ADP1876-EVALZ AD [Analog Devices], ADP1876-EVALZ Datasheet - Page 17

• Manufacturer Part Number: ADP1876-EVALZ
• Description: 600 kHz Dual Output Synchronous Buck
• Manufacturer: AD [Analog Devices]

Data Sheet
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 V
(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, R
in 67 µA of divider current. For R
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 R
the following equation:
where:
R
R
V
V
The minimum output voltage is dependent on f
DHx on time. The maximum output voltage is dependent on
f
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.
SW
TOP
BOT
OUT
FB
, the minimum DHx off time, the IR drop across the high-
is the feedback regulation threshold, 0.6 V.
is the high-side voltage divider resistance.
is the low-side voltage divider resistance.
R
is the regulated output voltage.
TOP
=
R
BOT




INLDO
V
OUT
V
BOT
exceeds the input undervoltage lockout
−
FB
TOP
, must be less than 9 kΩ, which results
V
FB
to set the output voltage by using




BOT
, use a 1 kΩ to 20 kΩ resistor.
SW
and minimum
Rev. A | Page 17 of 24
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
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,
R
sources nominally 50 μA to this external resistor. This creates an
offset voltage of R
across the low-side MOSFET R
this offset voltage, the
Because the ILIMx current and the MOSFET R
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 R
expected temperature, and the minimum ILIM current. Keep in
mind that the temperature coefficient of the MOSFET R
typically 0.4%/°C.
where:
I
ACCURATE CURRENT-LIMIT SENSING
R
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 R
with R
accurate current-limit sensing scheme.
LPK
ILIM,
DSON
is the peak inductor current.
t
between ILIMx and SWx. The current sense pin, ILIMx,
R
of the MOSFET can vary by more than 50% over the
SS
SENSE
ILIM
=
=
. See Figure 28 for the implementation of this
6
0
5 .
6 .
I
LPK
µ
V
A
×
C
ILIM
40
R
SS
DSON
μA
multiplied by 50 μA. When the drop
ADP1876
_ MAX
DSON
of the MOSFET at its highest
DSON
flags a current-limit event.
ILIM
ADP1876
is equal to or greater than
by replacing R
prevents reverse
DSON
ADP1876
vary over
DSON_MAX
DSON
is