PM6600TR datasheet(27/43 Pages) STMICROELECTRONICS

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PM6600

Operation description

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7.4

System stability

The boost section of the PM6600 is a Fixed Frequency, Peak Current-Mode converter.

During normal operation, a minimum voltage selection circuit compares all the voltage drops

across the active current generators and provides the minimum one to the error amplifier.

The output voltage of the error amplifier determines the inductor peak current in order to

keep its inverting input equal to the reference voltage (400 mV typ). The compensation

network consists of a simple RC series (RCOMP - CCOMP) between the COMP pin and

ground.

The calculation of RCOMP and CCOMP is fundamental to achieve optimal loop stability and

dynamic performance of the boost converter and is strictly related to the operating

conditions.

7.4.1

Loop compensation

The compensation network can be quickly calculated using equations 4 through 9. Once

both RCOMP and CCOMP have been determined, a fine-tuning phase may be required in

order to get the optimal dynamic performance from the application.

The first parameter to be fixed is the switching frequency. Normally, a high switching

frequency allows reducing the size of the inductor but increases the switching losses and

negatively affects the dynamic response of the converter. For most of applications, the fixed

value (660 kHz) represents a good trade-off between power dissipation and dynamic

response, allowing to save an external resistor at the same time. In low-profile applications,

the inductor value is often kept low to reduce the number of turns; an inductor value in the

4.7 µH-15 µH range is a good starting choice.

Even if the loop bandwidth of the boost converter should be chosen as large as possible, it

should be set to 20 % of the switching frequency, taking care not to exceed the CCM-mode

Right Half-Plane Zero (RHPZ).

Equation 5

Equation 6

Where VIN,min is the minimum input voltage, IOUT is the overall output current,

Note that, the lower the inductor value (or the lower the switching frequency) the higher the

bandwidth can be achieved. The output capacitor is directly involved in the loop of the boost

converter and must be large enough to avoid excessive output voltage drop in case of a

sudden line transition from the maximum to the minimum input voltages (

∆V

OUT should not

exceed 50-100 mV):

⋅

≤

OUT

min

⋅

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅

≤

OUT

min

OUT

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