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PRELIMINARY TECHNICAL DATA
a
1 MHz, 750 mA Buck Regulator
Preliminary Technical Data ADP3088
FEATURES
1 MHz PWM Frequency
Automatic PWM to Power Saving Mode at Light Load
Fully Integrated 1 A Power Switch
3% Output Regulation Accuracy over Temperature,
Line, and Load
100% Duty Cycle Operation
Simple Compensation
Output Voltage: 1.25 V to 10.5 V
Small Inductor and MLC Capacitors
Low Quiescent Current while Pulse Skipping
Thermal Shutdown
Fully Integrated Soft Start
Cycle-by-cycle Current Limit
APPLICATONS
PDAs and Palmtop Computers
Notebook Computers
PCMCIA Cards
Bus Products
Portable Instruments
IN
IN
COMP
GND
GND
FUNCTIONAL BLOCK DIAGRAM
CURRENT
SENSE
AMP
PWM
COMPARATOR
PROTECTION
LOGIC
(ILIM, OT)
ERROR
AMP
g
m
+
REF
1.245V
1MHz
SQ
R
RUN/STOP
COMPARATORS
SOFT-START
TIMER
SW
DRV
FB
GENERAL DESCRIPTION
The ADP3088 is a high frequency, non-synchronous PWM
step-down DC-DC regulator with an integrated 1A power
switch in a space-saving MSOP8 package. It provides high
efficiency, excellent dynamic response, and is very simple
to use.
The ADP3088’s 1 MHz switching frequency allows for
small, inexpensive external components, and the current
mode control loop is simple to compensate and eases noise
filtering. It operates in PWM current mode under heavy
loads and saves energy at lighter loads by switching automatically into Power Saving mode. Soft start is integrated
completely on chip, as is the cycle-by-cycle current limit.
Capable of operating from 2.5 V to 11 V input, it is ideal for
many applications, including portable, battery power applications, where local point-of-use power regulation is required. Supporting output voltages down to 1.25 V, the
ADP3088 is ideal to generate low voltage rails, providing
the optimal solution in its class for delivering power efficiently, responsively, and simply with minimal printed circuit board area.
The device is specified over the industrial temperature
range of -40 °C to +85°C.
VIN
3.3V
10µF
0.1µF
4.7pF
220pF
20kΩ
ADP3088
IN
IN
GND
COMP
SW
DRV
GND
FB
3.3µH
1N5817
10.0kΩ
22.4kΩ
Figure 1. Typical Application
10µF
VOUT
1.8V
REV. PrK 3/28/02
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor
for any infringements of patents or other rights of third parties which may
result from its use. No license is granted by implication or otherwise under any
patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700 World Wide Web Site: http://www.analog.com
Fax: 781/326-8703 Analog Devices, Inc., 2002
PRELIMINARY TECHNICAL DATA
1
ADP3088–SPECIFICATIONS
Parameter Symbol Conditions Min Typ Max Units
SUPPLY
Input Voltage Range V
Quiescent Current Operating I
Quiescent Current Operating I
Shutdown I
Ground Current I
Normal Operation VIN = 11 V, IL = 500 mA, 2.5 3.6 mA
Thermal Shutdown Threshold T
OSCILLATOR
Oscillator Frequency f
Minimum Sleep Duty Cycle D
Maximum Duty Cycle D
Wake up Hysteresis V
OUTPUT SWITCH
Switch On Voltage V
Current Limit Threshold I
Leakage Current VIN = 12 V 0.5 µA
ERROR AMPLIFIER
Reference Voltage Accuracy V
Reference Voltage Line FB = COMP, .02 %/V
Regulation V
Feedback Input Bias Current I
Sink/Source Current I
Short Circuit Current I
Transconductance gm,
MODULATOR
Transconductance gm,
Control Offset Voltage V
Soft Start Time t
Shutdown Threshold Voltage V
Slope Compensation m
NOTES
1 All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC).
2 For higher efficiency operation, tie the DRV pin to the output for IL < 250 mA, and VIN > 3 V.
3 V(IN) - V(SW), includes voltage drop across internal current sensor.
Specifications subject to change without notice.
IN
Q
Q
SD
2
GND
SD
SW
PSM
MAX
HYST
3
IO
LIM
REF
FB
COMP
COMP, SD
EA
MOD
PWM, OS
SS
COMP, SD
SC
(VIN = +3.3 V, TA = -40°C to +85°C, unless otherwise noted)
DRV = GND 2.5 11 V
VIN = 10 V, IL = 500 mA, 6 mA
DRV = GND
No load 150 250 µA
V
= 0 V 15 40
COMP
DRV = 2 V
160 °C
0.75 1 1.25 MHz
IL = 500 mA 14 %
100 %
FB voltage drops below V
REF
20 30 40 mV
IL = 500 mA, FB and DRV = 0.25 0.4 V
GND
1.0 1.2 1.4 A
FB = COMP 1.222 1.245 1.265 V
= 3 V to 12 V
IN
soft start expired -50 1 50 nA
35 60 85 µA
V
= 0 V, activating 20 40 µA
COMP
shutdown
FB = COMP 480 µA/V
V
COMP
to I
L
1 A/V
0.90 V
250 600 µs
340 750 mV
Effectively summed to I
SW
0.7 A/µs
–2– REV. PrK
PRELIMINARY TECHNICAL DATA
PRELIMINARY TECHNICAL DATA
ADP3088
ABSOLUTE MAXIMUM RATINGS*
Input Supply Voltage ............................... –0.3 V to +12 V
Voltage on any pin with respect to GND .. –0.3 V to +12 V
(voltage on any pin may not exceed V
)
IN
Operating Ambient Temperature Range .. –40°C to +85°C
Operating Junction Temperature ......................... +125°C
(4-layer board)........................................... +116°C/W
θ
JA
θ
(2-layer board)........................................... +159°C/W
JA
Storage Temperature Range .................. –65°C to +150°C
Lead Temperature Range (Soldering, 10 sec.) ..... +300°C
Vapor Phase (60 sec) ........................................ +215°C
Infrared (15 sec) ............................................... +220°C
*This is a stress rating only; operation beyond these limits can cause the device
to be permanently damaged. Unless otherwise specified, all voltages are
referenced to GND.
ORDERING GUIDE
Temperature Package Branding
Model Range Option Information
ADP3088ARM -40°C to +85°C MSOP-8 P0A
PIN FUNCTION DESCRIPTIONS
Pin Mnemonic Function
1, 2 IN Power Supply Input. Both pins
must be connected.
3,6 GND Ground. Both pins must be con-
nected.
4 COMP Feedback Loop Compensation
and Shutdown Input. An open
drain or collector used to pull
the pin to ground will shutdown
the device.
5 FB Feedback Voltage Sense Input.
This pin senses the voltage via
an external resistor divider.
7 DRV This pin provides a separate
path for drive current to be connected to ground.
8 SW Switching Output.
PIN CONFIGURATION
GND
COMP
1
IN
2
IN
ADP3088
3
4
8
SW
7
DRV
6
GND
5
FB
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the device features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
–3–REV. PrK
ADP3088
PRELIMINARY TECHNICAL DATA
THEORY OF OPERATION
The ADP3088 is a fixed frequency buck switching regulator in an MSOP-8 package using an external Schottky rectifier. It features an integrated 1A power switch and switches
at 1MHz. ADP3088 utilizes PWM operation and incorporates soft-start for controlled start-up sequence and over
temperature switch protection. The ADP3088 draws low
current while running in power saving mode, and even
lower current in shutdown.
Refer to the functional block diagram on page 1. The system shown is configured for a 1.8 V output using a 10 µH
inductor. At the beginning of a cycle the 1 MHz oscillator
enables an SR latch, enabling the internal 1 A power
switch. The current sense amplifier and the protection logic
block monitor the current flowing between the IN and SW
pins. The switch is turned off when the current reaches a
level determined by the protection logic block or PWM
comparator, whichever is lower. The error amplifier measures the output voltage through an external resistor divider
tied to the FB pin. This amplifier servos the switch current
to regulate the FB pin voltage to 1.245 V. An internal regulator provides power to the control circuitry. The COMP
pin can be used to shutdown the ADP3088. When pulled
low it turns off the internal regulator, thus biasing down the
chip, reducing the input current and disconnecting the
output from the input. Anti-saturation circuitry is used to
drive the switch to the edge of saturation. This allows the
driver to quickly switch at 1 MHz and maintain good efficiency. And for improved efficiency, the DRV pin may be
connected to the output provided that the input voltage is
at least 1 V greater than the output.
If the output load increases, the error amplifier will detect a
lower voltage on the FB pin, via the resistor divider on the
output, and send a signal to the PWM comparator to increase the on time of the switch. This in effect increases the
duty cycle and provides more current to drive the increased
load during the transient event, until a new operating point
is established.
Reference
The ADP3088 incorporates an internal bandgap reference,
it includes curvature correction for extremely low temperature coefficient. The reference can be disabled by grounding the COMP pin which also turns off the bias for the rest
of the chip.
Error Amplifier
The error amplifier provides a control voltage to the PWM
stage to set the peak inductor current which sets the output
current of the regulator. It is a g
is a current to the COMP pin.
Protection Logic
The protection logic block provides current limit and overtemperature protection. The over temperature protection is
enabled when the temperature of the chip exceeds a specified preset temperature; the switch will be disabled until the
temperature drops below a specified level, then normal
operation will resume. The thermal shutdown only stops
switching, but does not put the chip to power saving mode,
nor does it re-initiate soft start. As the chip cools slightly, it
will cycle in and out of thermal shutdown rapidly, maintaining the die temp at 150°C, but allowing the output voltage
amplifier in that its output
m
and current to swing up and down. The current limit protection overrides the PWM comparator; if this occurs then
the switch pulse will be terminated and the soft start mode
will be reset.
Current Sense Amplifier
The voltage on the internal current sense resistor is sensed
and passed to the ramp input of the PWM comparator.
This current sense signal is also passed to the current limit
comparator for peak current limit shutdown . At current
limit the soft-start capacitor is reset and soft-start is reinitiated. The current limit is normally 1.2 peak switch
current. Slope compensation is added to ADP3088 to stabilize the loop. A generated ramped signal is summed with
the current sense signal to provide slope compensation.
Slope compensation is needed to close the inner loop so
subharmonic oscillation is avoided. The ramp is reset with
each clock cycle so that the ADP3088 is capable of true
100% duty cycle.
Run/Stop Comparators
This block creates the 1 MHz signal sent to the SR latch
which is used for the switching frequency. It also takes the
FB voltage and decides when to go into and wake up from
power saving mode. The decision to induce power saving
mode is based upon duty ratio. During steady-state continuous operation, the duty ratio of a PWM buck regulator
is simply a function of input/output voltage ratio, with second order effects including the voltage drop of the internal
switch and the external diode. Once the load drops to a
certain point, discontinuous operation occurs, and the duty
ratio begins to modulate to maintain regulation. In the
ADP3088, the regulator goes to sleep when the integrated
duty ratio measurements drops to less than half of the minimum expected integrated duty ratio. The minimum expected duty ratio occurs at max input voltage, min output
voltage in continuous mode operation.
PWM Comparator
The PWM comparator looks at the signal from the current
sense amplifier and the error amplifier to determine the
correct switch on time to regulate the output voltage under
a given load.
Soft-Start Timer
Soft start will prevent saturating the inductor which could
cause uncontrolled overshoot of the output voltage and
electrical stress to the system at start up. When first powered up, an internal soft start capacitor is discharged and
the soft start circuitry provides a gradually decaying offset
to the error amplifier to prevent it from saturating and from
commanding maximum switch current to charge the output
capacitor. The output voltage approaches the final regulation voltage with a smooth exponential decay. This will
reduce electrical stress to the system.
Output
The output stage contains the bipolar power switch, and
the circuits necessary to switch it on and off quickly. The
pass switch is driven to the edge of saturation and the result
is a fast switching response and low switch resistance. For
improved efficiency, the DRV pin may be connected to the
output provided that the input voltage is at least 1 V higher
than the output. This will send the current needed to drive
the bipolar switch to the output load instead of routing it to
–4– REV. PrK
PRELIMINARY TECHNICAL DATA
X
ADP3088
ground. For some VIN and I
configurations, the DRV
LOAD
pin must be grounded for reliable operation.
APPLICATION INFORMATION
Output Voltage Setting
In its standard usage, the output voltage of the ADP3088 is
programmed to a desired fixed value by a resistor divider
from the output voltage into the feedback node, the pin FB,
at which node the control loop ensures regulation at the
reference level, V
. The divider should be designed to
REF
satisfy the formula:
(1)
VV
=×+
OUT REF
R
B
1
R
A
where RA is the upper divider resistor (between the output
and FB) and R
and RB are recommended to have values in the range of
R
A
is the lower one (between FB and ground).
B
2~200 kΩ and are likely to require a 1% tolerance or better
to attain acceptable output voltage tolerance.
In less conventional applications described separately, the
resistor feedback configuration can be modified or tapped
with other resistors to affect current flow into the FB node
that, in turn, influences the output voltage. Even a switched
voltage can be summed into the FB node as long as it is
sufficiently integrated and does not intolerably compromise
the transient response. This latter application is considered
further below in an application for powering a DSP.
Input Voltage, Power Dissipation Considerations, and
Power Savings Mode
The input voltage range is not typically considered a critical
parameter for electrical functionality, but there are several
considerations, upon which there is further elaboration
below:
must never exceed the maximum rated voltage
1. V
IN
2. V
must be within the specified operating range when
IN
normal operation is expected
3. V
must be greater than V
IN
by at least the specified
OUT
headroom when DC regulation is expected
4. V
, if not sufficiently greater than V
IN
, may limit the
OUT
large signal transient response of a buck converter
5. V
, if much greater than V
IN
, may give rise to such a
OUT
low duty ratio that it activates power savings mode even
at static higher load conditions or upon dynamic load
changes when it is not desired.
6. V
affects the device power dissipation (a lower value
IN
causes higher dissipation), which in turn affects die
temperature that must be kept below a maximum rating.
The lowest input voltage together with the maximum output voltage and maximum current create the conditions for
maximum power dissipation in the device, which determine
maximum temperature rise that should be checked against
the maximum junction temperature rating. The formula for
maximum power dissipation in the device is given by:
VV
+
@,
PIV
DMAX O MAX SW IO MA
OFIOMAX
=××
V
IN
,@,
(2)
where VF is the diode forward voltage drop and VSW is the
drop across the internal switch and current sensing resistor
that appears between the VIN and SW pins of the
ADP3088 during the on state of the switch. Both of these
variables can be approximated from a combination of
worst-case specs and typical graphs. Multiply the power
dissipation by the thermal resistance from junction to case
or ambient, as desired, to determine internal temperature
rise.
If the input voltage were so much higher than the output
voltage that it required an average duty ratio less than an
internally preset threshold, then power savings mode
(“PSM”) – that is characterized by periodic shutdown and
wakeup of the device that reduces average quiescent current – would be active for all load conditions rather than
only at lighter loads, for which it is intended. PSM operation is characterized by low-frequency ripple on the output
that appears similar to the behavior of a hysteretic regulator. This is usually not a factor for consideration and may
be ignored if PSM operation is acceptable for all load conditions, but in case it is relevant, the following recommendation is offered:
IN
D
PSM MAX
()
(3)
+
VV
OF
<
V
It is not possible to prevent the duty ratio from tending
towards zero in non-synchronous buck converters below a
certain minimum load current level called "borderline current" or "critical current" for the power converter. That
corresponds to the inductor ripple current reaching zero at
its bottom peak - sometimes called the "valley current". If
PSM activation strains the lower regulation limit due to the
hysteretic ripple, the output voltage can be offset slightly
upward by readjusting the nominal voltage setpoint with the
resistor divider.
Even though a buck converter may have a low dropout
voltage that allows the static regulation to be maintained as
the input voltage drops near to the output voltage, in buck
converters the slew rate limitation of inductor current can
compromise the dynamic regulation in response to load
current step increases. That is because the maximum rate
that current can be increased in the inductor is proportional
to the voltage available to impress across it, which is compromised as the input voltage reduces toward the output
voltage. This is not a limitation of the device but of buck
converters in general. The limitation is considered as part
of the output filter design, although it could also be considered in terms of a minimum acceptable input voltage for a
given output filter that will ensure that the dynamic response is acceptably maintained.
Output Filter Components
In most applications it is desirable to use the smallest inductor value that does not introduce practical problems, as this
tends to yield the lowest cost inductor. One reason for using
an even larger inductor than the minimum tolerable might
be to reduce output ripple voltage further, but cost being
–5–REV. PrK
ADP3088
PRELIMINARY TECHNICAL DATA
equal, this is generally better accomplished with a better
quality or proportionally larger output capacitor instead,
since a larger inductor degrades the large-signal transient
performance capability.
A conservative nominal design target value for the inductor
of a typical application circuit is that which creates a peakto-peak ripple current, ∆I
, for the nominal input voltage
L
that is approximately a third of the nominal 500mA rating
of the ADP3088. The reason for not suggesting to base the
ripple current on the maximum load current is for concern
of overload protection. Scaling of the ripple currents with
lower load currents would yield higher inductor values that
might give satisfactory operation, but in order for overload
operation up to the current limit level of the ADP3088 to
be satisfactory, it would be necessary to choose an inductor
rated up to that higher current, which would likely yield an
unsatisfactory inductor size and cost. In any case, having
chosen a target level for ∆I
, the recommended inductor
L
value is given by:
−× +
1
DVV
()
L
=
()
OF
×∆
fI
SW L
(4)
where D is the duty ratio - the suffix indicating continuous
inductor current - and is given by:
VV
+
D
=
OF
VVV
+−
IN F SW
(5)
and VSW and VF are assessed at full load, and fSW is the fixed
switching frequency of the ADP3088. The formula suggests the calculation of L using a nominal input voltage, and
for applications requiring a large ranges of V
tions of transient response at V
ripple at V
may warrant deeper consideration of how
IN(MAX)
versus the higher
IN(MIN)
the limita-
IN
to optimize the design. In applications where load transients are not severe, this conservative design for L is recommended. A more aggressive minimization of L is
outlined below, but a few restrictions are noted.
As inductance becomes smaller the ripple current becomes
larger. If the ripple becomes particularly large or, as an
additional factor, if the load is particularly dynamic, then
there is an increasing possibility that the peak inductor current will undesirably reach the current limit shutdown
threshold, ICL. This should be avoided by restricting the
minimum inductor value to keep the ripple current moderated. An alternate way to prevent excessive dynamic overshoot of inductor current during a load transient is to
reduce the DC gain of the error amplifier by adding resistive feedback; this idea is discussed below.
Another important restriction of the minimum inductor
value may apply. The design should ensure against possible
subharmonic oscillation that can occur in all fixed-frequency current-controlled switching power supplies when
switching at high duty ratios. The subharmonic oscillation
phenomenon will not be explained here - there are plenty
of papers written on the subject - except to say that it is
characterized by alternating high and low duty ratios - i.e.,
every other cycle - that produces additional ripple on the
output. To prevent subharmonic oscillation the following
restriction for the minimum inductor value is recommended:
VV
H
2
µ
LVV
> ×+×
VV
The value used for V
()
OF
should be only the minimum
IN(MIN)
+
OF
IN MIN
()
−
0.35
(6)
input voltage for which normal high performance operation
must be assured. Note the value returned for L may be
negative in which case the restriction does not apply. If the
preceding formula yields a lower inductor value than the
conservative recommendation given previously, as is likely
for most applications, then it is time to consider further
limitations to see how low the value can be minimized.
For a given inductor selection, the earlier formula is rearranged for convenience and skewed to the worst case input
voltage to determine the maximum inductor ripple current,
∆I
:
L
VVV
∆= ×
I
LMAX
()
IN MAX
VVVfL
IN MAX
−−
()
()
OSW
+− ×
FSWSW
VV
+
F
O
(7)
Performance degradation of the inductor - consisting of
some loss of inductance or excessive power loss - may be
encountered at higher ripple currents, so the ripple current
figure, together with knowledge of the expected DC current should be checked against specifications of the inductor.
If the ESR of the output capacitor is substantial - as it is
likely to be if a MLC capacitor is not used - then the ripple
voltage on the output, dominated by the ESR, may be
substantial and of concern for regulation specifications.
The resistive component of the output voltage ripple is
simply the ripple current times the ESR, and if it is more
than a few millivolts it will dominate the output capacitance
in contributing to output ripple voltage.
The boundary condition of the inductor reaching the borderline current, I
VVV VV
+−−
OF INOSW
=×
I
OBL
()
2
fLV V V
SW IN F SW
can be determined by the formula:
O(BL)
+−
(8)
Below this output current level, the inductor current will be
discontinuous and the duty ratio will be modulated to lower
values, by factors substantially more than thus the losses
that cause only a small amount of modulation in continuous
inductor current operation. PSM is initiated automatically
by a proprietary technique comprising a duty ratio amplifier
with an internal time constant. As load current drops well
into the low current region and the duty ratio passes below
the threshold of D
for a sufficient time, PSM is acti-
PSM
vated. The corresponding level of output current is given
by:
VVV VVV
1
ID
=× ×
OPSM
()
PSM
2
+− −−
2
IN F SW IN SW
VV f L
+×
OF SW
0
(9)
It can be seen in the formula that this current threshold is
inversely proportional to inductance, so although it is usually not a relevant concern, it is noted that an aggressively
–6– REV. PrK
PRELIMINARY TECHNICAL DATA
ADP3088
low output inductance should be avoided to keep the PSM
threshold current at a desirably low level.
For the user's reference, when current is below the borderline level, the duty ratio is modulated according to the formula:
VV f L
+×
DI
2
=×× ×
DO
OF SW
VVV
+−
IN F SW
−−
VVV
IN O SW
(10)
where the suffix indicates that the inductor current is discontinuous.
For controlling the capacitive component of the output
ripple voltage, the following constraint on the minimum
output capacitance should be applied:
I
∆
>
fV
8
SW R
L
∆
(11)
C
O
where ∆VR is the tolerable ripple voltage. However, this
constraint is rarely relevant, as the typical capacitance requirement is driven more by dynamic response requirements than by ripple concerns. In a typical application
circuit, a 10 µF capacitor produces a capacitive output
voltage ripple component of only about 2 mV. 10 µF is
usually sufficient for applications that do not impose particularly HF load transients, which imposes additional constraints that are elaborated upon in the next section.
Load Characterization
Optimization of the compensation, as well as the output
filter, requires some knowledge of a fundamental characteristic of the load. Qualitatively, there are two types of
loads with which we are concerned: fast-slew-rate and
slow-slew-rate. These slew rates are assessed with respect
to the minimum [absolute] inductor [current] slew rate as
given by:
VVV
IN MIN
MIN and
,
dt L L
=<
−−
()
SW O
MAX MAX
VVdI
+
OFL
(12)
where the "<" sign indicates a selection of whichever of the
bracketed terms is the lesser.
If the slew rate of the load is fast compared to the minimum
inductor slew rate, then the ability of the power converter
to contain the output voltage deviation following a load
change is limited not only by the response of the control
loop - i.e., by its speed to demand zero or maximum duty
ratio from the modulator - but by the power stage as well.
In such a case, beginning with the recognition that output
voltage deviation would be substantial even if the loop response were instantaneous, it can be shown that one can
achieve better overall voltage containment by degenerating
the DC loop gain. As a technical matter, it should be noted
that there will always be some minimum output voltage
deviation downward due to a load step even if the inductor
slew is as fast as the load slew rate, because, during a
switching cycle, the modulator latches its "decision" to turn
off the switch and it cannot rescind that decision but must
wait for the next clock cycle to turn on the switch again and
begin slewing the inductor current upward. This is only a
second order consideration.
Slow slew rate loads may be referred to simply as conventional loads, since these have been the more prevalent type
of load. Optimally compensating a conventional load is
synonymous with small signal AC considerations: the objective is to maximize the AC gain up to the crossover frequency, ensure sufficient phase margin at the unity gain
crossover frequency, and keep the gain rolling off at higher
frequencies to avoid gain margin problems.
Fast slew rate loads may be referred to as digital loads
since, from the perspective of the power converter, they
have a digital characteristic when changing between two
extremes, and also because such fast slew rates tends to
characterize modern digital circuits, which often feature
power management interrupts - i.e., interrupt signals used
to turn on and off circuitry on an as-needed basis during
normal system operation. Optimally compensating a digital
load is more a task of impedance matching and DC gain
determination than a task of AC loop optimization.
Returning to constraints for choosing the output capacitor,
for digital loads another criteria for ensuring sufficient output capacitance applies:
2
∆
I
C
>
O
2
O
dI
∆
L
VMIN
O
dt
(13)
where ∆IO is the maximum HF load step. It should be
noted that the formula results strictly from the physical
limitation of the output filter; the compensation must also
be optimized to maximize the response of the control loop
to avoid substantial additional output voltage deviation.
The formula might be also written in to describe a maximum inductance for a given capacitance, but it is generally
better practice to choose the inductor first and add capacitance as needed.
The he impedance of the output capacitor together with a
digital load creates some limiting considerations, also. Series resistance (ESR) rather than capacitance can be a
dominant design consideration with non-MLC capacitors.
If the load is essentially digital, then the dynamic deviation
of the output voltage cannot be limited to any better than
the dynamic load current step times the ESR. In a formula:
VIESR
∆≥∆×
OO
(14)
In such a case, it is often important to choose a capacitor
that controls the ESR to a sufficiently small value, and
MLC capacitors are often chosen to practically eliminate
the consideration of ESR entirely.
Closing the Loop - Compensation
The factors determining the response of the power converter are noted: the feedback input resistor divider, a lead
network if applicable, the transconductance of the error
–7–REV. PrK
ADP3088
PRELIMINARY TECHNICAL DATA
amplifier, its frequency response limitation (i.e., as adequately modeled by a capacitance from output to ground)
its external termination impedance (i.e., the compensation,
that may or may not include DC feedback), the modulator
transconductance, and the power converter's termination
impedance (i.e., the output capacitor and load resistance).
Since the ADP3088 has a current controlled loop, the particular inductor value does not by first order consideration
affect small signal stability. However, slew rate limitations
as discussed earlier - a large signal limitation consideration set boundaries that are often relevant for optimizing compensation of the feedback loop. If the compensation of the
current control signal, i.e., the COMP pin, is designed to
promote a current response that is faster than the inductor
current can slew, then when a step load is applied the control signal will tend to initially respond in excess (of the
actual current change that is occurring) and then allow an
overshoot of the current and output voltage as it is delayed
in correcting its excess.
For conventional loads, the following describes how the
frequency corners (poles and zeroes) are positioned or
should be chosen to optimize the loop gain, beginning in
the low frequency spectrum:
1. The DC loop gain is limited by the applied load resistance and the output resistance of the error amplifier,
but it is not important to determine how high the DC
gain is.
2. Two poles in the LF spectrum begin to roll off the gain,
one determined by the load resistance and output capacitor, C
, and the other by the error amplifier's out-
O
put resistance and its termination capacitance - the
equivalent feedback capacitance and the added compensation capacitance C
; determining the location of
HF
these poles is not relevant to compensation design - it
suffices to know that both are decades below the crossover frequency.
3. A lead network is especially desirable for a variable output voltage application in order to keep a fairly constant
crossover frequency and phase margin for all output
voltages; if used, this lead network consists of simply a
capacitor, C
vider resistor, R
, in parallel with the upper feedback di-
FF
; this creates a closely spaced zero/pole
A
pair that provides a gain boost before crossover so that,
above the pole frequency, the loop gain and phase are
similar for all output voltages; if the lead network is used
for a fixed voltage application, the pole should be chosen to align with the following described zero; for variable voltage applications, the maximum frequency of
the pole should be placed as high as is comfortable without substantially degrading phase margin (e.g., not
within an octave or, more conservatively, a half decade
of the crossover frequency).
4. A zero turns the gain rolloff back to 1-pole sufficiently in
advance of the crossover frequency to create ample
phase margin, e.g., half a decade; the zero could feasibly be that of the output capacitor itself - i.e., the zero
formed by the ESR and the capacitance C
- but that is
O
both unlikely (since the zero frequency will likely be
higher than where the loop zero is desired) and generally imprudent (since the loop performance would de-
pend on the stability of the ESR, which often is poor or
unknown); as recommended, the zero, f
an R
circuit terminating the COMP pin (a resistor, RC,
C
in series with a capacitor, C
terminating the error amplifier, C
with R
to cancel the zero of the output capacitor, or, if
C
), while the capacitance
C
HF
, is created by
Z
, forms a pole, fP,
the zero is well above the crossover frequency, as may
be the case when using an MLC output capacitor, that
pole is set high enough above the crossover frequency,
again e.g., half a decade, so that it doesn't cut substantially into the phase margin at crossover, but still ensures
continued gain rolloff so that the gain margin is acceptably high; note that the previous guidelines suggest that
≥ 10 × CHF.
C
C
5. The gain crosses 0 dB (unity) at a crossover frequency
that is typically a tenth and advisably not greater than a
fourth of the switching frequency - one primary reason
for this approximate upper limit being the extra phase
margin loss due to the switching interval that is not predicted by the linear model.
Assuming no lead network is used, the open loop gain is
given by:
V
where V
600
µ× ×
A
≈
OL
is the nominal DC level. This equation to-
OUT
ZZ
COMP O
2
Ω
V
OUT
(15)
gether with the preceding recommendations should suffice
to determine compensation component selection for users
familiar with loop design. This begins with deciding the
crossover frequency, f
frequency, and setting the open loop gain, A
By example, f
= 125 kHz is chosen.
C
Assuming a well chosen C
, evaluating the impedances at that
C
as described previously - i.e.,
HF
, to unity.
OL
such that it creates a pole well above crossover or approximately matches the zero of the output capacitor, the following equation approximates the calculation of the
crossover frequency:
=+
1
f
C
where k1 = CO×V
()
Ω×
21 /
kAk
()
OUT/RC
and fZ = 1/2πRCCC - the zero
1
Z
(16)
1
+Ω××
150 /
kAfk
frequency set by the compensation - and the units are
shown with the constants in the equation for clarification.
The preceding equation cannot readily be solved in terms
of k
, but it can be solved closely enough by a few iterations
1
beginning with values for k
example below, set the zero about a half decade below f
previously advised, that is, choose f
Using the previously stated values for f
= 800 p(FA) satisfies the equation. RA and RB are pre-
k
1
around 1×109 (FA). For the
1
~ fC /√10 = 40 kHz.
Z
and fC, the value of
Z
C
sumed to be already chosen per earlier guidelines to set the
output voltage. As an example, R
= RB = 10 kΩ (imply-
A
ing an output voltage of 2.5 V). Similarly it is presumed
that C
then also C
simple formulas previously given. The example yields R
47 kΩ and C
was chosen; let CO = 15 µF. Then, finally, RC and
O
can be determined by rearrangement of
C
~ 82 pF. Assuming an MLC output capaci-
C
C
as
~
–8– REV. PrK
PRELIMINARY TECHNICAL DATA
ADP3088
tor of reasonable quality, the pole setting capacitor could
be chosen to be C
= 4.7 pF.
HF
A general purpose application circuit is shown in Figure 2.
6.8µH
1A
SCHOTTKY
10µF
MLCC
VOUT
1.5V
R
A
10kΩ
R
B
48.7kΩ
5V
MLCC
C
4.7pF
1µF
HF
C
C
470pF
R
C
10kΩ
ADP3088
IN
IN
GND
COMP
SW
DRV
GND
FB
Figure 2. +5 V to 1.5 V, General Purpose Application
Another application circuit features a voltage inversion and
regulation design such that the output voltage is negative,
see Figure 3. Negative output voltages are allowed in the
case that the input plus the output voltage does not exceed
the rating of the device. In the voltage inverting configuration, the ground reference of the ADP3088 is the negative
output voltage and the conventional output voltage point is
tied to ground. Operation is bootstrapped: the power converter behaves as if the input voltage were equal to the
actual input voltage plus the magnitude of the output voltage and as if the output voltage were not inverted. This
implies that it is possible to have the input voltage be less
than the magnitude of the output voltage - provided that
the input voltage alone is sufficient to start the operation of
the IC - i.e., before the negative output voltage has been
developed. (The circuit below with a -3.3V output works
fine over an input range from 2.5 V to 7.5 V.) Since the
ADP3088 features a current controlled loop, the feedback
effect of essentially boosting the input voltage atop the output (with respect to the ground connection of the
ADP3088) is reduced to a negligible second-order effect.
just enough so that the static load regulation allows a similar
voltage deviation with current as would be the peak voltage
deviation, ∆V
, that could not be avoided in the event that
O
a step change of current were to occur even if the loop
response were instantaneous. The reason for even an instantaneous response in the control loop allowing an output
voltage deviation is that the slew-rate of current in the output is limited by to the inductor, and a corresponding dynamic burden is placed on the output capacitor to maintain
the output voltage. Therefore, inductor value minimization
is desired both for concern of its size and cost, and also to
maximize the slew rate of current to the output so that a
smaller output capacitor is needed.
To implement voltage positioning, a resistor, R
, should
FB
be placed between the COMP and FB pins according to the
formula:
IR
∆
OA
=
R
where g
FB
gV
×∆
MOD O
is the modulator gain and ∆IO must be assessed
MOD
(17)
over the entire operating load range as the difference between maximum and minimum load. C
least large enough to support the targeted ∆V
must be chosen at
O
according to
O
the earlier stated formula governing the relationship between minimum output capacitance, voltage deviation, and
load current. In order to ensure that the output voltage will
be constrained within the limitations of ∆V
, the limitations
O
noted earlier for PSM hysteretic ripple if applicable in the
operating load range and ESR. Also an experimental adjustment downward to the value of R
may be needed, as
B
the DC bias point of the COMP node is usually a little
higher than V
, which would result in a slight downward
REF
shift of the nominal output voltage.
Having chosen this design approach, the series RC of the
compensation network can be removed and the single remaining capacitor, C
, should be increased to approxi-
HF
mately:
4.7µH
1A
SCHOTTKY
10µF
MLCC
+
R
A
10kΩ
+
R
B
3.04kΩ
-3.3V
5V
MLCC
C
4.7pF
1µF
HF
C
C
220pF
R
C
20kΩ
ADP3088
IN
IN
GND
COMP
SW
DRV
GND
FB
Figure 3. +5V to -3.3V, General Purpose Inverting
Application
Voltage Positioning Designs
For digital loads a different compensation technique is recommended that involves implementing "voltage positioning", that is now commonly used on CPUs but is equally
applicable to any dynamic device. Voltage positioning is the
intentional and controlled variation of output voltage with
load current, such that the power supply appears to have a
substantial output resistance. The key to voltage positioning
optimization for a digital load is to degenerate the loop gain
CESR
×
O
=
C
HF
R
FB
(18)
If an MLC capacitor is used for CO, the value of CHF might
be calculated to be less than a few picofarads, in which case
it is recommended to use a 4.7~10 pF capacitor. The formula is derived from a patented design technique called
ADOPT
TM
- Analog Devices' Optimal Positioning Technology. This creates AC and DC impedance matching,
and the increased complexity of the DC regulation design is
moderated by the simplicity of the frequency compensation.
In this design approach, at higher currents the output voltage will be appreciably lower than at low currents. This is
equivalent to saying that the load regulation appears to be
poor. But, paradoxically perhaps to the user unfamiliar with
voltage positioning, the overall containment of voltage
within a given window will be improved, and that tends to
be of particularly importance in many highly dynamic
loads.
The application circuit in Figure 4 features a 3.3 V input
and a 2.5 V output at 100~400 mA which constrains the
–9–REV. PrK
ADP3088
PRELIMINARY TECHNICAL DATA
output voltage within a ~100 mV range with only a 4.7 µF
output capacitor, even when the load slew rate is extremely
fast. This does not include the initial tolerance of the voltage setting that is separately accounted with voltage positioning designs. Note that the lower resistor, R
, of the
B
feedback divider is reduced from the 10 kΩ value that one
would use for a standard (non-voltage-positioned) design
that had no voltage positioning resistor R
3.3µH
1A
SCHOTTKY
3.3V
C
HF
4.7pF
1µF
MLCC
ADP3088
IN
IN
GND
COMP
R
VP
51kΩ
SW
DRV
GND
FB
VP
4.7µF
MLCC
.
-2.5V
100-400 mA
R
A
10kΩ
R
B
8.75kΩ
Figure 4. Application Circuit using Voltage Positioning,
Allowing Small Output Capacitance
Extra-Low-Voltage Outputs
Some newer power management applications require voltage levels below the normal adjustable voltage range of the
ADP3088, i.e., below 1.25 V. Such applications can be
accommodated using the ADP3088 by modifying the application circuit to sum in a resistor-weighted portion of another regulated system voltage, e.g., 3.3 V, to the feedback
node (FB). The tolerance of the ADP3088's output voltage
will increase by an amount proportional to the tolerance of
the summed in system voltage times the ratio of the conductance from that node to that of the output voltage. The
below example in Figure 5 shows an implementation of this
technique together with another special implementation
described below. The resistor R
sums from a 2.5 V sys-
TT
tem voltage to the FB node that will reduce the output voltage according to the formula:
feature in the ADP3088, it can readily be accommodated
with a few components. Dynamic voltage control can be
implemented either by parallel bus control or by PWM. In
both cases, the output voltage is modified by summing
either switched bits with, presumably binary, weighting
resistors or a switched PWM node via a single resistor into
the FB pin. (The switched PWM node refers to an external
PWM control signal, not the switched node of the power
converter itself.) Since the PWM technique modulates a
current into the FB node, it is necessary both to integrate
that signal and to avoid slowing down the response of the
power converter to output voltage transitions. This can be
accomplished by placing a capacitor between the output
voltage and the feedback node, which serves to provide a
zero/pole pair in the main regulation loop, and appears as
an integration pole to the PWM signal.
The design of either parallel bit or PWM type of voltage
control must consider whether the interface node(s) - from
parallel switched bits or a single PWM signal - has an active
pullup state (in which case it must be to a known voltage)
or a passive pullup (open drain) that floats up to the FB
node voltage, 1.25 V, in its high state. If at least the lower
extreme of the desired output voltage range must be lower
than 1.25 V, either technique can be combined with the
technique for lowering the output voltage below 1.25 V.
Such an example of an application having this requirement
is the BlackFin™ DSP. Figure 5 shows an implementation
of this technique.
Input Voltage: 4.75 V ~ 7.5 V
Output Voltage: 0.9 V ~ 1.5 V
Dynamic voltage control interface technique: PWM,
active high to V
IO
System voltage used for lowering output voltage below
1.25 V: V
= VIO = 2.5 V
TT
Maximum output current: 700 mA
R
VV
∆=−×
OUT TT
A
R
TT
(19)
Dynamic Voltage Control
Some newer power management applications also require
an ability to adjust the voltage being delivered to a load
during operation. Although there is no integration of this
C
C
470pF
R
C
20kΩ
ADP3088
IN
IN
GND
COMP
SW
DRV
GND
FB
5V - 8V
C
HF
10pF
V
IN
MLCC
2.2µF
Figure 5. BlackFin DSP Application
10µH @ 1A
1N5817
C
FF
2.2nF
3×10µF
MLCC
R
A
10.0kΩ
R
TT
287kΩ
R
PWM
41.2kΩ
V
OUT
0.9V-1.5V
@700 mA
V
2.5V
TT
PWM 0-2.5V
–10– REV. PrK
PRELIMINARY TECHNICAL DATA
RM-8
8-Lead Mini/micro SOIC Package [Mini_SO]
0.122 (3.10)
0.114 (2.90)
ADP3088
0.122 (3.10)
0.114 (2.90)
0.006 (0.15)
0.002 (0.05)
SEATING
PLANE
85
1
PIN 1
0.0256 (0.65) BSC
0.120 (3.05)
0.112 (2.84)
0.018 (0.46)
0.008 (0.20)
0.199 (5.05)
0.187 (4.75)
4
0.043 (1.09)
0.037 (0.94)
0.011 (0.28)
0.003 (0.08)
0.120 (3.05)
0.112 (2.84)
33ⴗ
27ⴗ
0.028 (0.71)
0.016 (0.41)
–11–REV. PrK
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