AS3687 datasheet(10/55 Pages) AMSCO

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AS3687/87XM

Datasheet

www.austriamicrosystems.com/AS3687

1v3-4

9 - 54

Table

3 – Step Up DC/DC Converter Parameters

Symbol

Parameter

Min

TYP

Max

Unit

Note

Vrsense_max

e.g., 0.66A for 0.1

Ω sense

resistor

Vrsense_max_st

art

For fixed startup time of

500us

Vrsense_max_lc

Current Limit Voltage at

RSENSE (R1)

If stepup_lowcur= 1

RSW

Switch Resistance

ON-resistance of external

switching transistor.

At 16V output voltage.

Iload

Load Current

At 25V 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

4.7

μF 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=4.7uF,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.

7.1.1 Feedback Selection

The feedback for the DC/DC converter can be selected either by current sinks (CURR1, CURR2, CURR6) 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, CURR2 and CURR6 (the lowest voltage of these current sinks is used).

Setting step_up_fb enables feedback on the pins CURR1, CURR2 or CURR6. 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)

Note

: Always choose the path with the highest voltage drop as feedback to guarantee adequate supply for the

other (unregulated) paths or enable the register bit step_up_fb_auto.

7.1.2 Overvoltage Protection in Current Feedback Mode

The overvoltage protection in current feedback mode (step_up_fb = 01, 10 or 11 or step_up_fb_auto = 1) works

as follows: Only resistor R3 and C10/C11 is soldered and R4 is omitted. An internal current source (sink) is

used to generate a voltage drop across the resistor R3. If then the voltage on DCDC_FB is above 1.25V, the

DCDC is momentarily disabled to avoid too high voltages on the output of the DCDC converter.

The protection voltage can be calculated according to the following formula:

VPROTECT = 1.25V + IDCDC_FB * R2

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