ADP1853ACPZ-R7 datasheet(17/28 Pages) AD

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Data Sheet

ADP1853

Rev. 0 | Page 17 of 28

APPLICATIONS INFORMATION

ADIsimPower DESIGN TOOL

The ADP1853 is supported by the ADIsimPower design tool

set. ADIsimPower is a collection of tools that produce complete

power designs optimized to a specific design goal. The tools

allow the user to generate a full schematic, bill of materials, and

calculate performance in minutes. ADIsimPower can optimize

designs for cost, area, efficiency, and parts count while taking

into consideration the operating conditions and limitations of

the IC and all real external components. The ADIsimPower tool

can be found at www.analog.com/ADIsimPower and the user

can request an unpopulated board through the tool.

SETTING THE OUTPUT VOLTAGE

The output voltage is set using a resistive voltage divider from

the output to FB. For RBOT, use a 1 kΩ to 20 kΩ resistor. Choose

RTOP to set the output voltage by using the following equation:













−

OUT

BOT

TOP

where:

RTOP is the high-side voltage divider resistance.

RBOT is the low-side voltage divider resistance.

VOUT is the regulated output voltage.

VFB is the feedback regulation threshold, 0.6 V.

SOFT START

The soft start period is set by an external capacitor between

SS and AGND. The soft start function limits the input inrush

current and prevents output overshoot. When EN is enabled,

a current source of 6.5 µA starts charging the capacitor, and

the regulation voltage is reached when the voltage at SS reaches

0.6 V. The soft start time is approximated by

μA

The SS pin reaches a final voltage equal to VCCO.

When a controller is disabled, for instance, if EN is pulled low

or experiences an overcurrent limit condition, the soft start

capacitor is discharged through an internal 3 kΩ pull-down

resistor.

SETTING THE CURRENT LIMIT

The current-limit comparator measures the voltage across the

low-side MOSFET to determine the load current.

The current limit is set by an external current-limit resistor,

RILIM, between ILIM and CS. The current sense pin, ILIM,

sources nominally 50 μA to this external resistor. This creates

an offset voltage of RILIM multiplied by 50 μA. When the drop

across the current sense element RCS (a sense resistor or low-

side MOSFET, RDSON) is equal to or greater than this offset

voltage, the ADP1853 flags a current-limit event.

μA

LPK

ILIM

where:

ILPK is the peak inductor current.

ACCURATE CURRENT-LIMIT SENSING

RDSON of the MOSFET can vary by more than 50% over the

temperature range. Accurate current-limit sensing is achieved

by adding a current sense resistor from the source of the low-

side MOSFET to PGND. Make sure that the power rating of the

current sense resistor is adequate for the application. Figure 25

illustrates the implementation of accurate current-limit sensing.

Figure 25. Accurate Current-Limit Sensing

INPUT CAPACITOR SELECTION

Use two parallel capacitors placed close to the drain of the high-

side switch MOSFET (one bulk capacitor of sufficiently high

current rating and a 10 μF ceramic decoupling capacitor).

Select an input bulk capacitor based on its ripple current

rating. The minimum input capacitance required for a

particular load is

ESR

MIN

)

(

)

(

−

where:

IO is the output current.

D is the duty cycle.

VPP is the desired input ripple voltage.

RESR is the equivalent series resistance of the capacitors.

VIN

ADP1853

ILIM

RILIM

RSENSE

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