L6911E datasheet(10/34 Pages) STMICROELECTRONICS

10 / 34 page

Device description

L6911E

10/34

Device description

The device is an integrated circuit realized in BCD technology. It provides complete control

logic and protections for a high performance step-down DC-DC converter optimized for

microprocessor power supply. It is designed to drive N Channel Mosfets in a synchronous-

rectified buck topology. The device works properly with Vcc ranging from 5V to 12V and

regulates the output voltage starting from a 1.26V power stage supply voltage (Vin). The

output voltage of the converter can be precisely regulated, programming the VID pins, from

1.050V to 1.825V with 25mV binary steps, with a maximum tolerance of ±1% over

temperature and line voltage variations. The device provides voltage-mode control with fast

transient response. It includes a 200kHz free-running oscillator that is adjustable from

50kHz to 1MHz. The error amplifier features a 15MHz gain-bandwidth product and 10V/ms

slew rate which permits high converter bandwidth for fast transient performance. The

resulting PWM duty cycle ranges from 0% to 100%. The device protects against over-

current conditions entering in HICCUP mode. The device monitors the current by using the

rds(ON) of the upper MOSFET which eliminates the need for a current sensing resistor.

The device is available in SO20 package.

5.1

Oscillator

The switching frequency is internally fixed to 200kHz. The internal oscillator generates the

triangular waveform for the PWM charging and discharging with a constant current an

internal capacitor. The current delivered to the oscillator is tipically 50

µA (F

SW = 200KHz)

and may be varied using an external resistor (RT) connected between RT pin and GND or

VCC. Since the RT pin is maintained at fixed voltage (typ. 1.235V), the frequency is varied

proportionally to the current sinked (forced) from (into) the pin.

In particular connecting it to GND the frequency is increased (current is sinked from the pin),

according to the following relationship:

Equation 1

200kHz

4.94 10

⋅

T kΩ

()

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