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Fabricante

LT3505

1.2A, Step-Down Switching Regulator in 3mm ×3mm DFN

Linear Technology 

LT3505

APPLICATIONS INFORMATION

Finite transistor bandwidth limits the speed at which the

power switch can be turned on and off, effectively setting

the minimum on-time of the LT3505. For a given output volt-

age, the minimum on-time determines the maximum input

voltage to remain in continuous mode operation, VIN(PS).

See the “Input Voltage Range” section of the datasheet for

more information on determining VIN(PS). For switching

frequencies below 750kHz, operation above VIN(PS) (up

to 40V) is safe provided that the system will tolerate the

pulse-skipping behavior outlined in the “Minimum On

Time” section of the datasheet. At switching frequencies

exceeding 750kHz, edge loss limits operation to input

voltages below VIN(PS).

Finite transition time results in a small amount of power

dissipation each time the power switch turns on and off

(edge loss). Edge loss increases with frequency, switch

current, and input voltage.

Input Voltage Frequency Foldback

In constant frequency operation (below VIN(PS)) edge

loss only reduces the application efficiency. However, at

high switching frequencies exceeding 750kHz and input

voltages exceeding VIN(PS), the part operates in pulse-skip-

ping mode and the switch current can increase above the

current limit of the part, 1.75A. This further increases the

power dissipated during switch transitions and increases

die temperature. To remedy the situation a single resis-

tor (R5) and a zener diode (D3) can be added to a typical

LT3505 circuit as shown in Figure 4.

When the input voltage is below 16V, the zener diode

path conducts no current and the current flowing out

of the RT pin (and through R4) is nominally 0.5V/20k =

25µA, which programs a 2.2MHz switching frequency.

As the input voltage is increased above 16V, the zener

diode begins to conduct and gradually reduces the cur-

rent flowing out of the RT pin. This mechanism reduces

the switching frequency as the input voltage is increased

above 16V (up to 36V) to ensure that the part constantly

operates in continuous mode without skipping pulses,

thereby preventing the excessive die temperature rise

encountered in pulse-skipping mode.

Although the circuit can be operated indefinitely above

VZENER, this frequency foldback method is intended to

protect circuits during temporary periods of high input

voltage. For example, in many automotive systems, the

normal operating input range might be 9V to 16V, and

the LT3505 can be programmed to operate above the

AM band (>1.8MHz). At the same time, the circuit must

be able to withstand higher input voltages due to load

dump or double-battery jump starts. During these brief

periods, it is usually acceptable to switch at a frequency

within the AM band.

If the output is shorted while the input voltage is greater than

VZENER, the switching frequency will be reduced to 30kHz

and the part will not be able to recover from the short until

the input voltage is reduced below VZENER (see the following

discussion).

VIN

6.7V TO 36V

ON OFF

D3

BZT52C16T

16V

R5

806k

C2

1µF

VIN

BOOST

SHDN

SW

LT3505

FB

RT

GND

VC

D2

1N4148

C3

0.1µF

L1

6.8µH

R1

61.9k

D1

R2

MBRM140 11.5k

R4

20.0k

R3

100k

C4

22pF

VOUT

5V

C5

22pF

C1

10µF

3505 F04

2.50

2.25

2.00

1.75

1.50

1.25

1.00

0.75

Switching

0.50

Frequency

0.25

Maximum

Load Current

0

0 5 10 15 20 25 30 35 40

Input Voltage [V]

LTC3505 • F04b

Figure 4. 2.2MHz, 5V Application with Input Voltage Frequency Foldback Circuit

3505fc

11

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