C12 datasheet(15/72 Pages) AMSOSRAM

15 / 72 page

AS3688

austriami crosyst ems

Datasheet, Confidential

www.austriamicrosystems.com (ptr,tje)

Revision 1.1.1 / 20060707

13 - 70

Table

7 – Step Up DC/DC Converter Parameters

Symbol

Parameter

Min

TYP

Max

Unit

Note

IVDD

Quiescent Current

140

µ A

Pulse skipping mode.

VFB1

Feedback Voltage for

External Resistor Divider

1.20

1.25

1.30

For constant voltage control.

step_up_res=1

VFB2

Feedback Voltage for Current

Sink Regulation

0.4

0.5

0.6

on CURR1 or CURR2 in

regulation.

step_up_res=0

Additional Tuning Current at

Pin DCDC_FB and

overvoltage protection

µ A

IDCDC_FB

Accuracy of Feedback

Current

-4

Adjustable by software using

1µA step size

VPROTECT = 1.25V +

IDCDC_FB * R3

Design Target

Vrsense_max

e.g., 0.66A for 0.1Ω sense

resistor.

Vrsense_max_st

art

For fixed startup time of

500us

Vrsense_max_lc

Current Limit Voltage at

RSENSE (R2)

If stepup_lowcur=1

RSW

Switch Resistance

Ω

ON-resistance of external

switching transistor.

At 15V output voltage.

Iload

Load Current

At 17V output voltage.

fIN

Switching Frequency

0.9

1.1

MHz Internally trimmed.

Cout

Output Capacitor

0.7

4.7

µ F

Ceramic, ±20%. Use nominal

2.2µF capacitors to obtain at least

0.7µF under all conditions (voltage

dependance of capacitors)

Inductor

µ H

Use inductors with small Cparasitic

(<100pF) to get high efficiency.

tMIN_ON

Minimum on Time

140

190

MDC

Maximum Duty Cycle

Voltage ripple >20kHz

160

Vripple

Voltage ripple <20kHz

Cout=2.2uF,Iout=0..45mA,

Vbat=3.0...4.2V

Efficiency

Iout=20mA,Vout=17V,Vbat=3.8V

To ensure soft startup of the dcdc converter, the overcurrent limits are reduced for a fixed time after enabling the

dcdc converter. The total startup time for an output voltage of e.g. 25V is less than 2ms.

7.2.1 Feedback Selection

The feedback for the DC/DC converter can be selected either by current sinks CURR1 or CURR2 or by a voltage

feedback at pin DCDC_FB. If the register bit step_up_fb_auto is set, the feedback path is automatically selected

between CURR1 and CURR2 (the lowest voltage of these current sinks is used).

Setting step_up_fb = 01 enables feedback at pin 19 (CURR1); setting step_up_fb = 10 enables feedback at pin

20 (CURR2). The Step Up DC/DC Converter is regulated such that the required current at the feedback path can

be supported. (Bit step_up_res should be set to 0 in this configuration)

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