AS3675 datasheet(13/81 Pages) AMSCO

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AS3675

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. 25V is less than 2ms.

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.

8.2.2

Over voltage Protection in Current Feedback Mode

The over voltage protection in current feedback mode (step_up_fb = 01, 10 or 11 or step_up_fb_auto = 1) works as fol-

lows: Only resistor R2 and C7/C8 is soldered and R3 is omitted. An internal current source (sink) is used to generate a

voltage drop across the resistor R2. If then the voltage on DCDC_FB is above 1.25V, the DCDC is momentarily dis-

abled 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

(EQ 1)

Note: The voltage on the pin DCDC_FB is limited by an internal protection diode to VBAT + one diode forward volt-

age (typ. 0.6V).

If the over voltage protection is not used in current feedback mode, connect DCDC_FB to ground.

Vrsense_max

Current Limit Voltage

at R1

e.g., 0.66A for 0.1

Ω sense resistor.

For fixed startup time of

500us

If Step up_lowcur= 1

RSW

Switch Resistance

ON-resistance of external switching transistor.

ILOAD

Load Current

At 16V output voltage

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

Table 8. Step Up DC/DC Converter Parameters

Symbol

Parameter

Condition

Min

Typ

Max

Unit

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