Datasheet

Data Sheet ADP1877
Rev. D | Page 17 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, R
BOT
, must be less than 9 kΩ, which results
in 67 µA of divider current. For R
BOT
, use a 1 k to 20 k 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 FB
pin, while lower values cause increased quiescent current
consumption. 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.
The minimum output voltage is dependent on f
SW
and minimum
DH on time. The maximum output voltage is dependent on f
SW
,
the minimum DH off time, and the IR drop across the high-side
N-channel MOSFET (NMOSFET) and the DCR of the inductor.
For example, with an f
SW
of 600 kHz (or 1.67 µs) and minimum on
time of 130 ns, the minimum duty cycle is approximately 7.8%
(130 ns/1.67 µs). If V
IN
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 V
IN
is 5 V, f
SW
is 600 kHz, and the minimum DH off
time is 390 ns (330 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 f
SW
and duty cycle.
SOFT START
Program the soft start by connecting a capacitor from SSx to
AGND. The soft start function limits the input inrush current
and prevents the output overshoot.
On startup, a 6.5 µA current source charges the SSx capacitor.
The soft start period is approximated by
SSSS
C
A
V
t
μ
=
5.6
6.0
The SSx pin reaches a final voltage equal to VCCO. If the output
voltage is precharged prior to turn-on, the ADP1877 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.
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
ILIM
. The current sense pin, ILIMx, sources 50 A to this external
resistor. This creates an offset voltage of R
ILIM
multiplied by
50 A. When the drop across the low-side MOSFET, R
DSON
, is
equal to or greater than this offset voltage, the ADP1877 flags a
current limit event.
Because the ILIMx current and the MOSFET, R
DSON
, 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 R
DSON
of the MOSFET at its
highest expected temperature, and the minimum ILIM current.
A
RI
R
MAXDSON
LPK
ILIM
μ
×
=
40
_
where:
I
LPK
is the peak inductor current.
The buck converters usually run a fairly high current. PCB
layout and component placement may affect the current limit
setting. An iteration of the R
ILIM
value may be required for a
particular board layout and MOSFET selection. If alternative
MOSFETs are substituted at some point in production, these
resistor values may also need an iteration. Keep in mind that the
temperature coefficient of the MOSFET, R
DSON
, is typically
0.4%/
o
C.