LT3503 データシートの表示（PDF） - Linear Technology

部品番号

コンポーネント説明

メーカー

LT3503

1A, 2.2MHz Step-Down Switching Regulator in 2mm × 3mm DFN

Linear Technology 

LT3503

APPLICATIO S I FOR ATIO

Ceramic capacitors have very low equivalent series resis-

tance (ESR) and provide the best ripple performance. A

good value is:

COUT = 24/VOUT

where COUT is in µF. Use X5R or X7R types and keep in

mind that a ceramic capacitor biased with VOUT will have

less than its nominal capacitance. This choice will provide

low output ripple and good transient response. Transient

performance can be improved with a high value capacitor,

but a phase lead capacitor across the feedback resistor R1

may be required to get the full benefit (see the Compen-

sation section). Using a small output capacitor results in

an increased loop crossover frequency and increased

sensitivity to noise. A 22pF capacitor connected between

VOUT and the FB pin is required to filter noise at the FB pin

and ensure stability.

High performance electrolytic capacitors can be used for

the output capacitor. Low ESR is important, so choose

one that is intended for use in switching regulators. The

ESR should be specified by the supplier and should be

0.1Ω or less. Such a capacitor will be larger than a

ceramic capacitor and will have a larger capacitance, be-

cause the capacitor must be large to achieve low ESR.

Table 2 lists several capacitor vendors.

Figure 5 shows the transient response of the LT3503 with

a few output capacitor choices. The output is 3.3V. The

load current is stepped from 0.5A to 1.1A and back to 0.5A,

and the oscilloscope traces show the output voltage. The

upper photo shows the recommended value. The second

photo shows the improved response (less voltage drop)

resulting from a phase lead capacitor. The last photo

shows the response to a high performance electrolytic

capacitor. Transient performance is improved due to the

large output capacitance.

BOOST Pin Considerations

Capacitor C3 and diode D2 are used to generate a boost

voltage that is higher than the input voltage. In most cases

a 0.1µF capacitor and fast switching diode (such as the

1N4148 or 1N914) will work well. Figure 6 shows two

ways to arrange the boost circuit. The BOOST pin must be

at least 2.3V above the SW pin for best efficiency. For

outputs of 3.3V and above, the standard circuit (Figure 6a)

is best. For outputs between 3V and 3.3V, use a 0.22µF

capacitor. For outputs between 2.5V and 3V, use a 0.47µF

capacitor and a small Schottky diode (such as the BAT-54).

For lower output voltages tie a Schottky diode to the input

(Figure 6b). The circuit in Figure 6a is more efficient

because the BOOST pin current comes from a lower

voltage source. You must also be sure that the maximum

voltage rating of the BOOST pin is not exceeded.

Table 2. Capacitor Vendors

Vendor

Phone

Panasonic (714) 373-7366

Kemet

Sanyo

(864) 963-6300

(408) 749-9714

Murata

AVX

(404) 436-1300

Taiyo Yuden (864) 963-6300

URL

www.panasonic.com

www.kemet.com

www.sanyovideo.com

www.murata.com

www.avxcorp.com

www.taiyo-yuden.com

Part Series

Ceramic,

Polymer,

Tantalum

Ceramic,

Tantalum

Ceramic,

Polymer,

Tantalum

Ceramic

Ceramic,

Tantalum

Ceramic

Comments

EEF Series

T494, T495

POSCAP

TPS Series

3503f

11

Share Link: