EM5305VT datasheet(10/12 Pages) EXCELLIANCE

10 / 12 page

2013/7/11

A.2

EM5305

Feedback Compensation

If fESR is lower than fcross and close to fLC, the

phase lead of the capacitor ESR zero almost cancels

the phase loss of one of the complex poles of the

LC filter around the crossover frequency. Use a

Type II compensation network with a one zero and

a high-frequency pole to stabilize the loop. In

Figure 2, RF and CF introduce a one zero (fZ1). RF

and CCF in the Type II compensation network

provide

high-frequency

pole

(fP1),

which

mitigates the effects of the output high-frequency

ripple.

Fig.2 Compensation

II for Voltage Mode Buck Converter

When using a low-ESR tantalum or OSCON type,

the ESR-induced zero frequency is usually above

the targeted zero crossover frequency (fcross). Use

Type III compensation Fig3.Type III compensation

provides two zeros to cancel the pair of complex

poles introduced by LC filter.

Fig.2 Compensation

III for Voltage Mode Buck Converter

Fig.3 shows the Bode plot for the control loop. The

compensation

gain

uses

external

impedance

networks ZIN and ZFB to provide a stable loop. A

stable control loop has a gain crossing with

-20db/decade slope and phase margin greater than

45 degrees.

Fig.3 Bode Plot of Voltage Mode Buck Converter

Output Inductor Selection

The output inductor is selected to meet the output

voltage ripple requirements and minimize the

response time to the load transient. The inductor

value determines the current ripple and voltage

ripple. The ripple current is approximately the

following equation:

OUT

ΔI

∗

−

Output Capacitor Selection

An output capacitor is required to filter the output

and supply the load transient. The selection of

output capacitor depends on the output ripple

voltage. The output ripple voltage is approximately

bounded by the following equation:

)

OUT

(ESR

ΔI

ΔV

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