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MAX1809EEE+ |MAX1809EEEMAXN/a35avai3A, 1MHz, DDR Memory Termination Supply
MAX1809EEE+TMAXIM/PBFN/a4000avai3A, 1MHz, DDR Memory Termination Supply
MAX1809EEE+TG068MAXIMN/a734avai3A, 1MHz, DDR Memory Termination Supply
MAX1809EEE-T |MAX1809EEETMAXIMN/a2345avai3A, 1MHz, DDR Memory Termination Supply
MAX1809EGI-T |MAX1809EGITMAXIMN/a1089avai3A, 1MHz, DDR Memory Termination Supply
MAX1809ETI+MAXIMN/a80avai3A, 1MHz, DDR Memory Termination Supply
MAX1809ETI+TMAXIMN/a2avai3A, 1MHz, DDR Memory Termination Supply


MAX1809EEE+T ,3A, 1MHz, DDR Memory Termination Supplyfeatures an adjustable soft-start toOrdering Informationlimit surge currents during startup, a 100% ..
MAX1809EEE+TG068 ,3A, 1MHz, DDR Memory Termination SupplyELECTRICAL CHARACTERISTICS(V = V = 3.3V, V = 1.1V, T = 0°C to +85°C, unless otherwise noted. Typica ..
MAX1809EEE-T ,3A, 1MHz, DDR Memory Termination Supplyfeatures an♦ 93% Efficiencyinternal PMOS power switch and internal synchronousrectifier for high ef ..
MAX1809EGI ,0.3-6V; 3A, 1MHz, DDR memory power supply. For DDR memory termination, active termination busesApplicationsDDR Memory Termination Pin ConfigurationsActive Termination BusesTOP VIEWTypical Operat ..
MAX1809EGI-T ,3A, 1MHz, DDR Memory Termination SupplyApplicationsDDR Memory Termination Pin ConfigurationsActive Termination BusesTOP VIEWTypical Operat ..
MAX1809ETI ,3A / 1MHz / DDR Memory Termination SupplyELECTRICAL CHARACTERISTICS (continued)(V = V = 3.3V, V = 1.1V, T = 0°C to +85°C, unless otherwise n ..
MAX4666EPE ,5 / Quad / SPST / CMOS Analog SwitchesELECTRICAL CHARACTERISTICS—Dual Supplies(V+ = +15V, V- = -15V, V = +5V, V = +2.4V, V = +0.8V, T = T ..
MAX4666ESE ,5 / Quad / SPST / CMOS Analog SwitchesFeaturesThe MAX4664/MAX4665/MAX4666 quad analog switch-♦ Low On-Resistance (5Ω max)es feature 5Ω ma ..
MAX4666ESE+ ,5 Ohm, Quad, SPST, CMOS Analog SwitchesELECTRICAL CHARACTERISTICS—Dual Supplies(V+ = +15V, V- = -15V, V = +5V, V = +2.4V, V = +0.8V, T = T ..
MAX4667ESE+T ,2.5Ω, Dual, SPST, CMOS Analog SwitchesApplicationsMAX4667CPE 0°C to +70°C 16 Plastic DIPMAX4667ESE -40°C to +85°C 16 Narrow SOReed Relay ..
MAX466CWI ,Two-Channel, Triple/Quad RGB Video Switches and BuffersFeaturesThe MAX463–MAX470 series of two-channel, ' 100MHz Unity-Gain Bandwidthtriple/quad buffered ..
MAX4670ETJ+ ,Integrated T1/E1/J1 Short-Haul and Long-Haul Protection SwitchesApplications Pin ConfigurationOptical Multiplexers (ADMs, M13s, etc.)TOP VIEWEdge RoutersMultiservi ..


MAX1809EEE+-MAX1809EEE+T-MAX1809EEE+TG068-MAX1809EEE-T-MAX1809EGI-T-MAX1809ETI+-MAX1809ETI+T
3A, 1MHz, DDR Memory Termination Supply
General Description
The MAX1809 is a reversible energy flow, constant-off-
time, pulse-width modulated (PWM), step-down DC-DC
converter. It is ideal for use in notebook and subnote-
book computers that require 1.1V to 5V active
termination power supplies. This device features an
internal PMOS power switch and internal synchronous
rectifier for high efficiency and reduced component
count. The internal 90mΩPMOS power switch and
70mΩNMOS synchronous-rectifier switch easily deliver
continuous load currents up to 3A. The MAX1809 accu-
rately tracks an external reference voltage, produces
an adjustable output from 1.1V to VIN,and achieves
efficiencies as high as 93%.
The MAX1809 uses a unique current-mode, constant-
off-time, PWM control scheme that allows the output to
source or sink current. This feature allows energy to
return to the input power supply that otherwise would
be wasted. The programmable constant-off-time archi-
tecture sets switching frequencies up to 1MHz, allowing
the user to optimize performance trade-offs between
efficiency, output switching noise, component size, and
cost. The MAX1809 features an adjustable soft-start to
limit surge currents during startup, a 100% duty-cycle
mode for low-dropout operation, and a low-power shut-
down mode that disables the power switches and
reduces supply current below 1µA. The MAX1809 is
available in a 28-pin QFN with an exposed backside
pad, a 28-pin thin QFN, or a 16-pin QSOP.
Applications

DDR Memory Termination
Active Termination Buses
Features
Source/Sink 3A±1% Output AccuracyUp to 1MHz Switching Frequency93% EfficiencyInternal PMOS/NMOS Switches
90mΩ/70mΩOn-Resistance at VIN= 4.5V
110mΩ/80mΩOn-Resistance at VIN= 3V
1.1V to VINAdjustable Output Voltage3V to 5.5V Input Voltage Range<1µA Shutdown Supply CurrentProgrammable Constant-Off-Time OperationThermal ShutdownAdjustable Soft-Start Inrush Current LimitingOutput Short-Circuit Protection
MAX1809
3A, 1MHz, DDR Memory Termination Supply

N.C.
N.C.
EXTREF
PGND
PGND
PGND
VCC
GND
N.C.N.C.LXN.C.LXN.C.
REF
GNDN.C.N.C.N.C.
TOFF
TOP VIEW
THIN QFN

SHDN272625242322911121314
MAX1809
Pin Configurations
Ordering Information

MAX1809
VOUT
VCC
VIN
VSET
SHDN
EXTREF
TOFF
PGND
GND
REF
Typical Operating Circuit

19-2142; Rev 1; 9/02
PARTTEMP RANGEPIN-PACKAGE

MAX1809EGI*-40°C to +85°C28 QFN
MAX1809EEE-40°C to +85°C16 QSOP
MAX1809ETI-40°C to +85°C28 Thin QFN
Pin Configurations continued at end of data sheet.

*Contact factory for availability.
MAX1809
3A, 1MHz, DDR Memory Termination Supply
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS

(VIN= VCC= 3.3V, VEXTREF= 1.1V, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Note 1:
LX has clamp diodes to PGND and IN. If continuous current is applied through these diodes, thermal limits must be
observed.
VCC, IN to GND........................................................-0.3V to +6V
IN to VCC.............................................................................±0.3V
GND to PGND.....................................................................±0.3V
SHDN, SS, FB, TOFF, RREF,
EXTREF to GND.......................................-0.3V to (VCC+ 0.3V)
LX Current (Note 1).............................................................±4.7A
REF Short Circuit to GND Duration............................Continuous
Continuous Power Dissipation (TA= +70°C)
28-Pin QFN (derate 20mW/°C above +70°C;
part mounted on 1in2of 1oz copper)..............................1.6W
16-Pin QSOP (derate 12.5mW/°C above +70°C;
part mounted on 1in2of 1oz copper).................................1W
Operating Temperature Range...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS

Input VoltageVIN,VCC3.05.5V
Feedback Voltage Accuracy
(VFB - VEXTREF)
VIN = VCC = 3V to 5.5V, ILOAD = 0,
VEXTREF = 1.25V (Note 2)-12+12mV
Feedback Load Regulation ErrorΔVFBILOAD = -3A to +3A, VEXTREF = 1.25V20mV
External Reference Voltage
RangeVEXTREFVIN = VCC = 3V to 5.5VVREF -
VIN -
1.7V
Reference VoltageVREF1.0781.1001.122V
Reference Load RegulationIREF = -1µA to +10µA0.52.0mV
VIN = 4.5V90200PMOS Switch
On-ResistanceRPMOSILX = 0.5AVIN = 3V110250mΩ
VIN = 4.5V70150NMOS Switch
On-ResistanceRNMOSILX = 0.5AVIN = 3V80200mΩ
Current-Limit ThresholdILIMITVIN > VLX3.54.14.7A
Switching FrequencyfSW(Note 3)1MHz
ICCfSW = 500kHz1No Load Supply CurrentIINfSW = 500kHz16mA
Shutdown Supply CurrentISHDNSHDN = GND, ICC + IIN<115µA
Thermal-Shutdown ThresholdHysteresis = 15°C160°C
Undervoltage Lockout ThresholdVCC falling, hysteresis = 90mV2.52.62.7V
FB Input Bias CurrentIFBVFB = VEXTREF + 0.1V060250nA
RTOFF = 30.1kΩ0.240.300.37
RTOFF = 110kΩ0.91.01.1Off-TimetOFF
RTOFF = 499kΩ3.84.55.2
Startup Off-Time4 x tOFFµs
On-TimetON(Note 3)0.35µs
MAX1809
3A, 1MHz, DDR Memory Termination Supply
ELECTRICAL CHARACTERISTICS

(VIN= VCC= 3.3V, VEXTREF= 1.1V, TA= -40°C to +85°C, unless otherwise noted.) (Note 4)
Note 2:
The output voltage will have a DC-regulation level lower than the feedback error comparator threshold by 50% of the ripple.
Note 3:
Recommended operating frequency, not production tested.
Note 4:
Specifications from 0°C to -40°C are guaranteed by design, not production tested.
ELECTRICAL CHARACTERISTICS (continued)

(VIN= VCC= 3.3V, VEXTREF= 1.1V, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS

SS Source CurrentISS456µA
SS Sink CurrentISSVSS = 1V150mA
SHDN Input CurrentV SHDN = 0, VCC-1+1µA
VIL0.8SHDN Logic LevelsVIH2V
Maximum Output RMS CurrentIOUT(RMS)3.1ARMS
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS

Input VoltageVIN, VCC3.05.5V
Feedback Voltage Accuracy
(VFB - VEXTREF)
VIN = VCC = 3V to 5.5V, ILOAD = 0,
VEXTREF = 1.25V-24+24mV
External Reference Voltage
RangeVEXTREFVIN = VCC = 3 V to 5.5VVREF -
0.01V
VIN -
1.9VV
Reference VoltageVREF1.0671.133V
VIN = 4.5V200PMOS Switch
On-ResistanceRPMOSILX = 0.5AVIN = 3V250mΩ
VIN = 4.5V150NMOS Switch
On-ResistanceRNMOSILX = 0.5AVIN = 3V200mΩ
Current-Limit ThresholdILIMITVIN > VLX3.34.9A
FB Input Bias CurrentIFBVFB = VEXTREF + 0.1V300nA
Off-TimetOFFRTOFF = 110kΩ0.851.15µs
MAX1809
3A, 1MHz, DDR Memory Termination Supply

EFFICIENCY vs. OUTPUT CURRENT
(SOURCING)
MAX1809 toc01
OUTPUT CURRENT (A)
EFFICIENCY (%)21
RDROOP = 0Ω
VIN = 5V,
VOUT = 2.5V
VIN = 3.3V,
VOUT = 1.25V
VIN = 5V,
VOUT = 1.25V
EFFICIENCY vs. OUTPUT CURRENT
(SINKING)

MAX1809 toc02
OUTPUT CURRENT (A)
EFFICIENCY (%)21RDROOP = 0Ω
VIN = 5V,
VOUT = 1.25V
VIN = 5V,
VOUT = 2.5V
VIN = 3.3V,
VOUT = 1.25V
NORMALIZED OUTPUT ERROR
vs. OUTPUT CURRENT
MAX1809 toc03
OUTPUT CURRENT (A)
NORMALIZED OUTPUT ERROR (%)
VIN = 3.3V
VIN = 5V31456
NO-LOAD
SUPPLY CURRENT vs. SUPPLY VOLTAGE

MAX1809 toc04
VIN (V)
NO-LOAD SUPPLY CURRENT (I
+ I
(mA))
OFF-TIME vs. RTOFF
MAX1809 toc05
RTOFF (kΩ)
tOFF
SWITCHING FREQUENCY
vs. OUTPUT CURRENT
MAX1809 toc06
OUTPUT CURRENT (A)
FREQUECNY (kHz)-1-20123
VIN = 5V
VIN = 3.3V
STARTUP AND SHUTDOWN

MAX1809 toc07
1A/div
VOUT
1V/div
VSHDN
5V/div
IIN
1ms/div
VSS
2V/div
VIN = 3.3V, ROUT = 0.5Ω
LOAD-TRANSIENT RESPONSE

MAX1809 toc08
50mV/div
IOUT
5A/div
V(LX)
5V/div
VOUT
(AC-COUPLED)
10μs/div
VEXTREF = 1.25V, VIN = 3.3V, IOUT = -2A to +2A to -2A
RDROOP = 0Ω
Typical Operating Characteristics

(Circuit of Figure 1, VOUT= 1.25V, for VIN= 5V: L = 1µH, RTOFF= 130kΩ; for VIN= 3.3V: L = 0.68µH, RTOFF= 73.2kΩ.)
MAX1809
3A, 1MHz, DDR Memory Termination Supply
LOAD-TRANSIENT RESPONSE

MAX1809 toc09
50mV/div
IOUT
5A/div
V(LX)
5V/div
VOUT
(AC-COUPLED)
10μs/div
VEXTREF = 1.25V, VIN = 3.3V, IOUT = -2A to +2A to -2A
RDROOP 12mΩ
LINE-TRANSIENT RESPONSE

MAX1809 toc10
50mV/div
VIN
2V/div
VOUT
(AC-COUPLED)
20μs/div
IOUT = 2A, VIN = 5V to 3.3V to 5V
SWITCHING WAVEFORMS (SOURCING)

MAX1809 toc11
50mV/div
V(LX)
5V/div
VOUT
(AC-COUPLED)
400ns/div
IOUT = 2A, VIN = 5V
I(LX)
2A/div
SWITCHING WAVEFORMS (SINKING)

MAX1809 toc12
50mV/div
V(LX)
5V/div
VOUT
(AC-COUPLED)
400ns/div
IOUT = -2A, VIN = 5V
I(LX)
2A/div
Typical Operating Characteristics (continued)

(Circuit of Figure 1, VOUT= 1.25V, for VIN= 5V: L = 1µH, RTOFF= 130kΩ; for VIN= 3.3V: L = 0.68µH, RTOFF= 73.2kΩ.)
MAX1809
3A, 1MHz, DDR Memory Termination Supply
Detailed Description

The MAX1809 synchronous, current-mode, constant-
off-time, PWM DC-DC converter steps down input volt-
ages of 3V to 5.5V to an adjustable output voltage from
1.1V to VIN, as set by the voltage applied at EXTREF. It
sources and sinks up to 3A of output current. Internal
switches composed of a 90mΩPMOS power switch
and a 70mΩNMOS synchronous-rectifier switch
improve efficiency, reduce component count, and elim-
inate the need for an external Schottky diode across
the synchronous switch.
The MAX1809 operates in a constant-off-time mode
under all loads. A single resistor-programmable con-
stant-off-time control sets switching frequencies up to
1MHz, allowing the user to optimize performance trade-
offs in efficiency, switching noise, component size, and
cost.
When power is drawn from a regulated supply, con-
stant-off-time PWM architecture essentially provides
constant-frequency operation. This architecture has the
inherent advantage of quick response to line and load
transients. The MAX1809’s current-mode, constant-off-
time PWM architecture regulates the output voltage by
changing the PMOS switch on-time relative to the con-
stant off-time.
Constant-Off-Time Operation

In the constant-off-time architecture, the FB voltage
comparator turns the PMOS switch on at the end of
each off-time, keeping the device in continuous-con-
duction mode. The PMOS switch remains on until the
Pin Description
PIN (QFN)PIN (QSOP)NAMEFUNCTION

1, 5, 10, 11, 12,
22, 24, 26, 28—N.C.No Connection. Not internally connected.
2, 42, 4INSupply Voltage Input for the Internal PMOS Power Switch. Not internally
connected. Externally connect all pins for proper operation.
3, 18, 19, 23, 253, 14, 16LX
Inductor Connection. Connection for the drains of the PMOS power switch
and NMOS synchronous-rectifier switch. Connect the inductor from this
node to the output filter capacitor and load. Not internally connected.
Externally connect all pins for proper operation.SSSoft-Start. Connect a capacitor from SS to GND to limit inrush current
during startup.6EXTREFExternal Reference Input. Feedback input regulates to VEXTREF. The PWM
controller remains off until EXTREF is greater than REF.7TOFFOff-Time Select Input. Sets the PMOS power switch constant-off-time.
Connect a resistor from TOFF to GND to adjust the PMOS switch off-time.FBFeedback Input. Connect directly to output for fixed-voltage operation or to
a resistive-divider for adjustable operating modes.
13, backside pad,
corner tabs9GNDAnalog Ground. Connect exposed backside pad and corner tabs to analog
GND.10REFReference Output. Bypass REF to GND with a 1µF capacitor.11GNDTie to GND (pin 13 QFN; pin 9 QSOP)12VCCAnalog Supply Voltage Input. Supplies internal analog circuitry. Bypass
VCC with a 10Ω and 2.2µF low-pass filter (see Figure 1).
17, 20, 2113, 15PGNDPower Ground. Internally connected to the internal NMOS synchronous-
rectifier switch.1SHDN
Shutdown Control Input. Drive SHDN low to disable the reference, control
circuitry, and internal MOSFETs. Drive high or connect to VCC for normal
operation.
MAX1809
3A, 1MHz, DDR Memory Termination Supply

feedback voltage exceeds the external reference voltage
(VEXTREF) or the positive current limit is reached. When
the PMOS switch turns off, it remains off for the pro-
grammed off-time (tOFF). To control the current under
short-circuit conditions, the PMOS switch remains off for
approximately 4 ✕tOFFwhen VFB< VEXTREF/ 4.
Synchronous Rectification

In a stepdown regulator without synchronous rectification,
an external Schottky diode provides a path for current to
flow when the inductor is discharging. Replacing the
Schottky diode with a low-resistance NMOS synchro-
nous switch reduces conduction losses and improves
efficiency.
The NMOS synchronous-rectifier switch turns on follow-
ing a short delay (approximately 50ns) after the PMOS
power switch turns off, thus preventing cross-conduc-
tion or “shoot-through.” In constant-off-time mode, the
synchronous-rectifier switch turns off just prior to the
PMOS power switch turning on. While both switches
are off, inductor current flows through the internal body
diode of the NMOS switch.
Current Sourcing and Sinking

By operating in a constant-off-time, pseudo-fixed-fre-
quency mode, the MAX1809 can both source and sink
current. Depending on the output current requirement,
the circuit operates in two modes. In the first mode the
output draws current and the MAX1809 behaves as a
regular buck controller, sourcing current to the output
from the input supply rail. However, when the output is
supplied by another source, the MAX1809 operates in
a second mode as a synchronous boost, taking power
from the output and returning it to the input.
Thermal Resistance

Junction-to-ambient thermal resistance, θJA, is highly
dependent on the amount of copper area immediately
surrounding the IC leads. The MAX1809 QFN package
has 1in2of copper area and a thermal resistance of
50°C/W with no forced airflow. The MAX1809 16-pin
QSOP evaluation kit has 0.5in2of copper area and a
thermal resistance of 80°C/W with no forced airflow.
Airflow over the board significantly reduces the junction-
to-ambient thermal resistance. For heat sinking pur-
poses, it is essential to connect the exposed backside
pad of the QFN package to a large analog ground plane.
Shutdown

Drive SHDNto a logic-level low to place the MAX1809 in
low-power shutdown mode and reduce supply current to
less than 1µA. In shutdown, all circuitry and internal
MOSFETs turn off, so the LX node becomes high imped-
ance. Drive SHDNto a logic-level high or connect to VCC
for normal operation.
Power Dissipation

Power dissipation in the MAX1809 is dominated by
conduction losses in the two internal power switches.
Power dissipation due to charging and discharging the
gate capacitance of the internal switches (i.e., switch-
ing losses) is approximately:
PD(CAP)= C ✕VIN2✕fSW
MAX1809
VCC
VIN
VSET
SHDN
EXTREF
TOFF
33μF
10Ω
10kΩ
10kΩ
RTOFF
RDROOP
VDDQ
(2.5V)
VSSQ
0.01μF
1μF
270μF
15mΩ
2.2μF
1000pF
1000pF
PGND
GND
REF
FOR VIN = 5V: L = 1μH, RTOFF = 130kΩ
FOR VIN = 3.3V: L = 0.68μH, RTOFF = 73.2kΩ
VDDQ
2( )VOUT =
Figure 1. Typical Application Circuit
MAX1809
3A, 1MHz, DDR Memory Termination Supply

where C = 2.5nF and fSWis the switching frequency.
Resistive losses in the two power switches are approxi-
mated by:
PD(RES)= IOUT2✕RPMOS
where RPMOSis the on-resistance of the PMOS switch.
The junction-to-ambient thermal resistance required to
dissipate this amount of power is calculated by:
θJA= (TJ,MAX- TA,MAX) / (PD(CAP)+ PD(RES))
where:
θJA= junction-to-ambient thermal resistance
TJ,MAX= maximum junction temperature
TA,MAX= maximum ambient temperature
Design Procedure

For typical applications, use the recommended compo-
nent values in Figure 1. For other applications, take the
following steps:Select the desired PWM-mode switching frequency.
See Figure 4 for maximum operating frequency.Select the constant off-time as a function of input
voltage, output voltage, and switching frequency.Select RTOFFas a function of off-time.Select the inductor as a function of output voltage,
off-time, and peak-to-peak inductor current.
Setting the Output Voltage

The output voltage of the MAX1809 is set by an external
voltage applied to the EXTREF pin. This can come directly
from another voltage source or external reference.
As an active termination supply in DDR applications
(see Active Bus Termination in the Applications
Informationsection), the output of the MAX1809 is reg-
ulated at half the DDR supply voltage. In mobile sys-
tems, the DDR supply voltage is 2.5V, and the
termination voltage is 1.25V ±40mV. To regulate to
1.25V, an external divide-by-2 resistor network is
placed across the DDR supply voltage to generate
1.25V. This 1.25V is connected to EXTREF, which sets
the output voltage of the MAX1809. When FB is directly
tied to the output (Figure 5), the output voltage range is
limited by the external reference’s input voltage limits
(see EC table). External reference may not be set within
1.7V of the minimum supply voltage. VEXTREFshould
be limited to less than 1.4V for 3.3V input voltage.
Failure to comply can cause the part to operate abnor-
mally and may cause part damage.
Alternatively, the output can be adjusted up to VINby
connecting FB to a resistor-divider between the output
voltage and ground (Figure 6). Use 50kΩfor R1. R2 is
given by:V
OUT
EXTREF1=−
Figure 2. Functional Diagram
VCC
2.2μF
1μF
0.01μF
PWM LOGIC
AND
DRIVERSVIN
(3.0V TO 5.5V)L
CIN
CERAMIC
PGND
RTOFF
GND
REFREF
SHDN
EXTREF
TIMER
VIN
CURRENT
SENSEMAX1809
COUT
MAX1809
3A, 1MHz, DDR Memory Termination Supply
Programming the Switching Frequency
and Off-Time and On-Time

The MAX1809 features a programmable PWM-mode
switching frequency, which is set by the input and out-
put voltage and the value of RTOFF, connected from
TOFF to GND. RTOFFsets the PMOS power switch off-
time in PWM mode. Use the following equation to select
the off-time while sourcing current according to the
desired switching frequency in PWM mode:
where:
tOFF= the programmed off-time
VIN= the input voltage
VOUT= the output voltage
VPMOS= the voltage drop across the internal
PMOS power switch |IOUT✕RPMOS|
VNMOS= the voltage drop across the internal
NMOS synchronous-rectifier switch |IOUT✕
RNMOS|
fSW= switching frequency
Make sure that tONand tOFFare greater than 400ns
when sourcing current. Select RTOFFaccording to the
formula:
RTOFF= (tOFF- 0.07µs) ✕(117kΩ/1.00µs)
Recommended values for RTOFFrange from 36kΩto
430kΩfor off-times of 0.4µs to 4µs.
When sinking current, the switching frequency increas-
es due to the on-resistances of the internal switches
adding to the voltage across the inductor, reducing the
on-time. Calculate tONwhen sinking current using the
equation:
Check that tONin the current sinking mode is greater
than 350ns.
Inductor Selection

The key inductor parameters must be specified: inductor
value (L) and peak current (IPEAK). The following equa-
tion includes a constant, denoted as LIR, which is the
ratio of peak-to-peak inductor AC current (ripple current)VVVONOFFOUTNMOSOUTPMOS=+−VVVVVOFFINOUTPMOSINPMOSNMOS=()()
Figure 3. Sourcing and Sinking Capabilities of the MAX1809
SYNCHRONOUS BUCK MODE
(SOURCING CURRENT)
VIN
VOUTISOURCE
SYNCHRONOUS BOOST MODE
(SINKING CURRENT)
VIN
VSOURCE > VOUTISINK
Figure 4. Maximum Recommended Operating Frequency vs.
Input Voltage
VIN (V)
OPERATING FREQUENCY (kHz)
VOUT = 2.5V
VOUT = 1.25V
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