9.2.2.7 Loop Stability

The TPS61376 requires external compensation, which allows the loop response to be optimized for each

application. The COMP pin is the output of the internal error amplifier. An external compensation network,

comprised of resistor RC, and ceramic capacitors CC and CP, is connected to the COMP pin.

The power stage small signal loop response of constant off-time (COT) with peak current control can be

modeled by Equation 12.

GPS(S)=KCOMP×

RO×:1-D;

2

×

l1+

S

2NBESRZp

×

l1-

S

2NBRHPZp

1+

S

2NBP

(12)

where

• D is the switching duty cycle.

• RO is the output load resistance.

• KCOMP is power stage trans-conductance (inductor peak current / comp voltage), which is 13.5 A/V.

P

O

O

2

¦

2

R

C

S u

u

(13)

where

• CO is effective output capacitance.

ESRZ

ESR

O

1

¦

2

R

C

S u

u

(14)

where

• RESR is the equivalent series resistance of the output capacitor.

2

O

RHPZ

R

1

D

¦

2

L

u

S u

(15)

The COMP pin is the output of the internal transconductance amplifier. Equation 16 shows the small signal

transfer function of compensation network.

COMZ

EA

EA

REF

OUT

COMP1

COMP2

S

1

¦

G

R

V

Gc(S)

V

S

S

1

1

¦

¦

§

·

¨

¸

u S u

u

u

©

¹

u

§

·§

·

¨

¸¨

¸

u S u

u S u

©

¹©

¹

(16)

where

• GEA is the transconductance of the amplifier, which is 240 uS.

• REA is the output resistance of the amplifier, which is 100 MΩ.

• VREF is the reference voltage at the FB pin.

• VOUT is the output voltage.

• ƒCOMP1, ƒCOMP2 are the frequency of the poles of the compensation network.

• ƒCOMZ is the zero's frequency of the compensation network.

The next step is to choose the loop crossover frequency, ƒC. The higher frequency that the loop gain stays

above zero before crossing over, the faster the loop response is. It is generally accepted that the loop gain cross

TPS61376

SLVSGQ1A – JANUARY 2022 – REVISED SEPTEMBER 2022

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