LT1946

5

sn1946 1946fs

OPERATIO

The LT1946 uses a constant frequency, current mode

control scheme to provide excellent line and load regula-

tion. Please refer to Figure 2 for the following description

of the part’s operation. At the start of the oscillator cycle,

the SR latch is set, turning on the power switch Q1. The

switch current flows through the internal current sense

resistor generating a voltage. This voltage is added to a

stabilizing ramp and the resulting sum is fed into the

positive terminal of the PWM comparator A2. When this

voltage exceeds the level at the negative input of A2, the SR

latch is reset, turning off the power switch. The level at the

(A1) and is simply an amplified version of the difference

between the feedback voltage and the reference voltage of

1.250V. In this manner, the error amplifier sets the correct

peak current level to keep the output in regulation.

Two functions are provided to enable a very clean start-up

for the LT1946. Frequency foldback is used to reduce the

oscillator frequency by a factor of 3 when the FB pin is

below a nominal value of 0.5V. This is accomplished via

comparator A3. This feature reduces the minimum duty

cycle that the part can achieve thus allowing better control

of the switch current during start-up. When the FB pin

voltage exceeds 0.5V, the oscillator returns to the normal

frequency of 1.2MHz. A soft-start function is also provided

by the LT1946. When the part is brought out of shutdown,

4

µA of current is sourced out of the SS pin. By connecting

an external capacitor to the SS pin, the rate of voltage rise

on the pin can be set. Typical values for the soft-start

capacitor range from 10nF to 200nF. The SS pin directly

peak switch current. Current limit is not shown in Figure 2.

The switch current is constantly monitored and not al-

lowed to exceed the nominal value of 2.1A. If the switch

current reaches 2.1A, the SR latch is reset regardless of

the output of comparator A2. This current limit helps

protect the power switch as well as the external compo-

nents connected to the LT1946.

APPLICATIO S I FOR ATIO

Inductor Selection

Several inductors that work well with the LT1946 are listed

in Table 1. This table is not exclusive; there are many other

manufacturers and inductors that can be used. Consult

each manufacturer for more detailed information and for

their entire selection of related parts, as many different

sizes and shapes are available. Ferrite core inductors

should be used to obtain the best efficiency, as core losses

at 1.2MHz are much lower for ferrite cores than for the

cheaper powdered-iron ones. Choose an inductor that can

handle at least 1.5A without saturating, and ensure that the

inductor has a low DCR (copper wire resistance) to mini-

µH to 10µH inductor will be

the best choice for most LT1946 designs. Note that in

some applications, the current handling requirements of

the inductor can be lower, such as in the SEPIC topology

where each inductor only carries one-half of the total

switch current.

The inductors shown in Table 1 were chosen for small size.

For better efficiency, use similar valued inductors with a

larger volume.

Table 1. Recommended Inductors

MAX

SIZE

L

DCR

L

× W × H

PART

(

µH) (mΩ)

(mm)

VENDOR

CDRH5D18-4R1

4.1

57

5.7

× 5.7 × 2

Sumida

CDRH5D18-5R4

5.4

76

(847) 956-0666

CDRH5D28-5R3

5.3

38

5.7

× 5.7 × 3

www.sumida.com

CDRH5D28-6R2

6.2

45

CDRH5D28-8R2

8.2

53

ELL6SH-4R7M

4.7

50

6.4

× 6 × 3

Panasonic

ELL6SH-5R6M

5.6

59

(408) 945-5660

ELL6SH-6R8M

6.8

62

www.panasonic.com

RLF5018T-

4.7

45

5.6

× 5.2 × 1.8 TDK

4R7M1R4

(847) 803-6100

www.tdk.com