MAXIM MAX1928EUB18

19-2527; Rev 0; 7/02
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
The MAX1927/MAX1928 800mA step-down converters
power low-voltage microprocessors in compact equipment requiring the highest possible efficiency. The
MAX1927/MAX1928 are optimized for generating low
output voltages (down to 750mV) at high efficiency
using small external components. The supply voltage
range is from 2.6V to 5.5V and the guaranteed minimum
output current is 800mA. 1MHz pulse-width modulation
(PWM) switching allows for small external components.
A unique control scheme minimizes ripple at light loads,
while maintaining a low 140µA quiescent current.
The MAX1927/MAX1928 include a low on-resistance
internal MOSFET switch and synchronous rectifier to
maximize efficiency and minimize external component
count. No external diode is needed. 100% duty-cycle
operation allows for a dropout voltage of only 340mV at
800mA. Other features include internal soft-start,
power-OK (POK) output, and selectable forced PWM
operation for lower noise at all load currents.
The MAX1928 is available with several preset output
voltages: 1.5V (MAX1928-15), 1.8V (MAX1928-18), and
2.5V (MAX1928-25). The MAX1927R has adjustable
output range down to 0.75V. The MAX1927/MAX1928
are available in a tiny 10-pin µMAX package.
Features
♦ 800mA Output Current
♦ Output Voltages from 0.75V to 5V
♦ 2.6V to 5.5V Input Voltage Range
♦ Power-OK Output
♦ No Schottky Diode Required
♦ Selectable Forced PWM Operation
♦ 1MHz Fixed-Frequency PWM Operation
♦ 140µA Quiescent Current
♦ Soft-Start
♦ 10-Pin µMAX Package
Ordering Information
PRESET
TEMP
PINOUTPUT
PACKAGE
RANGE
VOLTAGE
Adj. to 0.75V -40°C to +85°C 10 µMAX
PART
MAX1927REUB
Applications
WCDMA Handsets
MAX1928EUB15
1.5V
-40°C to +85°C 10 µMAX
MAX1928EUB18
1.8V
-40°C to +85°C 10 µMAX
MAX1928EUB25
2.5V
-40°C to +85°C 10 µMAX
PDAs and Palmtops
DSP Core Power
Battery-Powered Equipment
Pin Configuration
Typical Operating Circuit
VIN
2.6V TO 5.5V
TOP VIEW
PWM
BATT
L1
C1
PWM 1
10 POK
GND
2
9 BATT
REF
3
FB
4
7 PGND
COMP
5
6 SHDN
MAX1927R
MAX1928
µMAX
VOUT
0.75V AT 800mA
LX
SHDN
RC
COMP
8 LX
CC
C2
FB
MAX1927R
POK
Cf
REF
PGND
GND
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1
MAX1927/MAX1928
General Description
MAX1927/MAX1928
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
ABSOLUTE MAXIMUM RATINGS
BATT, PWM, POK, COMP, SHDN to GND ...............-0.3V to +6V
PGND to GND .......................................................-0.3V to +0.3V
LX, REF, FB to GND ................................-0.3V to (VBATT + 0.3V)
Continuous Power Dissipation (TA = +70°C)
10-Pin µMAX (derate 5.6mW/°C above +70°C) ...........444mW
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
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.
ELECTRICAL CHARACTERISTICS
(VBATT = 3.6V, SHDN = BATT, CREF = 0.1µF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
BATT Input Voltage
Undervoltage Lockout Threshold
Quiescent Current
MIN
MAX
UNITS
5.5
V
2.35
2.55
V
140
240
µA
2.6
VBATT rising or falling (35mV hysteresis)
2.15
No load, pulse skipping, PWM = GND
1MHz switching
2
Quiescent Current in Dropout
Shutdown Supply Current
TYP
SHDN = GND
mA
190
340
µA
0.1
10
µA
REFERENCE AND ERROR AMP
FB Voltage Accuracy
FB Input Current
Transconductance (gm)
MAX1927R
0.738
0.75
0.762
MAX1928-15
1.477
1.5
1.523
MAX1928-18
1.773
1.8
1.827
MAX1928-25
2.462
2.5
2.538
5
10
15
µA
150
nA
1.231
10
250
210
175
125
1.25
1.269
V
2.6V < VBATT < 5.5V
0.5
2
mV
VBATT = 3.6V
0.25
0.4
VBATT = 2.6V
0.3
0.5
VBATT = 3.6V
0.17
0.3
VBATT = 2.6V
0.2
0.35
MAX1928
MAX1927R
MAX1927R
MAX1928-15
MAX1928-18
MAX1928-25
Reference Voltage Accuracy
Reference Supply Rejection
V
µS
PWM CONTROLLER
P-Channel On-Resistance
N-Channel On-Resistance
Current-Sense Transresistance (RCS )
0.48
Ω
Ω
V/A
P-Channel Current-Limit Threshold
1.1
1.3
1.6
A
P-Channel Pulse-Skipping Current Threshold
0.11
0.13
0.15
A
N-Channel Negative Current-Limit Threshold
2
-0.55
_______________________________________________________________________________________
A
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
(VBATT = 3.6V, SHDN = BATT, CREF = 0.1µF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
MIN
N-Channel Synchronous Rectifier Turn-Off
Threshold
LX Leakage Current
Maximum Duty Cycle
-20
0.1
+20
15
0.85
15°C hysteresis
µA
%
0
PWM = BATT
UNITS
mA
100
PWM = GND
Internal Oscillator Frequency
Thermal Shutdown Threshold
MAX
20
VBATT = 5.5V, LX = GND or BATT
Minimum Duty Cycle
TYP
1
1.15
160
%
MHz
Degrees
POK COMPARATOR
BATT Operating Voltage Range
IPOK = 0.1 mA
Output Low Voltage
VFB = 0.5V, IPOK = 1mA
1
Output High Leakage Current
VPOK = 5.5V
5.5
0.01
V
1
µA
MAX1927R
0.650
0.675
0.700
MAX1928-15
1.305
1.350
1.395
MAX1928-18
1.566
1.620
1.674
MAX1928-25
2.175
2.250
2.325
POK transitions to high impedance 20ms
after VFB > VPOK
15
20
25
Logic Input High
2.6V < VBATT < 5.5 V
1.6
Logic Input Low
2.6V < VBATT < 5.5 V
Logic Input Current
VBATT = 5.5V
POK Threshold
Output Valid to POK Release Delay
V
0.1
V
ms
LOGIC INPUTS (SHDN, PWM)
V
0.6
V
1
µA
MIN
MAX
UNITS
2.6
5.5
V
2.15
2.55
V
240
µA
340
µA
10
µA
0.1
ELECTRICAL CHARACTERISTICS
(VBATT = 3.6V, SHDN = BATT, CREF = 0.1µF, TA = -40°C to +85°C, unless otherwise noted.)
PARAMETER
CONDITIONS
BATT Input Voltage
Undervoltage Lockout Threshold
VBATT rising or falling (35mV hysteresis)
Quiescent Current
No load, pulse skipping, PWM = GND
Quiescent Current in Dropout
Shutdown Supply Current
SHDN = GND
REFERENCE AND ERROR AMP
MAX1927R
FB Voltage Accuracy
FB Input Current
0.732
0.768
MAX1928-15
1.47
1.53
MAX1928-18
1.764
1.836
MAX1928-25
2.45
2.55
5
15
MAX1928
V
µA
_______________________________________________________________________________________
3
MAX1927/MAX1928
ELECTRICAL CHARACTERISTICS (continued)
MAX1927/MAX1928
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
ELECTRICAL CHARACTERISTICS (continued)
(VBATT = 3.6V, SHDN = BATT, CREF = 0.1µF, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
FB Input Current
CONDITIONS
P-Channel On-Resistance
N-Channel On-Resistance
MAX
UNITS
150
nA
1.22
1.269
V
2
mV
MAX1927R
Reference Voltage Accuracy
Reference-Supply Rejection
PWM CONTROLLER
MIN
2.6V < VBATT < 5.5V
VBATT = 3.6V
0.4
VBATT = 2.6V
0.5
VBATT = 3.6V
0.30
VBATT = 2.6V
0.35
1.1
P-Channel Pulse-Skipping Current Threshold
0.10
0.16
A
-20
+20
µA
0
%
0.8
1.2
MHz
1
5.5
V
VBATT = 5.5V, LX = GND or BATT
Maximum Duty Cycle
Minimum Duty Cycle
1.6
Ω
P-Channel Current-Limit Threshold
LX Leakage Current
0.10
Ω
100
PWM = GND
Internal Oscillator Frequency
POK COMPARATOR
BATT Operating Voltage Range
IPOK = 0.1 mA
Output Low Voltage
VFB = 0.5V, IPOK = 1mA
Output High Leakage Current
VPOK = 5.5V
%
0.1
V
1
µA
MAX1927R
MAX1928-15
0.650
1.305
0.700
1.395
MAX1928-18
1.566
1.674
MAX1928-25
2.175
2.325
POK transitions to high impedance 20ms
after VFB > VPOK
15
25
Logic Input High
2.6V < VBATT < 5.5 V
1.6
Logic Input Low
2.6V < VBATT < 5.5 V
Logic Input Current
VBATT = 5.5V
POK Threshold
Output Valid to POK Release Delay
A
V
ms
LOGIC INPUTS (SHDN, PWM)
4
_______________________________________________________________________________________
V
0.6
V
1
µA
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
MAX1928-25
EFFICIENCY vs. LOAD CURRENT
VIN = 5V
60
50
VIN = 5V
70
60
10
100
40
40
30
VOUT = 1.8V
1
1000
10
100
1000
LOAD CURRENT (mA)
MAX1928-15
EFFICIENCY vs. LOAD CURRENT
MAX1927R
EFFICIENCY vs. LOAD CURRENT
MAX1928-25
DROPOUT VOLTAGE vs. LOAD CURRENT
100
VIN = 2.7V
90
80
EFFICIENCY (%)
80
VIN = 5V
70
VIN = 3.6V
70
VIN = 5V
VIN = 3.6V
60
50
50
40
40
500
450
DROPOUT VOLTAGE (mV)
MAX1927 toc04
90
VOUT = 1.5V
100
300
250
200
150
VIN = 2.5V
0
1
1000
350
50
30
10
400
100
VOUT = 1V
30
1
10
1
LOAD CURRENT (mA)
VIN = 2.7V
10
100
1000
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (A)
MAX1928-18
OUTPUT VOLTAGE vs. LOAD CURRENT
NO-LOAD INPUT CURRENT
vs. INPUT VOLTAGE
OSCILLATOR FREQUENCY
vs. INPUT VOLTAGE
1.84
1.82
1.80
1.78
1.76
300
250
200
150
100
1.74
1.72
VIN = 3.6V
1.70
50
LOAD CURRENT (mA)
TA = +85°C
1.04
1.02
1.00
TA = +25°C
0.98
TA = -40°C
0.96
0.94
0
0 100 200 300 400 500 600 700 800 900 1000
MAX1927 toc09
350
INPUT CURRENT (µA)
1.86
1.06
OSCILLATOR FREQUENCY (MHz)
1.88
MAX1927 toc08
400
MAX1927 toc07
1.90
OUTPUT VOLTAGE (V)
1000
100
LOAD CURRENT (mA)
100
60
VIN = 5V
60
50
MAX1927 toc05
1
VIN = 3.6V
70
50
VOUT = 3.3V
40
EFFICIENCY (%)
80
80
MAX1927 toc06
70
VIN = 2.7V
90
EFFICIENCY (%)
EFFICIENCY (%)
EFFICIENCY (%)
80
VIN = 3.6V
90
100
MAX1927 toc02
VIN = 3.6V
90
100
MAX1927 toc01
100
MAX1928-18
EFFICIENCY vs. LOAD CURRENT
MAX1927 toc03
MAX1927R
EFFICIENCY vs. LOAD CURRENT
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE (V)
2.6
3.1
3.6
4.1
4.6
5.1
5.6
INPUT VOLTAGE (V)
_______________________________________________________________________________________
5
MAX1927/MAX1928
Typical Operating Characteristics
(Circuits of Figure 3 and 4, TA = +25°C, unless otherwise noted.)
Typical Operating Characteristics (continued)
(Circuits of Figure 3 and 4, TA = +25°C, unless otherwise noted.)
MAXIMUM LOAD CURRENT
vs. INPUT VOLTAGE
POK WAVEFORM
STARTUP WAVEFORM
MAX1927 toc12
MAX1927 toc11
MAX1927 toc10
1.4
VOUT = 1V
MAXIMUM LOAD CURRENT (A)
MAX1927/MAX1928
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
1.2
SHDN
1.0
0.8
VOUT = 1.8V
5V/div
VOUT = 2.5V
5V/div
SHDN
POK
0.6
1V/div
VOUT
2V/div
0.4
0.2
IIN
200mA/div
VOUT
2V/div
0
2.6
3.1
3.6
4.1
4.6
5.1
5.6
20ms/div
1ms/div
INPUT VOLTAGE (V)
LIGHT-LOAD SWITCHING WAVEFORMS
HEAVY-LOAD SWITCHING WAVEFORMS
MAX1927 toc14
MAX1927 toc13
VOUT
(AC-COUPLED)
10mV/div
IL
200mA/div
LX
5V/div
VOUT
(AC-COUPLED)
10mV/div
LX
5V/div
200mA/div
IL
2ms/div
400ns/div
LINE TRANSIENT
LOAD TRANSIENT
MAX1927 toc16
MAX1927 toc15
VOUT
(AC-COUPLED)
100mV/div
VOUT
(AC-COUPLED)
10mV/div
4.2V
3V
900mA
VIN
500mA/div
250mA
ILOAD
100µs/div
6
2V/div
1ms/div
_______________________________________________________________________________________
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
PIN
NAME
FUNCTION
1
PWM
Forced-PWM Input. Drive to GND to use PWM at medium to heavy loads and pulse-skipping at light loads.
Drive to BATT to force PWM operation at all loads.
2
GND
Ground
3
REF
Internal 1.25V Reference. Bypass to GND with a 0.1µF capacitor.
4
FB
5
COMP
Compensation Input. See the Compensation, Stability, and Output Capacitor section for compensation
component selection.
6
SHDN
Shutdown Control Input. Drive low to shut down the converter. Drive high for normal operation.
7
PGND
8
LX
9
BATT
Supply Voltage Input. Connect to a 2.6V to 5.5V source. Bypass to GND with a low-ESR 10µF capacitor.
10
POK
Power-OK Open-Drain Output. Once the soft-start routine has completed, POK goes high impedance 20ms
after FB exceeds 90% of its expected final value.
Output Feedback Sense Input. To set the output voltage to the preset voltage (MAX1928), connect FB directly
to the output. To adjust the output voltage (MAX1927R), connect FB to the center of an external resistordivider between the output and GND. FB regulation voltage is 0.75V.
Power Ground
Inductor Connection to the drains of the internal power MOSFETs.
BATT
COMP
SLOPE
COMPENSATION
PWM
COMPARATOR
BIAS
P
P
LX
MAX1927
MAX1928
PFM CURRENT
COMPARATOR
1MHz
OSC
ILIM
COMPARATOR
PWM
PWM
CONTROL
N
N
SHDN
N-CHANNEL
CURRENT COMPARATOR
PGND
TO
COMP
REF
1.25V
REFERENCE
POWER-OK
CONTROL
POK
FB
MAX1927R
ONLY
MAX1928
ONLY
GND
Figure 1. Simplified Functional Diagram
_______________________________________________________________________________________
7
MAX1927/MAX1928
Pin Description
MAX1927/MAX1928
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
Detailed Description
The MAX1927/MAX1928 PWM step-down DC-DC converters accept inputs as low as 2.6V, while delivering
800mA to output voltages as low as 0.75V. These
devices operate in one of two modes to optimize noise
and quiescent current. Under heavy loads, MAX1927/
MAX1928 operate in pulse-width modulation (PWM)
mode and switch at a fixed 1MHz frequency. Under
light loads, they operate in PFM mode to reduce power
consumption. In addition, both devices provide selectable forced PWM operation for minimum noise at all
load currents.
PFM Operation and PWM Control Scheme
The PFM mode improves efficiency and reduces quiescent current to 140µA at light loads. The MAX1927/
MAX1928 initiate pulse-skipping PFM operation when
the peak inductor current drops below 130mA. During
PFM operation, the MAX1927/MAX1928 switch only as
necessary to service the load, reducing the switching
frequency and associated losses in the internal switch,
synchronous rectifier, and inductor.
During PFM mode, a switching cycle initiates when the
error amplifier senses that the output voltage has
dropped below the regulation point. If the output voltage is low, the P-channel MOSFET switch turns on and
conducts current to the output filter capacitor and load.
The PMOS switch turns off when the PWM comparator
is satisfied. The MAX1927/MAX1928 then wait until the
error amplifier senses a low output voltage to start
again. Some jitter is normal during the transition from
PFM to PWM with loads around 100mA. This has no
adverse impact on regulation.
At loads greater than 130mA, the MAX1927/MAX1928
use a fixed-frequency, current-mode, PWM controller
capable of achieving 100% duty cycle. Current-mode
feedback provides cycle-by-cycle current limiting,
superior load and line response, as well as overcurrent
protection for the internal MOSFET and synchronous
rectifier. A comparator at the P-channel MOSFET switch
detects overcurrent conditions exceeding 1.1A.
During PWM operation, the MAX1927/MAX1928 regulate output voltage by switching at a constant frequency
and then modulating the power transferred to the load
using the PWM comparator (Figure 1). The error-amp
output, the main switch current-sense signal, and the
slope compensation ramp are all summed at the PWM
comparator. The comparator modulates the output
power by adjusting the peak inductor current during the
first half of each cycle based on the output-error voltage. The MAX1927/MAX1928 have relatively low ACloop gain coupled with a high-gain integrator to enable
8
the use of a small, low-valued, output filter capacitor.
The resulting load regulation is 0.3% (typ) from 0 to
800mA.
Forced PWM Operation
To force PWM-only operation, connect PWM to BATT.
Forced PWM operation is desirable in sensitive RF and
data-acquisition applications to ensure that switching
noise does not interfere with sensitive IF and data sampling frequencies. A minimum load is not required during forced PWM operation because the synchronous
rectifier passes reverse inductor current as needed to
allow constant frequency operation with no load.
Forced PWM operation has higher quiescent current
than PFM (2mA typ compared to 140µA) due to continuous switching.
100% Duty-Cycle Operation
The maximum on-time can exceed one internal oscillator cycle, which permits operation at 100% duty cycle.
As the input voltage drops, the duty cycle increases
until the internal P-channel MOSFET stays on continuously. Dropout voltage at 100% duty cycle is the output
current multiplied by the sum of the internal PMOS onresistance (typically 0.25Ω) and the inductor resistance. Near dropout, switching cycles can be skipped,
reducing switching frequency. However, voltage ripple
remains small because the current ripple is still low.
Synchronous Rectification
An N-channel synchronous rectifier eliminates the need
for an external Schottky diode and improves efficiency.
The synchronous rectifier turns on during the second
half of each cycle (off-time). During this time, the voltage across the inductor is reversed, and the inductor
current falls. In normal mode, the synchronous rectifier
is turned off when either the output falls out of regulation (and another on-time begins) or when the inductor
current approaches zero. In forced PWM mode, the
synchronous rectifier remains active until the beginning
of a new cycle.
Shutdown Mode
Driving SHDN to GND places the MAX1927/MAX1928
in shutdown mode. In shutdown, the reference, control
circuitry, internal switching MOSFET, and synchronous
rectifier turn off and the output becomes high impedance. Drive SHDN high for normal operation. Input current falls to 0.1µA (typ) during shutdown mode.
POK Output
POK is an open-drain output that goes high impedance
20ms after the soft-start ramp has concluded and VFB
is within 90% of the threshold. POK is low impedance
when in shutdown.
_______________________________________________________________________________________
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
PART
LX
R1
MAX1927R
PRESET OUTPUT VOLTAGE
MAX1927R
FB
R2
50kΩ
Figure 2. Setting the Adjustable Output Voltage
Applications Information
Output Voltage Selection
The MAX1927/MAX1928 have preset output voltages.
In addition, the MAX1927R has an adjustable output.
To set the output voltage at the preset voltage, connect
FB to the output. See Table 1 for a list of the preset voltages and their corresponding part numbers.
The output voltage for the MAX1927R is adjustable
from 0.75V to the input voltage by connecting FB to a
resistor-divider between the output and GND (Figure
2). To determine the values of the resistor-divider, first
select a value for feedback resistor R2 between 5kΩ to
50kΩ. R1 is then given by:
V

R1 = R2 ×  OUT − 1
 VFB

where VFB is 0.75V.
Input Capacitor Selection
Capacitor equivalent series resistance (ESR) is a major
contributor to input ripple in high-frequency DC-DC
converters. Ordinary aluminum-electrolytic capacitors
have high ESR and should be avoided. Low-ESR aluminum electrolytic capacitors are acceptable and relatively inexpensive. Low-ESR tantalum capacitors or
polymer capacitors are better and provide a compact
solution for space-constrained surface-mount designs.
Ceramic capacitors have the lowest ESR overall.
The input filter capacitor reduces peak currents and
noise at the input voltage source. Connect a low-ESR
bulk capacitor (≥10µF typ) to the input. Select this bulk
capacitor to meet the input ripple requirements and
voltage rating rather than capacitance value. Use the
0.75V, Adjustable
MAX1928-15
1.5 V
MAX1928-18
1.8 V
MAX1928-25
2.5 V
following equation to calculate the maximum RMS input
current:
I
IRMS = OUT × VOUT × (VIN − VOUT )
VIN
Compensation, Stability, and
Output Capacitor
The MAX1927/MAX1928 are externally compensated
with a resistor and a capacitor (see Figure 3, RC and
CC) in series from COMP to GND. An additional capacitor (Cf) may be required from COMP to GND if highESR output capacitors are used. The capacitor integrates the current from the transimpedance amplifier,
averaging output capacitor ripple. This sets the device
speed for transient response and allows the use of
small ceramic output capacitors because the phaseshifted capacitor ripple does not disturb the current
regulation loop. The resistor sets the proportional gain
of the output error voltage by a factor g m ✕ R C .
Increasing this resistor also increases the sensitivity of
the control loop to output ripple.
The resistor and capacitor set a compensation zero
that defines the system’s transient response. The load
creates a dynamic pole, shifting in frequency with
changes in load. As the load decreases, the pole frequency decreases. System stability requires that the
compensation zero must be placed to ensure adequate
phase margin (at least 30° at unity gain). The following
is a design procedure for the compensation network:
1) Select an appropriate converter bandwidth (fC) to
stabilize the system while maximizing transient
response. This bandwidth should not exceed 1/10
of the switching frequency.
2) Calculate the compensation capacitor, CC, based
on this bandwidth:
For the MAX1927:
 V
  1  
R2   1 
OUT
CC = 
 ×
 ×  gm × R1+ R2  ×  2πf 
I
R


 C
 OUT(MAX) 
CS
_______________________________________________________________________________________
9
MAX1927/MAX1928
Table 1. FB Regulation Voltages
MAX1927/MAX1928
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
For the MAX1928:
cel out the dominant pole created by the output
load and the output capacitance:
 V
  1 
 1 
OUT
CC = 
× (gm ) × 
 ×


 2πfC 
 IOUT(MAX)   RCS 
1
1
=
2π × RL × COUT 2π × RC × CC
Resistors R1 and R2 are external to the MAX1927 (see
the Setting the Output Voltage section). IOUT(MAX) is the
maximum output current, R CS = 0.48V/A, and g m =
250µS for the MAX1927. See the Electrical Characteristics
table for MAX1928 gm values. Select the closest standard
CC value that gives an acceptable bandwidth.
3) Calculate the equivalent load impedance, RL, by:
RL =
Solving for RC gives:
R × COUT
RC = L
CC
5) Calculate the high-frequency compensation pole to
cancel the zero created by the output capacitor’s ESR:
VOUT
1
1
=
2π × RESR × COUT 2π × RC × Cf
IOUT(MAX)
4) Calculate the compensation resistance (RC) to can-
VIN
2.6V TO 5.5V
PWM
L1
CDRH4D18
4.7µH
BATT
C1
10µF
VOUT
1.8V AT 800mA
LX
C2
10µF
SHDN
MAX1928-18
CC
1200pF
COMP
RC
18kΩ
Cf
22pF
FB
POK
REF
PGND
GND
C3
0.1µF
Figure 3. Applications Circuit for the MAX1928
VIN
2.6V TO 5.5V
PWM
BATT
C1
10µF
VOUT
1V AT 800mA
LX
SHDN
MAX1927R
COMP
CC
680pF
L1
CDRH4D18
4.7µH
RC
15kΩ
Cf
22pF
FB
R1
16.5kΩ
1%
C2
10µF
POK
REF
PGND
GND
R2
49.9kΩ
1%
C3
0.1µF
Figure 4. Applications Circuit for the MAX1927
10
______________________________________________________________________________________
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
R
× COUT
Cf = ESR
RC
or 22pF, whichever is greater.
Standard Application Circuits
Figures 3 and 4 are standard applications circuits for
the MAX1927/MAX1928. Figure 3 illustrates the preset
output voltages (MAX1928), while Figure 4 shows the
adjustable configuration (MAX1927). Table 2 lists part
numbers and suppliers for the components used in
these circuits.
PC Board Layout and Routing
ground pins at a single common node in a star ground
configuration. The external voltage feedback network
should be very close to the FB pin, within 0.2in (5mm).
Keep noisy traces, such as those from the LX pin, away
from the voltage feedback network. Position the bypass
capacitors as close as possible to their respective pins
to minimize noise coupling. For optimum performance,
place input and output capacitors as close to the
device as possible. Connect GND and PGND to the
highest quality system ground. The MAX1928 evaluation kit illustrates an example PC board layout and routing scheme.
Chip Information
TRANSISTORS: 3282
PROCESS: BiCMOS
High switching frequencies and large peak currents
make PC board layout a very important part of design.
Good design minimizes EMI, noise on the feedback
paths, and voltage gradients in the ground plane, all of
which can result in instability or regulation errors.
Connect the inductor, input filter capacitor, and output
filter capacitor as close together as possible and keep
their traces short, direct, and wide. Connect their
Table 2. Suggested Parts/Suppliers
PART
PART NUMBER
MANUFACTURER
PHONE
CDRH3D16-4R7
Sumida
USA 847-956-0666
Japan 81-3-3607-5111
www.sumida.com
Input/Output Capacitors
JMK212BJ106MG
Taiyo Yuden
408-573-4150
www.t-yuden.com
COMP Capacitor
GRM1881X1H561J
Murata
770-436-1300
www.murata.com
EMK107BJ104KA
Taiyo Yuden
408-573-4150
www.t-yuden.com
Inductor
REF Capacitor
WEBSITE
______________________________________________________________________________________
11
MAX1927/MAX1928
Solving for Cf gives:
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)
e
10LUMAX.EPS
MAX1927/MAX1928
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
4X S
10
INCHES
10
H
ÿ 0.50±0.1
0.6±0.1
1
1
0.6±0.1
BOTTOM VIEW
TOP VIEW
D2
MILLIMETERS
MAX
DIM MIN
0.043
A
0.006
A1
0.002
A2
0.030
0.037
0.120
D1
0.116
0.118
0.114
D2
0.116
0.120
E1
E2
0.114
0.118
H
0.187
0.199
L
0.0157 0.0275
L1
0.037 REF
b
0.007
0.0106
e
0.0197 BSC
c
0.0035 0.0078
0.0196 REF
S
α
0∞
6∞
MAX
MIN
1.10
0.15
0.05
0.75
0.95
3.05
2.95
3.00
2.89
3.05
2.95
2.89
3.00
4.75
5.05
0.40
0.70
0.940 REF
0.177
0.270
0.500 BSC
0.090
0.200
0.498 REF
0∞
6∞
E2
GAGE PLANE
A2
c
A
b
D1
A1
α
E1
L
L1
FRONT VIEW
SIDE VIEW
PROPRIETARY INFORMATION
TITLE:
PACKAGE OUTLINE, 10L uMAX/uSOP
APPROVAL
DOCUMENT CONTROL NO.
21-0061
REV.
I
1
1
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2002 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.