ADP1876ACPZ-R7 datasheet(17/24 Pages) AD

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Data Sheet

ADP1876

Rev. A | Page 17 of 24

APPLICATIONS INFORMATION

INDEPENDENT LOW DROPOUT LINEAR

REGULATOR

The input voltage range to VINLDO of the independent LDO

regulator is 2.7 V to 5.5 V, and the output is fixed at 1.5 V with

a 150 mA maximum load current. The internal short-circuit

current limit is set to about 430 mA. Apply power to the VIN

pin to keep the LDO operating within specification. The LDO

is enabled when VINLDO exceeds the input undervoltage lockout

(UVLO) threshold. Safety features include short-circuit protection

and thermal overload shutdown.

SETTING THE OUTPUT VOLTAGE OF THE

CONTROLLER

The output voltage is set using a resistive voltage divider from

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

voltage to the 0.6 V FBx 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 FBx 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, whereas 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

DHx on time. The maximum output voltage is dependent on

fSW, the minimum DHx off time, the IR drop across the high-

side N-channel MOSFET, and the DCR of the inductor.

SOFT START

The soft start period is set by an external capacitor between SS1

or 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 the following equation:

The SSx pin reaches a final voltage of 3.2 V. If the output voltage

is precharged prior to turn on, the ADP1876 prevents reverse

inductor current, which discharges the output capacitor. When

the voltage at SSx exceeds the regulation voltage (typically at

0.6 V), the reverse current is enabled to allow the output voltage

regulation to be independent of load current.

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 1 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 ADP1876 flags a current-limit event.

Because the ILIMx current and the MOSFET RDSON vary over

process and temperature, set the minimum current limit 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%/°C.

μA

_ MAX

DSON

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 PGNDx. Make sure that the power rating of the

current sense resistor is adequate for the application. Apply the

previous equation and calculate RILIM by replacing RDSON_MAX

with RSENSE. See Figure 28 for the implementation of this

accurate current-limit sensing scheme.

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