LT1941 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT1941

Triple Monolithic Switching Regulator

Linear Technology 

LT1941

APPLICATIONS INFORMATION

can handle at least 1.5A of current without saturating and

ensure that the inductor has a low DCR (copper-wire resis-

tance) to minimize I2R power losses. If using uncoupled

inductors, each inductor need only handle one-half of the

total switch current so that 0.75A per inductor is sufficient.

A 4.7μH to 15μH coupled inductor or two 15μH to 20μH

uncoupled inductors will usually be the best choice for

most LT1941 inverter designs. A 4.7μH to 15μH inductor

will be the best choice for most LT1941 boost designs. In

this case, the single inductor must carry the entire 1.5A

peak switch current.

Table 4. Inductors

PART NUMBER

Coiltronics

TP3-4R7

TP4-100

Sumida

CDRH6D38NP-6R2

CDRH6D38NP-7R4

CDRH6D38NP-100

VALUE

(μH)

4.7

10

6.2

7.4

10

ISAT(DC)

(A)

1.5

1.5

2.5

2.3

2.0

DCR

(Ω)

0.181

0.146

20m

23m

28m

HEIGHT

(mm)

2.2

3.0

3.8

3.8

3.8

Output Capacitor Selection

Use low ESR (equivalent series resistance) capacitors at

the output to minimize the output ripple voltage. Multilayer

ceramic capacitors are an excellent choice; they have an

extremely low ESR and are available in very small pack-

ages. X7R dielectrics are preferred, followed by X5R, as

these materials retain their capacitance over wide voltage

and temperature ranges. A 4.7μF to 20μF output capacitor

is sufficient for most LT1941 applications. Solid tantalum

or OS-CON capacitors will work but they will occupy more

board area and will have a higher ESR than a ceramic

capacitor. Always use a capacitor with a sufficient volt-

age rating.

Diode Selection

A Schottky diode is recommended for use with the LT1941

inverter/boost regulator. The Microsemi UPS120 is a very

good choice. Where the input to output voltage differential

exceeds 20V, use the UPS140 (a 40V diode). These diodes

are rated to handle an average forward current of 1A. For

applications where the average forward current of the

diode is less than 0.5A, use an ON Semiconductor

MBR0520L diode. The load current for boost, SEPIC and

inverting configurations is equal to the average diode

current.

BIAS2 Pin Considerations

The BIAS2 pin provides the drive current for the inverter/

boost switch. The voltage source on the BIAS2 line should

be able to supply the rated current and be at a minimum

of 2.5V. For highest efficiency, use the lowest voltage

source possible (VOUT = 3.3V, for example) to minimize

the VBIAS2 • IBIAS2 power loss inside the part.

INPUT CAPACITOR SELECTION

Bypass the input of the LT1941 circuit with a 10μF or

higher ceramic capacitor of X7R or X5R type. A lower

value or a less expensive Y5V type will work if there is

additional bypassing provided by bulk electrolytic capaci-

tors, or if the input source impedance is low. The following

paragraphs describe the input capacitor considerations

in more detail.

Step-down regulators draw current from the input sup-

ply in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage ripple

at the LT1941 input and to force this switching current

into a tight local loop, minimizing EMI. The input capaci-

tor must have low impedance at the switching frequency

to do this effectively and it must have an adequate ripple

current rating. With two switchers operating at the same

frequency but with different phases and duty cycles, cal-

culating the input capacitor RMS current is not simple;

however, a conservative value is the RMS input current

for the channel that is delivering the most power (VOUT

times IOUT):

( ) CIN(RMS) = IOUT •

VOUT VIN – VOUT < IOUT

VIN

2

1941fb

15

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