3509 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

一致するリスト

3509

Dual 36V, 700mA Step-Down Regulator

Linear Technology 

LT3509

APPLICATIONS INFORMATION

and Z5U types lose a large fraction of their capacitance

with applied voltage and at temperature extremes. Because

loop stability and transient response depend on the value

of COUT, this loss may be unacceptable. Use X7R and X5R

types.

The value of the output capacitor greatly affects the

transient response to a load step. It has to supply extra

current demand or absorb excess current delivery until

the feedback loop can respond. The loop response is

dependent on the error amplifier transconductance, the

internal compensation capacitor and the feedback net-

work. Higher output voltages necessarily require a larger

feedback divider ratio. This will also reduce the loop gain

and slow the response time. Fortunately this effect can be

mitigated by use of a feed-forward capacitor, CPL, across

the top feedback resistor. The small-signal model shown

in Figure 10 can be used to model this in a simulator or

to give insight to an empirical design. Figure 11 shows

some load step responses with differing output capacitors

and CPL combinations.

Input Capacitor

The input capacitor needs to supply the pulses of charge

demanded during the on time of the switches. Little total

capacitance is required as a few hundred millivolts of ripple

at the VIN pin will not cause any problems to the device.

When operating at 2MHz and 12V, 2μF will work well. At

the lowest operating frequency and/or at low input voltages

a larger capacitor such as 4.7μF is preferred.

ILOAD

700mA

300mA

VOUT (AC)

50mV/DIV

ILOAD

700mA

300mA

VOUT (AC)

50mV/DIV

COUT = 10μF TIME 20μs/DIV

CPL = 0

COUT = 10μF TIME 20μs/DIV

CPL = 82pF

3509 F11

Figure 11. Transient Load Response with Different Combinations

of COUT and CPL Load Current Step from 300mA to 700mA

R1 = 10k, R2 = 32.4k, VIN = 12V, VOUT = 3.3V, fSW = 2.0MHz

3509fc

17

Share Link: