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LT3645

36V 500mA Step-Down Regulator and 200mA LDO

Linear Technology 

LT3645

APPLICATIONS INFORMATION

large enough to create an additional zero, and the phase

lead might not be necessary. If the output capacitor is

different than the recommended capacitor, stability should

be checked across all operating conditions, including input

voltage and temperature.

Figure 7 shows the transient response of the LT3645 with a

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

current is stepped from 0.25A to 0.5A and back to 0.25A,

and the oscilloscope traces show the output voltage. The

upper photo shows the recommended value. The second

photo shows the improved response (faster recovery)

resulting from a phase lead capacitor.

No Phase Lead Capacitor

pin and the FB2 pin. Capacitors up to 1nF can be used. This

bypass capacitor reduces system noise as well.

Extra consideration must be given to the use of ceramic

capacitors. Ceramic capacitors are manufactured with a

variety of dielectrics, each with different behavior across

temperature and applied voltage. The most common

dielectrics used are specified with EIA temperature char-

acteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and

Y5V dielectrics are good for providing high capacitances

in a small package, but they tend to have strong voltage

and temperature coefficients as shown in Figures 8 and 9.

When used with a 5V regulator, a 16V 10μF Y5V capaci-

tor can exhibit an effective value as low as 1μF to 2μF

for the DC bias voltage applied and over the operating

With Phase Lead Capacitor

Figure 7.

Frequency Compensation (LDO)

The LT3645 LDO requires an output capacitor for stability.

It is designed to be stable with most low ESR capacitors

(typically ceramic, tantalum or low ESR electrolytic). A

minimum output capacitor of 2.2μF with an ESR of 0.5Ω

or less is recommended to prevent oscillations. Larger

values of output capacitance decrease peak deviations

and provide improved transient response for larger load

current changes. Bypass capacitors, used to decouple

individual components powered by the LT3645, increase

the effective output capacitor value. For improvement in

transient performance, place a capacitor across the OUT2

20

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10μF

0

X5R

–20

–40

–60

Y5V

–80

–100

0 2 4 6 8 10 12 14 16

DC BIAS VOLTAGE (V)

3645 F08

Figure 8. Ceramic Capacitor DC Bias Characteristics

40

20

0

X5R

–20

–40

Y5V

–60

–80 BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10μF

–100

–50 –25 0 25 50 75

TEMPERATURE (°C)

100 125

3645 F09

Figure 9. Ceramic Capacitor Temperature Characteristics

3645f

15

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