ML1565 datasheet(13/21 Pages) MINILOGIC

13 / 21 page

P13/21

Rev. C, Sep 2005

ML1565

Detailed Description (4)

Low-Voltage Startup Oscillator

The ML1565 internal control and reference-voltage

circuitry receive power from OUTSU and do not function

when OUTSU is less than 2.5V. To ensure low-voltage

startup, the step-up employs a low-voltage startup

oscillator that activates at 0.9V. The startup oscillator

drives the internal N-channel MOSTFET at LXSU until

OUTSU reaches 2.5V, at which point voltage control is

passed to the current-mode PWM circuitry.

Once in regulation, the ML1565 operates with inputs as

low as 0.7V since internal power for the IC is supplied by

OUTSU. At low input voltages, the ML1565 can have

difficulty starting into heavy loads.

Soft-Start

The ML1565 step-down and AUX_ channels feature a

soft-start function that limits inrush current and prevents

excessive battery loading at startup by ramping the output

voltage to the regulation voltage. This is achieved by

increasing the internal reference inputs to the controller

transconductance amplifiers from 0V to the 1.25V

reference voltage over 4096 oscillator cycles (8ms at

500kHz) when initial power is applied or when a channel

is enabled. Soft-start is not included in the step-up

converter in order to avoid limiting startup capability with

Shutdown

The step-up converter is activated with a high input at

ONSU.

The

step-down

and

auxiliary

DC-to-DC

converters1, 2, and 3 activate with a high input at ONSD,

ON1,

ON2,

and

ON3,

respectively.

The

auxiliary

controllers and step-down cannot be activated until

OUTSU is in regulation. For automatic startup, connect

ON_ to OUTSU or a logic level greater than 1.6V.

Design Procedure (1)

Setting the Switching Frequency

Choose a switching frequency to optimize external

component size or circuit efficiency for any particular

ML1565

application.

Typically,

switching

frequencies

between 300kHz and 600kHz offer a good balance

between component size and circuit efficiency. Higher

frequencies generally allow smaller components and lower

frequencies give better conversion efficiency. The switching

frequency is set with an external timing resistor (Rosc) and

capacitor (Cosc). At the beginning of a cycle, the timing

capacitor charges thought the resistor until it reaches VREF.

The charge time, t1, is:

t1 = -Rosc Cosc In [ 1 - 1.25 / VOUTSU ]

Table 1. Voltage Setting Summary

Channel

FB_

FB THRESHOLD

(FBSEL_LOW)

FB THRESHOLD

(FBSEL_HIGH)

FBSU

3.35V

FBSD

1.5V

FB1

1.25V

FB2

Always 1.25V

(FBSEL

not

provided

for

these

channels)

The capacitor voltage is then given time (t2 = 300ns) to

discharge. The oscillator frequency is

fOSC = 1 / (t1 + t2)

fOSC can operate from 100kHz to 1MHz. Choose Cosc

between 47pF and 470pF. Determine Rosc from the

equation:

Rosc = (300ns – 1 / fOSC) / ( Cosc In [ 1-1.25/VOUTSU] )

See the Typical operating Characteristics for fOSC versus

ROSC using different values of COSC.

ML1565 датащи(PDF) 13 Page - Minilogic Device Corporation Limited

ML1565 датащи(HTML) 13 Page - Minilogic Device Corporation Limited

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

датащи скачать

ссылки URL