MAX1927REUB+T [MAXIM]
Switching Regulator, Current-mode, 0.8A, 1200kHz Switching Freq-Max, BICMOS, PDSO10, MICRO MAX PACKAGE-10;
| 型号： | MAX1927REUB+T |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | Switching Regulator, Current-mode, 0.8A, 1200kHz Switching Freq-Max, BICMOS, PDSO10, MICRO MAX PACKAGE-10 信息通信管理 开关 光电二极管 |
| 文件： | 总12页 (文件大小：303K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-2527; Rev 0; 7/02
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
General Description
Features
The MAX1927/MAX1928 800mA step-down converters
power low-voltage microprocessors in compact equip-
ment 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.
o 800mA Output Current
o Output Voltages from 0.75V to 5V
o 2.6V to 5.5V Input Voltage Range
o Power-OK Output
o No Schottky Diode Required
o Selectable Forced PWM Operation
o 1MHz Fixed-Frequency PWM Operation
o 140µA Quiescent Current
o Soft-Start
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.
o 10-Pin µMAX Package
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.
Ordering Information
PRESET
OUTPUT
VOLTAGE
TEMP
RANGE
PIN-
PACKAGE
PART
MAX1927REUB
MAX1928EUB15
MAX1928EUB18
MAX1928EUB25
Adj. to 0.75V -40°C to +85°C 10 µMAX
1.5V
1.8V
2.5V
-40°C to +85°C 10 µMAX
-40°C to +85°C 10 µMAX
-40°C to +85°C 10 µMAX
Applications
WCDMA Handsets
PDAs and Palmtops
DSP Core Power
Battery-Powered Equipment
Pin Configuration
Typical Operating Circuit
V
IN
2.6V TO 5.5V
PWM
BATT
TOP VIEW
V
OUT
L1
C1
0.75V AT 800mA
LX
FB
PWM
GND
REF
1
2
3
4
5
10 POK
SHDN
COMP
C2
9
8
7
6
BATT
LX
R
C
MAX1927R
MAX1927R
MAX1928
POK
C
C
f
C
FB
PGND
SHDN
COMP
PGND
REF
µMAX
GND
________________________________________________________________ Maxim Integrated Products
1
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.
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 (V
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
+ 0.3V)
BATT
Continuous Power Dissipation (T = +70°C)
A
10-Pin µMAX (derate 5.6mW/°C above +70°C)...........444mW
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
(V
BATT
= 3.6V, SHDN = BATT, C
= 0.1µF, T = 0°C to +85°C, unless otherwise noted. Typical values are at T = +25°C.)
REF A A
PARAMETER
CONDITIONS
MIN
2.6
TYP
MAX
5.5
UNITS
V
BATT Input Voltage
2.55
Undervoltage Lockout Threshold
V
rising or falling (35mV hysteresis)
2.15
2.35
140
2
V
BATT
No load, pulse skipping, PWM = GND
1MHz switching
240
µA
mA
µA
µA
Quiescent Current
Quiescent Current in Dropout
Shutdown Supply Current
190
0.1
340
10
SHDN = GND
REFERENCE AND ERROR AMP
MAX1927R
MAX1928-15
MAX1928-18
MAX1928-25
MAX1928
0.738
1.477
1.773
2.462
5
0.75
1.5
1.8
2.5
10
0.762
1.523
1.827
2.538
15
FB Voltage Accuracy
FB Input Current
V
µA
nA
MAX1927R
MAX1927R
MAX1928-15
MAX1928-18
MAX1928-25
10
150
250
210
175
125
1.25
µS
Transconductance (g )
m
Reference Voltage Accuracy
Reference Supply Rejection
PWM CONTROLLER
1.231
1.269
2
V
2.6V < V
< 5.5V
0.5
mV
BATT
V
V
V
V
= 3.6V
= 2.6V
= 3.6V
= 2.6V
0.25
0.3
0.4
0.5
BATT
BATT
BATT
BATT
P-Channel On-Resistance
Ω
Ω
0.17
0.2
0.3
N-Channel On-Resistance
0.35
Current-Sense Transresistance (R
)
0.48
1.3
V/A
A
CS
P-Channel Current-Limit Threshold
1.1
1.6
P-Channel Pulse-Skipping Current Threshold
N-Channel Negative Current-Limit Threshold
0.11
0.13
-0.55
0.15
A
A
2
_______________________________________________________________________________________
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
ELECTRICAL CHARACTERISTICS (continued)
(V
BATT
= 3.6V, SHDN = BATT, C
= 0.1µF, T = 0°C to +85°C, unless otherwise noted. Typical values are at T = +25°C.)
REF A A
PARAMETER
CONDITIONS
MIN
TYP
20
MAX
UNITS
N-Channel Synchronous Rectifier Turn-Off
Threshold
mA
LX Leakage Current
Maximum Duty Cycle
V
= 5.5V, LX = GND or BATT
-20
0.1
+20
0
µA
%
BATT
100
PWM = GND
PWM = BATT
Minimum Duty Cycle
%
15
1
Internal Oscillator Frequency
Thermal Shutdown Threshold
POK COMPARATOR
0.85
1
1.15
MHz
15°C hysteresis
160
Degrees
BATT Operating Voltage Range
Output Low Voltage
I
= 0.1 mA
5.5
0.1
V
V
POK
V
V
= 0.5V, I
= 1mA
0.01
FB
POK
Output High Leakage Current
= 5.5V
1
µA
POK
MAX1927R
0.650
1.305
1.566
2.175
0.675
1.350
1.620
2.250
0.700
1.395
1.674
2.325
MAX1928-15
MAX1928-18
MAX1928-25
POK Threshold
V
POK transitions to high impedance 20ms
after V > V
Output Valid to POK Release Delay
15
20
25
ms
FB
POK
LOGIC INPUTS (SHDN, PWM)
Logic Input High
2.6V < V
2.6V < V
< 5.5 V
< 5.5 V
1.6
V
V
BATT
BATT
Logic Input Low
0.6
1
Logic Input Current
V
= 5.5V
0.1
µA
BATT
ELECTRICAL CHARACTERISTICS
(V
BATT
= 3.6V, SHDN = BATT, C
= 0.1µF, T = -40°C to +85°C, unless otherwise noted.)
REF
A
PARAMETER
CONDITIONS
MIN
2.6
MAX
5.5
UNITS
V
BATT Input Voltage
2.55
Undervoltage Lockout Threshold
Quiescent Current
V
rising or falling (35mV hysteresis)
2.15
V
BATT
No load, pulse skipping, PWM = GND
240
340
10
µA
µA
µA
Quiescent Current in Dropout
Shutdown Supply Current
REFERENCE AND ERROR AMP
SHDN = GND
MAX1927R
MAX1928-15
MAX1928-18
MAX1928-25
MAX1928
0.732
1.47
1.764
2.45
5
0.768
1.53
1.836
2.55
15
FB Voltage Accuracy
FB Input Current
V
µA
_______________________________________________________________________________________
3
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
ELECTRICAL CHARACTERISTICS (continued)
(V
BATT
= 3.6V, SHDN = BATT, C
= 0.1µF, T = -40°C to +85°C, unless otherwise noted. Typical values are at T = +25°C.)
REF A A
PARAMETER
CONDITIONS
MIN
MAX
150
1.269
2
UNITS
nA
FB Input Current
MAX1927R
Reference Voltage Accuracy
Reference-Supply Rejection
PWM CONTROLLER
1.22
V
2.6V < V
< 5.5V
mV
BATT
V
V
V
V
= 3.6V
= 2.6V
= 3.6V
= 2.6V
0.4
0.5
BATT
BATT
BATT
BATT
P-Channel On-Resistance
N-Channel On-Resistance
Ω
Ω
0.30
0.35
1.6
P-Channel Current-Limit Threshold
P-Channel Pulse-Skipping Current Threshold
LX Leakage Current
1.1
0.10
-20
0.10
A
A
0.16
+20
V
= 5.5V, LX = GND or BATT
µA
%
BATT
Maximum Duty Cycle
100
Minimum Duty Cycle
PWM = GND
0
%
Internal Oscillator Frequency
POK COMPARATOR
0.8
1
1.2
MHz
BATT Operating Voltage Range
I
= 0.1 mA
5.5
0.1
V
V
POK
Output Low Voltage
V
V
= 0.5V, I
= 1mA
FB
POK
Output High Leakage Current
= 5.5V
1
µA
POK
MAX1927R
MAX1928-15
0.650
1.305
0.700
1.395
POK Threshold
MAX1928-18
MAX1928-25
1.566
2.175
1.674
2.325
V
POK transitions to high impedance 20ms
after V > V
Output Valid to POK Release Delay
15
25
ms
FB
POK
LOGIC INPUTS (SHDN, PWM)
Logic Input High
2.6V < V
2.6V < V
< 5.5 V
1.6
V
V
BATT
BATT
Logic Input Low
< 5.5 V
0.6
1
Logic Input Current
V
= 5.5V
µA
BATT
4
_______________________________________________________________________________________
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
Typical Operating Characteristics
(Circuits of Figure 3 and 4, T = +25°C, unless otherwise noted.)
A
MAX1928-25
EFFICIENCY vs. LOAD CURRENT
MAX1928-18
EFFICIENCY vs. LOAD CURRENT
MAX1927R
EFFICIENCY vs. LOAD CURRENT
100
90
80
70
60
50
40
100
90
80
70
60
50
40
30
100
90
80
70
60
50
40
V
= 2.7V
V
= 3.6V
IN
IN
V
= 3.6V
IN
V
= 3.6V
IN
V
= 5V
V
= 5V
IN
IN
V
= 5V
IN
V
= 3.3V
OUT
V
= 1.8V
OUT
1
10
100
1000
1
10
100
1000
1
10
100
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
MAX1927R
EFFICIENCY vs. LOAD CURRENT
MAX1928-25
DROPOUT VOLTAGE vs. LOAD CURRENT
MAX1928-15
EFFICIENCY vs. LOAD CURRENT
100
90
80
70
60
50
40
30
100
90
80
70
60
50
40
30
500
450
400
350
300
250
200
150
100
50
V
= 2.7V
IN
V
= 2.7V
IN
V
= 5V
IN
V
= 5V
IN
V
= 3.6V
V
= 3.6V
IN
IN
V
= 1V
V
= 1.5V
OUT
OUT
V
= 2.5V
IN
0
1
10
100
1000
1
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 (A)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
MAX1928-18
OUTPUT VOLTAGE vs. LOAD CURRENT
OSCILLATOR FREQUENCY
vs. INPUT VOLTAGE
NO-LOAD INPUT CURRENT
vs. INPUT VOLTAGE
1.90
1.88
1.86
1.84
1.82
1.80
1.78
1.76
1.74
1.72
1.70
1.06
1.04
1.02
1.00
0.98
0.96
0.94
400
350
300
250
200
150
100
50
T
= +85°C
A
T
= +25°C
A
T
= -40°C
A
V
= 3.6V
IN
0
0
100 200 300 400 500 600 700 800 900 1000
LOAD CURRENT (mA)
2.6
3.1
3.6
4.1
4.6
5.1
5.6
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)
INPUT VOLTAGE (V)
_______________________________________________________________________________________
5
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
Typical Operating Characteristics (continued)
(Circuits of Figure 3 and 4, T = +25°C, unless otherwise noted.)
A
MAXIMUM LOAD CURRENT
POK WAVEFORM
vs. INPUT VOLTAGE
STARTUP WAVEFORM
MAX1927 toc11
MAX1927 toc12
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
V
= 1V
OUT
5V/div
SHDN
POK
SHDN
5V/div
V
= 1.8V
V
= 2.5V
OUT
OUT
1V/div
V
2V/div
2V/div
OUT
I
200mA/div
V
IN
OUT
2.6
3.1
3.6
4.1
4.6
5.1
5.6
20ms/div
1ms/div
INPUT VOLTAGE (V)
HEAVY-LOAD SWITCHING WAVEFORMS
LIGHT-LOAD SWITCHING WAVEFORMS
MAX1927 toc13
MAX1927 toc14
V
V
OUT
(AC-COUPLED)
OUT
10mV/div
10mV/div
(AC-COUPLED)
I
200mA/div
5V/div
L
LX
5V/div
I
L
200mA/div
LX
400ns/div
2ms/div
LINE TRANSIENT
LOAD TRANSIENT
MAX1927 toc16
MAX1927 toc15
V
OUT
100mV/div
900mA
(AC-COUPLED)
V
OUT
10mV/div
(AC-COUPLED)
4.2V
3V
V
IN
500mA/div
250mA
I
2V/div
LOAD
1ms/div
100µs/div
6
_______________________________________________________________________________________
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
Pin Description
PIN
NAME
FUNCTION
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.
1
PWM
2
3
GND
REF
Ground
Internal 1.25V Reference. Bypass to GND with a 0.1µF capacitor.
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 resistor-
divider between the output and GND. FB regulation voltage is 0.75V.
4
FB
Compensation Input. See the Compensation, Stability, and Output Capacitor section for compensation
component selection.
5
COMP
6
7
8
9
SHDN Shutdown Control Input. Drive low to shut down the converter. Drive high for normal operation.
PGND Power Ground
LX
Inductor Connection to the drains of the internal power MOSFETs.
BATT
Supply Voltage Input. Connect to a 2.6V to 5.5V source. Bypass to GND with a low-ESR 10µF capacitor.
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.
10
POK
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
POK
FB
REF
1.25V
REFERENCE
POWER-OK
CONTROL
MAX1927R
ONLY
MAX1928
ONLY
GND
Figure 1. Simplified Functional Diagram
_______________________________________________________________________________________
7
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
the use of a small, low-valued, output filter capacitor.
Detailed Description
The resulting load regulation is 0.3% (typ) from 0 to
The MAX1927/MAX1928 PWM step-down DC-DC con-
800mA.
verters 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 selec-
table forced PWM operation for minimum noise at all
load currents.
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 sam-
pling frequencies. A minimum load is not required dur-
ing 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 contin-
uous switching.
PFM Operation and PWM Control Scheme
The PFM mode improves efficiency and reduces quies-
cent 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.
100% Duty-Cycle Operation
The maximum on-time can exceed one internal oscilla-
tor 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 continu-
ously. Dropout voltage at 100% duty cycle is the output
current multiplied by the sum of the internal PMOS on-
resistance (typically 0.25Ω) and the inductor resis-
tance. Near dropout, switching cycles can be skipped,
reducing switching frequency. However, voltage ripple
remains small because the current ripple is still low.
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 volt-
age 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.
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 volt-
age 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 regula-
tion (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.
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.
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 imped-
ance. Drive SHDN high for normal operation. Input cur-
rent falls to 0.1µA (typ) during shutdown mode.
During PWM operation, the MAX1927/MAX1928 regu-
late 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 volt-
age. The MAX1927/MAX1928 have relatively low AC-
loop gain coupled with a high-gain integrator to enable
POK Output
POK is an open-drain output that goes high impedance
20ms after the soft-start ramp has concluded and V
FB
is within 90% of the threshold. POK is low impedance
when in shutdown.
8
_______________________________________________________________________________________
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
Table 1. FB Regulation Voltages
PART
PRESET OUTPUT VOLTAGE
MAX1927R
0.75V, Adjustable
LX
FB
MAX1928-15
MAX1928-18
MAX1928-25
1.5 V
1.8 V
2.5 V
R1
MAX1927R
R2
50kΩ
following equation to calculate the maximum RMS input
current:
I
OUT
I
=
× V
× V − V
(
)
RMS
OUT
IN
OUT
V
Figure 2. Setting the Adjustable Output Voltage
IN
Compensation, Stability, and
Output Capacitor
Applications Information
The MAX1927/MAX1928 are externally compensated
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 volt-
ages and their corresponding part numbers.
with a resistor and a capacitor (see Figure 3, R and
C
C ) in series from COMP to GND. An additional capaci-
C
tor (C ) may be required from COMP to GND if high-
f
ESR output capacitors are used. The capacitor inte-
grates 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 phase-
shifted capacitor ripple does not disturb the current
regulation loop. The resistor sets the proportional gain
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:
of the output error voltage by a factor g ✕ R .
m
C
Increasing this resistor also increases the sensitivity of
the control loop to output ripple.
V
V
OUT
R1=R2×
−1
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 fre-
quency 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:
FB
where V is 0.75V.
FB
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 alu-
minum electrolytic capacitors are acceptable and rela-
tively 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.
1) Select an appropriate converter bandwidth (f ) to
C
stabilize the system while maximizing transient
response. This bandwidth should not exceed 1/10
of the switching frequency.
2) Calculate the compensation capacitor, C , based
C
on this bandwidth:
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
For the MAX1927:
V
1
R2
R1+R2
1
2πf
C
OUT
C
=
×
× g
×
×
C
m
I
R
OUT(MAX)
CS
_______________________________________________________________________________________
9
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
2πf
C
OUT
C
=
×
× g
×
(
)
C
m
1
1
I
R
=
OUT(MAX)
CS
2π ×RL ×COUT 2π ×RC ×CC
Resistors R1 and R2 are external to the MAX1927 (see
the Setting the Output Voltage section). I is the
Solving for R gives:
OUT(MAX)
= 0.48V/A, and g
C
maximum output current, R
=
m
CS
250µS for the MAX1927. See the Electrical Characteristics
table for MAX1928 g values. Select the closest standard
C
R ×C
L
OUT
R
=
m
C
C
C
C value that gives an acceptable bandwidth.
3) Calculate the equivalent load impedance, R , by:
5) Calculate the high-frequency compensation pole to
L
cancel the zero created by the output capacitor’s ESR:
V
OUT
OUT(MAX)
R =
L
1
1
I
=
2π ×R
×C
2π ×R ×C
C f
ESR
OUT
4) Calculate the compensation resistance (R ) to can-
C
V
IN
PWM
2.6V TO 5.5V
L1
BATT
CDRH4D18
4.7µH
V
C1
OUT
1.8V AT 800mA
10µF
LX
FB
C2
10µF
SHDN
MAX1928-18
GND
COMP
REF
POK
C
C
R
C
C
f
1200pF
18kΩ
22pF
PGND
C3
0.1µF
Figure 3. Applications Circuit for the MAX1928
V
IN
PWM
2.6V TO 5.5V
L1
BATT
V
CDRH4D18
4.7µH
OUT
1V AT 800mA
C1
10µF
LX
FB
C2
10µF
SHDN
COMP
REF
R1
16.5kΩ
1%
MAX1927R
GND
POK
PGND
C
C
R
C
C
f
R2
49.9kΩ
1%
680pF
15kΩ
22pF
C3
0.1µF
Figure 4. Applications Circuit for the MAX1927
10 ______________________________________________________________________________________
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
Solving for C gives:
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 evalua-
tion kit illustrates an example PC board layout and rout-
ing scheme.
f
R
×C
OUT
ESR
C =
f
R
C
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.
Chip Information
TRANSISTORS: 3282
PC Board Layout and Routing
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
WEBSITE
USA 847-956-0666
Japan 81-3-3607-5111
Inductor
CDRH3D16-4R7
Sumida
www.sumida.com
Input/Output Capacitors
COMP Capacitor
REF Capacitor
JMK212BJ106MG
GRM1881X1H561J
EMK107BJ104KA
Taiyo Yuden
Murata
408-573-4150
770-436-1300
408-573-4150
www.t-yuden.com
www.murata.com
www.t-yuden.com
Taiyo Yuden
______________________________________________________________________________________ 11
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
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
4X S
10
10
INCHES
MAX
MILLIMETERS
MAX
1.10
0.15
0.95
3.05
3.00
3.05
3.00
5.05
0.70
DIM MIN
MIN
-
A
-
0.043
0.006
0.037
0.120
0.118
0.120
0.118
0.199
A1
A2
D1
D2
E1
E2
H
0.002
0.030
0.116
0.114
0.116
0.114
0.187
0.05
0.75
2.95
2.89
2.95
2.89
4.75
0.40
H
ÿ 0.50±0.1
0.6±0.1
L
0.0157 0.0275
0.037 REF
L1
b
0.940 REF
0.007
0.0106
0.177
0.270
0.200
1
1
e
0.0197 BSC
0.500 BSC
0.6±0.1
c
0.0035 0.0078
0.0196 REF
0.090
BOTTOM VIEW
0.498 REF
S
α
TOP VIEW
0∞
6∞
0∞
6∞
D2
E2
GAGE PLANE
A2
c
A
E1
b
L
α
A1
D1
L1
FRONT VIEW
SIDE VIEW
PROPRIETARY INFORMATION
TITLE:
PACKAGE OUTLINE, 10L uMAX/uSOP
APPROVAL
DOCUMENT CONTROL NO.
REV.
1
21-0061
I
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.
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