Datasheet

Data Sheet ADP1851
Rev. 0 | Page 15 of 24
APPLICATIONS INFORMATION
ADIsimPower DESIGN TOOL
The ADP1851 is supported by the ADIsimPower™ design tool
set. ADIsimPower is a collection of tools that produce complete
power designs optimized for a specific design goal. The tools
allow the user to generate a full schematic and bill of materials
and to 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 R
BOT
, use a 1 kΩ to 20 kΩ resistor. Choose
R
TOP
to set the output voltage by using the following equation:








−
=
FB
FB
OUT
BOTTOP
V
VV
RR
where:
R
TOP
is the high-side voltage divider resistance.
R
BOT
is the low-side voltage divider resistance.
V
OUT
is the regulated output voltage.
V
FB
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
SSSS
Ct
μA5.6
V6.0
=
The SS pin reaches a final voltage equal to VCCO.
When a controller is disabled, for example, 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,
R
ILIM
, between ILIM and SW. The current sense pin, ILIM,
sources nominally 50 μA to this external resistor. This creates
an offset voltage of R
ILIM
multiplied by 50 μA. When the drop
across the current sense element R
CS
(low-side MOSFET, R
DSON
)
is equal to or greater than this offset voltage, the ADP1851 flags
a current-limit event.
μA50
06.1
CS
LPK
ILIM
RI
R
××
=
where:
I
LPK
is the peak inductor current.
ACCURATE CURRENT-LIMIT SENSING
The R
DSON
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 24
illustrates the implementation of accurate current-limit sensing.
Figure 24. 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 the input bulk capacitor based on its ripple current
rating. The minimum input capacitance required for a
particular load is
SWESRO
PP
O
MININ
fDRIV
DDI
C
)(
)1(
,
×−
−×
=
where:
I
O
is the output current.
D is the duty cycle.
V
PP
is the desired input ripple voltage.
R
ESR
is the equivalent series resistance of the capacitors.
V
IN
ADP1851
DH
SW
ILIM
DL
R
ILIM
R
SENSE
10595-024