LT1936H データシートの表示（PDF） - Linear Technology

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LT1936H

1.4A, 500kHz Step-Down Switching Regulator

Linear Technology 

LT1936

APPLICATIONS INFORMATION

complicated and the best values depend on the application

and in particular the type of output capacitor. A practical

approach is to start with one of the circuits in this data

sheet that is similar to your application and tune the com-

pensation network to optimize the performance. Stability

should then be checked across all operating conditions,

including load current, input voltage and temperature. The

LT1375 data sheet contains a more thorough discussion of

loop compensation and describes how to test the stability

using a transient load.

Figure 1 shows an equivalent circuit for the LT1936 control

loop. The error amplifier is a transconductance amplifier

with finite output impedance. The power section, consisting

of the modulator, power switch and inductor, is modeled

as a transconductance amplifier generating an output

current proportional to the voltage at the VC pin. Note that

the output capacitor integrates this current, and that the

capacitor on the VC pin (CC) integrates the error amplifier

output current, resulting in two poles in the loop. In most

cases a zero is required and comes from either the output

capacitor ESR or from a resistor RC in series with CC.

This simple model works well as long as the value of the

inductor is not too high and the loop crossover frequency

is much lower than the switching frequency. A phase lead

capacitor (CPL) across the feedback divider may improve

the transient response.

Figure 2 compares the transient response across several

output capacitor choices and compensation schemes.

In each case the load current is stepped from 200mA to

800mA and back to 200mA.

COUT = 22μF

(AVX 1210ZD226MAT)

(2a)

COMP

VC

VOUT

100mV/DIV

COUT = 22μF ×2

(2b)

COMP

VC

COUT = 150μF

(4TPC150M)

(2c)

COMP

VC

VOUT

100mV/DIV

VOUT

100mV/DIV

(2d)

COUT = 150μF

(4TPC150M)

COMP

VC

220k

100pF

VOUT

100mV/DIV

800mA

IOUT

500mA/DIV 200mA

50μs/DIV

1936 F02

Figure 2. Transient Load Response of the LT1936 with Different Output

Capacitors as the Load Current is Stepped from 200mA to 800mA. VOUT = 3.3V

1936fd

11

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