MCP1632-BAESLASHMS datasheet(20/34 Pages) MICROCHIP

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MCP1632

DS20005254A-page 20

 2013 Microchip Technology Inc.

5.2

Operation in Voltage Mode Control

The MCP1632 PWM controller can operate in Voltage

Mode Control using the internal slope compensation

ramp to generate the PWM signal. The current sense

resistor is not necessary for this application, thus the

overall efficiency of the converter can be improved.

Refer to Typical Application Circuit – Voltage Mode

Control. Certain limitations occur in this operating

mode. The compensation network for Voltage Mode

Control must be of Type III, increasing the number of

components.The closed-loop system is now a second

order system and stability can be difficult to achieve

over a wide range of operating conditions. The position

of the dominant pole (double pole) in boost-derived

converters varies with the operating conditions

(input/output voltages); maintaining acceptable phase

and gain margins across the entire operating range of

the converter becomes a difficult task in this case.

Note that there is no inherent protection mechanism

that can limit the inductor’s current during transients or

overloads. A resistor placed between the CS pin and

GND allows adjusting the maximum duty cycle by

controlling the amplitude of the ramp signal. Refer to

Figure 5-4 for details. If the RDC Adj resistor is not

placed, the maximum duty cycle is set to approximately

60% (typical). The duty cycle can be increased up to

85% (typical) by adjusting the value of the RDC Adj

resistor. The designer must limit the maximum

operating duty cycle of the converter to a safe value by

adjusting the value of this resistor. The DC offset of the

ramp enables operation with 0% duty cycle if the output

of the error amplifier divided by 3 is lower than DCLOW.

The Voltage Mode Control should be used only for

systems with low input voltages, low DC conversion

ratios and limited dynamics of the load (e.g., LED

drivers or battery chargers).

FIGURE 5-4:

Voltage Mode Operation Details.

Oscillator

300/600 kHz

VEXT

RAMP

PWM

RDC Adj

0.9 VPP

2.7V

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