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LT3504

Quad 40V/1A Step-Down Switching Regulator with 100% Duty Cycle Operation

Linear Technology 

LT3504

Applications Information

choice for an inductor value is 8.2µH. These values will

ensure that each buck regulator will have sufficient drive

current to saturate the power switch in all applications

and under all operating conditions.

A user desiring a lower inductor current value can calculate

their optimum inductor size based on their output cur-

rent requirements. Each buck regulator instantaneously

requires 20mA from the SKY pin per 1A of switch current.

The average current that each buck regulator draws from

the SKY pin is 20mA multiplied by the duty cycle. So if

all four buck regulators run at 100% duty cycle with each

channel supplying 1A of output current, then the SKY pin

should be able to source 80mA. However if each chan-

nel runs at 50% duty cycle then the SKY pin only has to

source 40mA. Alternatively if each channel runs at 100%

duty cycle but the output current requirement is 0.5A per

channel instead of 1A, then again the SKY pin only has to

source 40mA. To summarize, the SKY pin output current

requirement is calculated from the following equation:

⎛IOUT1 • VOUT1 + IOUT2 • VOUT2 +⎞

ISKY

=

⎜

⎝

IOUT

3

•

VOUT3 + IOUT4

50 • VIN

•

VOUT 4

⎟

⎠

where IOUTX is the desired output current from Channel

X, VOUTX is the programmed output voltage of Channel X,

and VIN is input voltage.

Once the SKY pin output current requirement is deter-

mined, the inductor value can be calculated based on

the maximum tolerable inductor current ripple from the

following equation:

Soft-Start/Tracking

The RUN/SS pin can be used to soft-start the correspond-

ing channel, reducing the maximum input current during

start-up. The RUN/SS pin is pulled up through a 1µA current

source to about 2.1V. A capacitor can be tied to the pin to

create a voltage ramp at this pin. The buck regulator will

not switch while the RUN/SS pin voltage is less than 0.1V.

As the RUN/SS pin voltage increases above 0.1V, the chan-

nel will begin switching and the FB pin voltage will track

the RUN/SS pin voltage (offset by 0.1V), until the RUN/SS

pin voltage is greater than 0.8V + 0.1V. At this point the

output voltage will be at 100% of it’s programmed value

and the FB pin voltage will cease to track the RUN/SS

pin voltage and remain at 0.8V (the RUN/SS pin will

continue ramping up to about 2.1V with no effect on the

output voltage). The ramp rate can be tailored so that the

peak start up current can be reduced to the current that

is required to regulate the output, with little overshoot.

Figure 4 shows the start-up waveforms with and without

a soft-start capacitor (CSS) on the RUN/SS pin.

IL

0.5A/DIV

VOUT

2V/DIV

100µs/DIV

3504 F04a

Figure 4a. Inductor Current Waveform During

Start-Up without a Soft-Start Capacitor

L=

VIN • DC5

2 • fSW • ⎡⎣0.3 • (1− 0.25 • DC5) − ISKY ⎤⎦

where fSW is the programmed switching frequency and

DC5 is the boost regulator duty cycle, given by: DC5 =

5V/(VIN + 5V).

For a 1MHz application, with VIN = 12V, VOUT1 = 5V, VOUT2

= 3.3V, VOUT3 = 2.5V, VOUT4 = 1.8V, and all channels

supplying 1A of output current, the required SKY pin cur-

rent is 21mA and the inductor value is 6µH.

IL

0.5A/DIV

VOUT

2V/DIV

100µs/DIV

3504 F04b

Figure 4b. Inductor Current Waveform During

Start-Up with a 1nF Soft-Start Capacitor (CSS)

3504fa

12

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