General Description

The MAX669 constant-frequency, pulse-width modulating (PWM), current-mode DC-DC controllers are designed for a wide range of DC-DC conversion applications including step-up, SEPIC, flyback, and isolated-output configurations. Power levels of 20W or more can be controlled with conversion efficiencies of over 90%. The 1.8V to 28V input voltage range supports a wide range of battery and AC-powered inputs. An advanced BiCMOS design features low operating current (220μA), adjustable operating frequency (100kHz to 500kHz), soft-start, and a SYNC input allowing the MAX669 oscillator to be locked to an external clock.
DC-DC conversion efficiency is optimized with a low 100mV current-sense voltage as well as with Maxim’s proprietary Idle Mode™ control scheme. The controller operates in PWM mode at medium and heavy loads for lowest noise and optimum efficiency, then pulses only as needed (with reduced inductor current) to reduce operating current and maximize efficiency under light loads. A logiclevel shutdown input is also included, reducing supply current to 3.5μA.
The MAX669, optimized for low input voltages with a guaranteed start-up voltage of 1.8V, requires bootstrapped operation (IC powered from boosted output). It supports output voltages up to 28V. The IC is available in an extremely compact 10-pin μMAX package.

 

Benefits and Features

● 1.8V Minimum Start-Up Voltage (MAX669)
● Wide Input Voltage Range (1.8V to 28V)
● Tiny 10-Pin μMAX Package
● Current-Mode PWM and Idle Mode™ Operation
● Efficiency over 90%
● Adjustable 100kHz to 500kHz Oscillator or SYNC Input
● 220μA Quiescent Current
● Logic-Level Shutdown
● Soft-Start

 

アプリケーション

● Cellular Telephones
● Telecom Hardware
● LANs and Network Systems
● POS Systems

 

詳細

The MAX669 current-mode PWM controllers operate in a wide range of DC-DC conversion applications, including boost, SEPIC, flyback, and isolated output configurations. Optimum conversion efficiency is maintained over a wide range of loads by employing both PWM operation and Maxim’s proprietary Idle Mode control to minimize operating current at light loads. Other features include shutdown, adjustable internal operating frequency or synchronization to an external clock, soft start, adjustable current limit, and a wide (1.8V to 28V) input range.

 

Bootstrapped Operation

With bootstrapped operation, the IC is powered from the circuit output (VOUT). This improves efficiency when the input voltage is low, since EXT drives the FET with a higher gate voltage than would be available from the low-voltage input. Higher gate voltage reduces the FET on-resistance, increasing efficiency. Other (undesirable) characteristics of bootstrapped operation are increased IC operating power (since it has a higher operating voltage) and reduced ability to start up with high load current at low input voltages. If the input voltage range extends below 2.7V, then bootstrapped operation with the MAX669 is the only option.

 

Setting the Operating Frequency

The MAX669 can be set to operate from 100kHz to 500kHz. Choice of operating frequency will depend on number of factors:
1) Noise considerations may dictate setting (or synchronizing) fOSC above or below a certain frequency or band of frequencies, particularly in RF applications.
2) Higher frequencies allow the use of smaller value (hence smaller size) inductors and capacitors.
3) Higher frequencies consume more operating power both to operate the IC and to charge and discharge the gate of the external FET. This tends to reduce efficiency at light loads; however, the MAX669’s Idle mode feature substantially increases light-load efficiency.
4) Higher frequencies may exhibit poorer overall efficiency due to more transition losses in the FET; however, this shortcoming can often be nullified by trading some of the inductor and capacitor size benefits for lower resistance components.

 

Determining Inductance Value

The MAX669 allow significant latitude in inductor selection if LIDEAL is not a convenient value. This may happen if LIDEAL is a not a standard inductance (such as 10μH, 22μH, etc.), or if LIDEAL is too large to be obtained with suitable resistance and saturation-current rating in the desired size. Inductance values smaller than LIDEAL may be used with no adverse stability effects; however, the peak-to-peak inductor current (ILPP) will rise as L is reduced. This has the effect of raising the required ILPK for a given output power and also requiring larger output capacitance to maintain a given output ripple. An inductance value larger than LIDEAL may also be used, but output-filter capacitance must be increased by the same proportion that L has to LIDEAL. See the Capacitor Selection section for more information on determining output filter values.
Due to the MAX669 high switching frequencies, inductors with any core materials that exhibit low core loss (ferrite, or equivalent) are recommended for best efficiency performance.