Single-Output PWM (Pulse Width Modulation) Controller ICs

Switchmode dc-dc converters require a means to vary their output voltage in response to changes in their load. One approach is to use pulse width modulation (PWM) that controls the input to the associated power switch. The PWM signal consists of two values, ON and OFF, as shown in Fig. 1. A low-pass filter connected to the output of the power switch provides a voltage proportional to the ON and OFF times of the PWM controller.

In operation, a small amount of the output voltage is fed back to the PWM controller, which varies its ON time in response to the feedback voltage. If the filtered output of the power switch tends to change, the negative feedback applied to the PWM controller regulates the output voltage.

Typical examples of PWM controller ICs include the following.

1. A fixed frequency voltage mode PWM controller IC targets off-line SMPS (switchmode power supply) and dc-dc converter applications requiring minimal external components. It features a trimmed oscillator for precise duty cycle control, a temperature compensated reference, an on/off control, a high gain error amplifier, a current sensing comparator, and a high current totem-pole output.

With a power saving mode current of 200µA (typical) and stand-by current of 100µA (typical), the IC's low power consumption means high efficiency in energy-saving designs.  The IC features a current sensing comparator, a trimmed oscillator for precise duty cycle control, a temperature compensated reference, and a high current totem pole output ideally suited for driving a power MOSFET.

The IC's operating temperature ranges from ?25°C to +125°C.  In addition, the it incorporates an on/off control and a soft-start circuit. Used in conjunction with complementary power MOSFETs and high performance power factor ICs, this PWM controller IC enables implementation of SMPS designs that provide high efficiency and allow for regulatory compliance with relevant standards for harmonic emission.

Among its features are pulse-by-pulse current limiting, undervoltage lockout ( UVLO), 7mA operating current (typ.), soft-start, on/off control, overload protection (OLP), overcurrent protection (OCP), and overvoltage protection (OVP)

2. This IC is a wide input range, forward, boost, flyback and SEPIC controller that drives an N-channel power MOSFET with few external components required. A resistor programmable duty cycle clamp can be used to generate a volt-second clamp for forward converter applications. An internal boost switcher is available for creating a separate supply for the output gate driver, allowing 10V gate drive from input voltages as low as 3V. An external resistor sets its operating frequency over a 100kHz to 500kHz range and a SYNC pin allows the part to be synchronized to an external clock. Additional programmability exists for leading edge blanking and slope compensation.

The internal boost switcher runs in burst mode operation, asynchronous to the main oscillator. If low VIN operation with high GATE drive is not required, the BOOST pin is left open and the VIN2 pin shorted to VIN. With VIN2 shorted to VIN the minimum operational VIN will increase from 3V to 8V (required at VIN2 to enable the GATE output driver). For GATE turn on, a PWM latch is set at the start of each main oscillator cycle. For GATE turn off, the PWM latch is reset when either the current sense comparator is tripped, the maximum duty cycle is reached, or the BLANK override threshold is exceeded.

A fast current sense comparator achieves 60ns current sense delay and the error amplifier is a true voltage mode error amplifier, allowing a wide range of compensation networks. An accurate shutdown<