LED7707 datasheet(26/48 Pages) STMICROELECTRONICS

26 / 48 page

Application information

LED7707

26/48

Application information

6.1

System stability

The boost section of the LED7707 is a fixed frequency, 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 (700 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.

6.1.1

Loop compensation

The compensation network can be quickly calculated using equations 11 to 16. 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 and positively affects the dynamic

response of the converter (wider bandwidth) but increases the switching losses. 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 4.7µH-15µH range is a good starting choice.

In order to avoid instability due to interaction between the DC-DC converter's loop and the

current generators' loop, the bandwidth of the boost should not exceed the bandwidth of the

current generators. A unity-gain frequency (fU) in the order of 30-40 kHz is acceptable. Also,

take care not to exceed the CCM-mode right half-plane zero (RHPZ).

Equation 11

Equation 12

Equation 13 a

⋅

≤

OUT

min

⋅

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅

≤

OUT

min

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