AS3676 datasheet(13/92 Pages) AMSCO

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AS3676

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. If C7 and C8 are mounted and the

bit step_up_prot is set, the total startup time can exceed 2ms.

Note: If the DCDC converter is only used in current feedback mode (CURR1, CURR2 or CURR6 - and not used for a

constant voltage source), the capacitors C7 and C8 can be removed.

8.2.1

Feedback Selection

The feedback for the DC/DC converter can be selected either by current sinks (CURR1, CURR2, CURR6) or by a volt-

age 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 regu-

lated 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.

IDCDC_FB

Additional Tuning

Current at Pin

DCDC_FB and over

voltage protection

Adjustable by software using Register DCDC

control1

1µA step size (0-31µA)

VPROTECT = 1.25V +

IDCDC_FB * R2

031

µA

Accuracy of Feedback

Current at full scale

-6

Vrsense_max

Current Limit Voltage

at R1

e.g., 1.32A for 0.1

 sense resistor R1.92

132

170

For fixed startup time of

500us

If step_up_lowcur= 1

114

RSW

Switch Resistance

ON-resistance of external switching transistor.



ILOAD

Load Current

At 26V 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

Table 8. Step Up DC/DC Converter Parameters

Symbol

Parameter

Condition

Min

Typ

Max

Unit

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