ADP1850ACPZ-R7 datasheet(16/32 Pages) AD

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ADP1850

Data Sheet

Rev. C | Page 16 of 32

APPLICATIONS INFORMATION

SETTING THE OUTPUT VOLTAGE

The output voltage is set using a resistive voltage divider from

the output to FB. The voltage divider divides down the output

voltage to the 0.6 V FB regulation voltage to set the regulation

output voltage. The output voltage can be set to as low as 0.6 V

and as high as 90% of the power input voltage.

The maximum input bias current into FB is 100 nA. For a 0.15%

degradation in regulation voltage and with 100 nA bias current,

the low-side resistor, RBOT, must be less than 9 kΩ, which results

in 67 µA of divider current. For RBOT, use a 1 kΩ to 20 kΩ resistor.

A larger value resistor can be used but results in a reduction in

output voltage accuracy due to the input bias current at the FBx

pin, while lower values cause increased quiescent current

consumption. 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.

The minimum output voltage is dependent on fSW and minimum

DH on time. The maximum output voltage is dependent on fSW,

the minimum DH off time, and the IR drop across the high-side

NMOSFET and the DCR of the inductor. For example, with fSW of

600 kHz (or 1.67 µs) and a minimum on time of 130 ns, the

minimum duty cycle is approximately 7.8% (130 ns/1.67 µs). If

VIN is 12 V and the duty cycle is 7.8%, then the lowest output is

0.94 V. As an example for the maximum output voltage, if VIN is

5 V, fSW is 600 kHz, and the minimum DH off time is 395 ns

(335 ns DH off time plus approximately 60 ns total dead time),

then the maximum duty cycle is 76%. Therefore, the maximum

output is approximately 3.8 V. If the IR drop across the high-

side NMOSFET and the DCR of the inductor is 0.5 V, then the

absolute maximum output is 4.5 V (5 V − 0.5 V), independent of

fSW and duty cycle.

SOFT START

The soft start period is set by an external capacitor between

SS1/SS2 and AGND. The soft start function limits the input

inrush current and prevents output overshoot.

When EN1/EN2 is enabled, a current source of 6.5 µA starts

charging the capacitor, and the regulation voltage is reached

when the voltage at SS1/SS2 reaches 0.6 V.

The soft start period is approximated by

μA

The SSx pin reaches a final voltage equal to VCCO. If the output

voltage is precharged prior to turn-on, the ADP1850 prevents

reverse inductor current, which discharges the output capacitor.

Once the voltage at SSx exceeds the regulation voltage (typically

0.6 V), the reverse current is reenabled to allow the output

voltage regulation to be independent of load current.

Furthermore, in dual-phase operation, where SS1 is shorted to

SS2, the current source is doubled to 13 µA during the soft start

sequence.

When a controller is disabled, for instance, EN1/EN2 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 ILIMx and SWx. The current sense pin, ILIMx,

sources nominally 50 μA to this external resistor. This creates an

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

across the low-side MOSFET, RDSON, is equal to or greater than

this offset voltage, the ADP1850 flags a current limit event.

Because the ILIMx current and the MOSFET, RDSON, vary over

process and temperature, the minimum current limit should be

set to ensure that the system can handle the maximum desired

load current. To do this, use the peak current in the inductor,

which is the desired output current limit level plus ½ of the

ripple current, the maximum RDSON of the MOSFET at its

highest expected temperature, and the minimum ILIM current.

Keep in mind that the temperature coefficient of the MOSFET,

RDSON, is typically 0.4%/oC.

MAX

DSON

LPK

ILIM

where:

ILPK is the peak inductor current.

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