AS3677 datasheet(12/68 Pages) AMSCO

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AS3677

Datasheet - D e ta i l e d D e s c r i p t i o n

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

converter. The total startup time for an output voltage of e.g. 26V is less than 2ms.

8.2.1

Feedback Selection

The feedback for the DC/DC converter can be selected to any of the current sinks (CURR1, CURR2, CURR6). If the

(the lowest voltage of these current sinks is used)

1. The Step Up DC/DC Converter is regulated such that the required

current at the feedback path can be supported.

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.

8.2.2

Over voltage Protection

The over voltage protection is controlled by the register step_up_vmax (can be programmed to 10V, 16V or 25V) to

protect the external components (especially the output capacitor C1. If the voltage on the pin SW exceeds this voltage,

the DCDC is immediately disabled and the register bit step_up_ov is set. To re-enable the DCDC set step_up_on=0

and afterwards step_up_on=1.

The voltage rating of the external components must be chosen to fit to the software setting of step_up_vmax

Note:

The voltage on CURR1, CURR2 and CURR6 must not exceed 26V (see page 20)

VFB

Feedback Voltage for

Current Sink

Regulation

on CURR1, CURR2 or CURR6 in regulation.

0.4

0.5

0.6

ICOIL_MAX

Coil current limit

step_up_lowcur=0

1200

step_up_lowcur=1

750

For fixed startup time of

500us

step_up_lowcur=0

600

step_up_lowcur=1

330

RSW

Switch Resistance

ON-resistance of external switching transistor.

0.42

1.0

ILOAD

Load Current

At 25V output voltage

fIN

Switching Frequency

Internally trimmed

0.9

1.1

MHz

COUT

Output Capacitor

Ceramic, ±20%. Use nominal 4.7µF capacitors

to obtain at least 0.7µF under all conditions

(voltage dependence of capacitors)

0.7

4.7

µF

Inductor

Use inductors with small Cparasitic (<100pF) to

get high efficiency.

710

µH

tMIN_ON

Minimum on Time

140

190

MDC

Maximum Duty Cycle

Vripple

Voltage ripple >20kHz

Cout=4.7µF,Iout=0..45mA, VBAT=3.0...4.2V

160

Voltage ripple <20kHz

Efficiency

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

1. It is recommended to leave step_up_fb_auto=1 (default) all the times.

2. If the voltage is the DCDC overvoltage protection is chosen above the voltage ratings of the external com-

ponents, permanent damage might result.

Table 7. Step Up DC/DC Converter Parameters (Continued)

Symbol

Parameter

Condition

Min

Typ

Max

Unit

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