LT1946

7

sn1946 1946fs

APPLICATIO S I FOR ATIO

Compensation—Theory

Like all other current mode switching regulators, the

LT1946 needs to be compensated for stable and efficient

operation. Two feedback loops are used in the LT1946: a

fast current loop which does not require compensation,

and a slower voltage loop which does. Standard Bode plot

analysis can be used to understand and adjust the voltage

feedback loop.

As with any feedback loop, identifying the gain and phase

contribution of the various elements in the loop is critical.

Figure 4 shows the key equivalent elements of a boost

converter. Because of the fast current control loop, the

power stage of the IC, inductor and diode have been

replaced by the equivalent transconductance amplifier

IC.

From Figure 4, the DC gain, poles and zeroes can be

calculated as follows:

Output Pole: P1=

2

2• •R

Error Amp Pole: P2 =

1

2• •R

Error Amp Zero: Z1=

1

2• •R

DC GAIN: A =

1.25

V

ESR Zero:

RHP Zero: Z3 =

High Frequency Pole: P3 >

L

O

C

OUT

π

π

π

=

π

π

•

•

•

••

•

•

••

•

•

••

•

C

C

C

gR

g

R

Z

ESR C

VR

VL

f

OUT

C

C

ma

O

mp

L

OUT

IN

L

OUT

S

2

1

2

2

3

2

2

–

+

R2

R1, R2: FEEDBACK RESISTOR DIVIDER NETWORK

1946 F04

R1

1.250V

REFERENCE

Figure 4. Boost Converter Equivalent Model

The Current Mode zero is a right half plane zero which can

be an issue in feedback control design, but is manageable

with proper external component selection.

Using the circuit of Figure 1 as an example, the following

table shows the parameters used to generate the Bode plot

shown in Figure 5.

Table 3. Bode Plot Parameters

Parameter

Value

Units

Comment

18.6

Ω

Application Specific

20

µF

Application Specific

10

M

Ω

Not Adjustable

470

pF

Adjustable

49.9

k

Ω

Adjustable

8

V

Application Specific

3.3

V

Application Specific

40

µmho

Not Adjustable

5

mho

Not Adjustable

L

5.4

µH

Application Specific

1.2

MHz

Not Adjustable

From Figure 5, the phase is 120

° when the gain reaches

0dB giving a phase margin of 60

°. This is more than

adequate. The crossover frequency is 25kHz, which is

about three times lower than the frequency of the right half

plane zero Z2. It is important that the crossover frequency

be at least three times lower than the frequency of the RHP

zero to achieve adequate phase margin.