. Setting Output Voltage

Efficiency Considerations

The efficiency of a switching regulator is equal to the out-

put 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. Efficiency can be expressed as:

Efficiency = 100% â (Î·1 + Î·2 + Î·3 + ...)

where Î·1, Î·2, etc. are the individual losses as a percent-

age of input power.

Although all dissipative elements in the circuit produce

losses, five main sources usually account for most of the

losses in LTC3772B circuits: 1) LTC3772B DC bias current,

2) MOSFET gate charge current, 3) I2R losses, 4) voltage

drop of the output diode and 5) external MOSFET transi-

tion losses.

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 that is typically

much larger than the DC supply current. In continuous

mode, IGATECHG = (f)(dQ).

3. I2R losses are predicted from the DC resistances of the

MOSFET, inductor and current shunt. In continuous

mode the average output current flows through L but is

âchoppedâ 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 Schot-

tky 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 volt-

ages. Transition losses can be estimated from:

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

Other losses including CIN and COUT ESR dissipative losses

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.

3772bfa

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