MAX1974EUB-T [MAXIM]
Switching Regulator, Current-mode, 2.4A, 1600kHz Switching Freq-Max, BICMOS, PDSO10, MICRO, SOP-10;
| 型号： | MAX1974EUB-T |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | Switching Regulator, Current-mode, 2.4A, 1600kHz Switching Freq-Max, BICMOS, PDSO10, MICRO, SOP-10 稳压器 |
| 文件： | 总15页 (文件大小：331K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-2547; Rev 0; 7/02
Smallest 1A, 1.4MHz Step-Down Regulators
General Description
Features
o Tiny Circuit Footprint of 0.19in2
o Ultra-Low Circuit Height of 1.8mm
The MAX1973/MAX1974 are constant-frequency
1.4MHz pulse-width-modulated (PWM) current-mode
step-down regulators. The output voltage can be set as
low as 0.75V using an external voltage-divider, or it can
be set to preset outputs of 1V, 1.5V (MAX1974), 1.8V,
or 2.5V (MAX1973) without requiring external resistors.
The MAX1973 also includes a voltage-margining fea-
ture that offsets the output voltage up or down 4% to
facilitate board-level production testing.
o 4.7µF Ceramic Input and Output Capacitors
o 2.6V to 5.5V Input Voltage
o 1A Output Current
o 1% Accurate
A fixed 1.4MHz operating frequency ensures operation
outside the DSL frequency band, provides fast transient
response, and allows the use of small external compo-
nents. Only 4.7µF input and output ceramic capacitors
are needed for 1A applications. Forced PWM operation
ensures a constant switching frequency over all load
conditions.
o Built-In 4% Logic-Controlled Voltage ꢀargining
(ꢀAX1973)
o Preset 1V, 1.5V, 1.8V, 2.5V, or 0.75V to V
IN
Adjustable Output
o Fixed-Frequency PWꢀ Current-ꢀode Operation
o 1.4ꢀHz Switching Frequency, Operate Outside
Output voltage accuracy is 1% over load, line, and
temperature operating ranges. The MAX1973 features
voltage margining; the MAX1974 provides a POK out-
put to indicate when the output has reached 90% of its
nominal regulation voltage. Both devices are available
in small 10-pin µMAX packages.
DSL Band
o 100% Duty-Cycle Dropout Capability
o Small External Components
Applications
Ordering Information
Network Equipment
PART
TEꢀP RANGE
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
10 µMAX
Cellular Base Stations
ꢀAX1973EUB
ꢀAX1974EUB
DSL and Wireless Modems/Routers
Optical Modules
10 µMAX
Central-Office DSL and Telecom
DSP/ASIC Core and IO supplies
Pin Configurations
TOP VIEW
Selector Guide appears at end of data sheet.
FBSEL
COMP
FB
1
2
3
4
5
10 CTL1
Typical Operating Circuit
9
8
7
6
IN
MAX1973
LX
SS
PGND
CTL2
OUTPUT
INPUT
2.6V TO 5.5V
1.25V TO V
1A
IN
GND
IN
LX
FB
µꢀAX
COMP
FBSEL
COMP
FB
1
2
3
4
5
10 ON
FBSEL MAX1973
CTL1
CTL2
VOLTAGE
MARGINING
ON/OFF
9
8
7
6
IN
SS
MAX1974
LX
GND
PGND
SS
PGND
POK
GND
MAX1974 OUTPUT DOWN TO 0.75V
µꢀAX
________________________________________________________________ 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.
Smallest 1A, 1.4MHz Step-Down Regulators
ABSOLUTE ꢀAXIꢀUꢀ RATINGS
IN, POK, CTL1, CTL2, FBSEL, ON to GND ..............-0.3V to +6V
Continuous Power Dissipation (T = +70°C)
A
COMP, FB, SS to GND ................................-0.3V to (V + 0.3V)
PGND to GND .......................................................-0.3V to +0.3V
LX Current (Note 1)...............................................-2.4A to +2.4A
10-Pin µMAX (derate 5.6mW/°C above +70°C).......... 444mW
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
IN
Note 1: LX has internal clamp diodes to IN and PGND. Applications that forward bias these diodes should take care not to exceed
the IC package power dissipation limit.
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 = V
IN
= 3.3V, FB = OUT, T = 0°C to +85°C, unless otherwise noted. Typical values are at T = +25°C.)
CTL_
A
A
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
IN
IN Voltage Range
2.6
2.2
5.5
2.5
V
V
IN Undervoltage Lockout
Threshold
Rising and falling, hysteresis is 25mV (typ)
Switching with no load
2.35
Supply Current
4.0
3
7.5
5
mA
mA
µA
Supply Current in Dropout
Shutdown Supply Current
FB
V
V
set for 3.6V
OUT
= 5.5V
0.1
10
IN
MAX1973
MAX1974
1.25
0.75
V
V
IN
IN
Output Voltage Range
V
V
FBSEL not connected
FBSEL = GND
1.2375
1.7820
2.4750
0.7425
0.99
1.25
1.8
1.2625
1.8180
2.5250
0.7575
1.01
MAX1973
MAX1974
FBSEL = IN
2.5
FB Regulation Voltage
FBSEL not connected
FBSEL = GND
0.75
1.00
1.500
FBSEL = IN
1.485
1.515
FB Regulation Voltage
Positive Voltage Margining
MAX1973, CTL1 = GND, CTL2 = IN
MAX1973, CTL1 = IN, CTL2 = GND
+3
-3
+4
-4
+5
-5
%
%
FB Regulation Voltage
Negative Voltage Margining
FB Input Resistance to GND in
Preset Output Modes
10
30
70
kΩ
FB Input Bias Current
FBSEL not connected
-0.1
0.01
+0.1
µA
SS (REFERENCE OUTPUT)
MAX1974
MAX1973
0.75
1.25
SS Voltage
V
2
_______________________________________________________________________________________
Smallest 1A, 1.4MHz Step-Down Regulators
ELECTRICAL CHARACTERISTICS (continued)
(V = V
= 3.3V, FB = OUT, T = 0°C to +85°C, unless otherwise noted. Typical values are at T = +25°C.)
IN
CTL_
A
A
PARAMETER
CONDITIONS
MIN
-25
10
TYP
-20
20
MAX
-15
UNITS
µA
SS Source Current
SS Sink Current
35
µA
SS to GND Resistance in
Shutdown
5
40
100
Ω
FBSEL
Low Input Threshold
0.3
V
V
V
0.3
-
IN
High Input Threshold
Input Bias Current
FBSEL = GND or IN, V = 5.5V
-20
10
+20
µA
IN
COMP
Transconductance from FB to
COMP
40
5
60
40
80
µS
COMP to GND Resistance in
Shutdown
100
Ω
Clamp Voltage Low
Clamp Voltage High
LX
0.6
0.9
1.2
V
V
1.35
1.75
2.15
On-Resistance High
On-Resistance Low
Current-Sense Transresistance
Positive Current-Limit Threshold
V
V
= 3.3V
= 3.3V
0.23
0.16
0.335
1.6
0.46
0.32
0.425
1.75
-0.4
Ω
Ω
IN
IN
0.275
1.1
V/A
A
Negative Current-Limit Threshold
LX Shutdown Leakage Current
Switching Frequency
-1.2
-0.8
A
V
= V = 5.5V
20
LX
IN
µA
LX = GND, V = 5.5V
IN
-20
1.2
1.4
1.6
MHz
CTL1, CTL2 (MAX1973), ON (MAX1974)
Logic-Low Input Threshold
Logic-High Input Threshold
Logic Input Current
0.6
-1
V
V
1.6
+1
µA
POK (MAX1974 only)
Output Low Voltage
POK sinking 1mA
10
92.5
90
100
95
mV
%
Rising
Falling
90
88
Percentage of nominal
regulation voltage
Output Valid Threshold for POK
92
_______________________________________________________________________________________
3
Smallest 1A, 1.4MHz Step-Down Regulators
ELECTRICAL CHARACTERISTICS (continued)
(V = V
= 3.3V, FB = OUT, T = 0°C to +85°C, unless otherwise noted. Typical values are at T = +25°C.)
IN
CTL_
A
A
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
THERMAL SHUTDOWN
Thermal-Shutdown Threshold
Thermal-Shutdown Hysteresis
+170
20
°C
°C
ELECTRICAL CHARACTERISTICS
(V = V = V
= 3.3V, T = -40°C to +85°C, unless otherwise noted.) (Note 2)
A
IN
FB
CTL_
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
IN
IN Voltage Range
2.6
2.2
5.5
2.5
V
V
IN Undervoltage Lockout
Threshold
Rising and falling, hysteresis is 25mV (typ)
Switching with no load
Supply Current
7.5
5
mA
mA
µA
Supply Current in Dropout
Shutdown Supply Current
FB
V
V
set for 3.6V
OUT
= 5.5V
10
IN
MAX1973
MAX1974
1.25
0.75
V
V
IN
IN
Output Voltage Range
V
V
FBSEL not connected
FBSEL = GND
1.2375
1.7820
2.4750
0.7425
0.99
1.2625
1.8180
2.5250
0.7575
1.01
MAX1973
MAX1974
FBSEL = IN
FB Regulation Voltage
FBSEL not connected
FBSEL = GND
FBSEL = IN
1.485
1.515
FB Regulation Voltage
Positive Voltage Margining
MAX1973, CTL1 = GND, CTL2 = IN
MAX1973, CTL1 = IN, CTL2 = GND
3
5
%
%
FB Regulation Voltage
Negative Voltage Margining
-3
-5
FB Input Resistance to GND in
Preset Output Modes
10
70
kΩ
FB Input Bias Current
SS (REFERENCE OUTPUT)
SS Source Current
FBSEL not connected
-0.15
+0.15
µA
-25
10
-15
35
µA
µA
SS Sink Current
4
_______________________________________________________________________________________
Smallest 1A, 1.4MHz Step-Down Regulators
ELECTRICAL CHARACTERISTICS
(V = V
= 3.3V, FB = OUT, T = -40°C to +85°C, unless otherwise noted.) (Note 2)
A
IN
CTL_
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
SS to GND Resistance
in Shutdown
5
40
100
Ω
FBSEL
Low Input Threshold
0.3
-20
V
V
V
0.4
-
IN
High Input Threshold
Input Bias Current
FBSEL = GND or IN, V = 5.5V
+20
µA
IN
COMP
Transconductance
from FB to COMP
40
5
80
µS
COMP to GND Resistance
in Shutdown
100
Ω
Clamp Voltage Low
0.6
1.3
1.2
2.2
V
V
Clamp Voltage High
LX
On-Resistance High
V
V
= 3.3V
= 3.3V
0.46
0.32
0.425
1.85
-0.35
20
Ω
Ω
IN
IN
On-Resistance Low
Current-Sense Transresistance
Positive Current-Limit Threshold
Negative Current-Limit Threshold
0.275
1.10
V/A
A
-1.20
A
V
= V = 5.5V
IN
LX
LX Shutdown Leakage Current
µA
LX = GND, V = 5.5V
IN
-20
1.2
Switching Frequency
1.6
MHz
CTL1, CTL2 (MAX1973), ON (MAX1974)
Logic-Low Input Threshold
Logic-High Input Threshold
Logic Input Current
0.6
-1
V
V
1.6
1
µA
POK (MAX1974 only)
Output Low Voltage
POK sinking 1mA
100
95
mV
%
Rising
Falling
90
88
Output Valid Threshold
for POK
Percentage of nominal
regulation voltage
92
Note 2: Specifications to -40°C are guaranteed by design and not production tested.
_______________________________________________________________________________________
5
Smallest 1A, 1.4MHz Step-Down Regulators
Typical Operating Characteristics
(Circuits of Figure 2, 3, and 4; T = +25°C, unless otherwise noted.)
A
EFFICIENCY vs. LOAD CURRENT
EFFICIENCY vs. LOAD CURRENT
DROPOUT VOLTAGE vs. LOAD CURRENT
100
100
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
V
= 2.5V
V
= 3.3V
OUT
OUT
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
V
= 2.5V
OUT
V
= 2.5V
OUT
V
OUT
= 1V
V
= 1.8V
OUT
V
= 3.3V
OUT
V
= 3.3V
IN
V
= 5V
IN
0.01
0.1
LOAD CURRENT (A)
1
0.01
0.1
LOAD CURRENT (A)
1
0
0.2
0.4
LOAD CURRENT (A)
0.6
0.8
CHANGE IN OUTPUT VOLTAGE
vs. LOAD CURRENT
NO-LOAD CURRENT vs. INPUT VOLTAGE
FB VOLTAGE vs. LOAD CURRENT
1.0
0.5
1.252
1.251
1.250
1.249
1.248
1.247
1.246
1.245
1.244
12
V
= 1.8V
OUT
10
8
0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
2.5V
1V
6
4
MAX1973
V
= 5V
IN
2
R1 = 22kΩ
V
= 5V
0.1
R2 = 13kΩ
IN
V
= 2.5V
1
OUT
0
0.01
1
10
100
1000
0
2
3
4
5
0
0.2
0.4
0.6
0.8
1.0
LOAD CURRENT (A)
INPUT VOLTAGE (V)
LOAD CURRENT (A)
SWITCHING FREQUENCY
vs. SUPPLY VOLTAGE
1.40
1.35
1.30
1.25
1.20
1.15
1.10
T
= +85°C
A
T
= +25°C
= -40°C
A
T
A
2
3
4
5
6
SUPPLY VOLTAGE (V)
6
_______________________________________________________________________________________
Smallest 1A, 1.4MHz Step-Down Regulators
Typical Operating Characteristics (continued)
(Circuits of Figure 2, 3, and 4; T = +25°C, unless otherwise noted.)
A
MAX1974
STARTUP WAVEFORMS
MAX1973
HIGH-CURRENT SWITCHING WAVEFORMS
MAX1973/74 toc08
MAX1973/74 toc09
10V/div
0
2V/div
0
ON
20mV/div
V
OUT
POK
V
LX
5V/div
0
1V/div
V
OUT
0
500mA/div
I
L
200mA/div
0
I
IN
0
500ns/div
V
= 5V, 100kΩ PULLUP RESISTOR POK TO V
IN
OUT
V
= 5V, V
= 2.5V, I
= 800mA
OUT
IN
OUT
MAX1973
LOAD TRANSIENT
MAX1973
LOW-CURRENT SWITCHING WAVEFORMS
MAX1973/74 toc11
MAX1973/74 toc10
V
20mV/div
OUT
20mV/div
V
OUT
V
5V/div
0
LX
500mA/div
I
L
500mA/div
0
I
L
0
500ns/div
500ns/div
V
= 5V, V
= 2.5V, I
= 400mA TO 800mA
V
IN
= 5V, V
= 2.5V, I
= 10mA
OUT
IN
OUT
OUT
OUT
MAX1973
LINE TRANSIENT
MAX1973
LOAD TRANSIENT
MAX1973/74 toc13
MAX1973/74 toc12
50mV/div
V
200mV/div
V
OUT
OUT
V
I
L
IN
2V/div
0
500mA/div
0
200µs/div
= 3.3V TO 5V TO 3.3V, I
20µs/div
V
IN
= 800mA
OUT
V
= 5V, V
= 2.5V, I
= 600mA TO 800mA
IN
OUT
OUT
_______________________________________________________________________________________
7
Smallest 1A, 1.4MHz Step-Down Regulators
Typical Operating Characteristics (continued)
(Circuits of Figure 2, 3, and 4; T = +25°C, unless otherwise noted.)
A
MAX1974
POK AND INPUT VOLTAGE
MAX1973
VOLTAGE MARGIN STEP CHANGE RESPONSE
MAX1973/74 toc15
MAX1973/74 toc14
V
OUT
1V/div
V
POK
2V/div
V
0
CTL1
0
10V/div
0
V
OUT
1V/div
10V/div
0
V
CTL2
V
I
IN
IN
0
500mA/div
2V/div
0
0
20ms/div
1ms/div
= 800mA, -4% TO +4% TO -4%
OUT
MAX1974 WITH 100kΩ PULLUP RESISTOR
V
= 5V, VOUT = 2.5V, I
IN
FROM POK TO IN, I
= 100mA
LOAD
Pin Description
NAME
PIN
FUNCTION
MAX1973
MAX1974
Feedback-Mode Selector. Connect FBSEL to GND to set the output voltage to 1.8V
(MAX1973) or 1V (MAX1974). Connect FBSEL to IN to set the output voltage to 2.5V
(MAX1973) or 1.5V (MAX1974). Leave FBSEL unconnected to set the output voltage
using a resistor-divider at FB.
1
FBSEL
FBSEL
Compensation. Connect a series RC network to GND. COMP is internally pulled to
GND when the device is in shutdown or in undervoltage lockout (see the Compen-
sation Components section).
2
3
COMP
FB
COMP
FB
Feedback Input. Connect to the output if a preset voltage is used, or to a resistor-
divider from the output to GND for an adjustable output voltage.
Soft-Start Pin and Reference Output. Bypass to GND with at least 0.01µF. Connect
0.1µF to GND for a soft-start ramp time of 6.25ms for the MAX1973, or 3.75ms for the
MAX1974. SS is internally pulled to GND when the device is shut down or in
undervoltage lockout.
4
SS
SS
5
6
GND
CTL2
GND
Ground
—
Control Input 2. Controls enable/disable and voltage margining (see Table 1).
Power-OK Output. Open-drain output goes low when output is below 90% of nominal
output. POK is also low when the device is shut down or in undervoltage lockout.
—
POK
7
8
PGND
LX
PGND
LX
Power Ground
Inductor Connection. Connect an inductor from LX to the output.
Input Supply Voltage. Input voltage range is 2.6V to 5.5V. Connect a 4.7µF capacitor
from IN to PGND.
9
IN
CTL1
—
IN
—
Control Input 1. Controls Enable/Disable and voltage margining (see Table 1).
10
Enable Input. Connect to IN or drive high for normal operation. Drive low to put device
in shutdown.
ON
8
_______________________________________________________________________________________
Smallest 1A, 1.4MHz Step-Down Regulators
the load. Under overload conditions, when the inductor
Detailed Description
current exceeds the current limit, the high-side MOSFET
The MAX1973/MAX1974 are 1.4MHz fixed-frequency
is not turned on at the rising edge of the clock, and the
PWM current-mode step-down DC/DC converters. A
low-side MOSFET remains on to let the inductor current
high 1.4MHz switching frequency allows use of small
ramp down.
inductors and small capacitors for filtering and decou-
pling. An internal synchronous rectifier improves effi-
ciency and eliminates the need for an external Schottky
freewheeling diode. On-chip current sensing uses the
on-resistance of the internal MOSFETs, eliminating cur-
rent-sensing resistors and improving efficiency.
100% Duty-Cycle Operation
The MAX1973/MAX1974 can operate at 100% duty
cycle. In this state, the high-side P-channel MOSFET is
turned on (not switching). The dropout voltage in 100%
duty-cycle operation is the output current multiplied by
the sum of the on-resistance of the P-channel MOSFET
The input voltage range is 2.6V to 5.5V. The output volt-
age is selectable to one of two presets, or adjustable by
using a resistor-divider. The output voltage of the
MAX1973 is preset to 1.8V or 2.5V by connecting FBSEL
to GND or IN, respectively. The MAX1974 is preset to
1.0V or 1.5V by connecting FBSEL to GND or IN, respec-
tively. In adjustable mode (see the Output Voltage
Selection section), the output voltage is programmable
down to 0.75V on the MAX1974, and down to 1.25V on
the MAX1973.
(R
) and the inductor resistance (R ).
L
DS(ON)P
V
= I
✕ ( R + R )
DS(ON)P L
DROPOUT
OUT
Current Sense and Current Limit
The current-sense circuit amplifies the current-sense
voltage generated by the high-side MOSFET’s on-resis-
tance and the inductor current (R
✕ INDUCTOR).
DS(ON)
This amplified current-sense signal and the internal
slope compensation signal are summed together at the
PWM comparator’s inverting input. The PWM compara-
tor turns off the internal high-side MOSFET when this
sum exceeds the integrated feedback voltage.
PWM Control Scheme
The MAX1973/MAX1974 use a fixed-frequency PWM
current-mode control scheme. The heart of the PWM
current-mode controller is an open-loop comparator
that compares the integrated voltage feedback signal
against the sum of the amplified current-sense signal
and the slope compensation ramp (see Figure 1). At
each rising edge of the internal clock, the internal high-
side MOSFET turns on until the PWM comparator trips.
During this on-time, current ramps up through the
inductor, sourcing current to the output and storing
energy in a magnetic field.
The internal high-side MOSFET has a current limit of
1.6A (typ). If the current flowing out of LX exceeds this
maximum, the high-side MOSFET turns off and the syn-
chronous rectifier MOSFET turns on. This lowers the
duty cycle and causes the output voltage to droop until
the current limit is no longer exceeded. There is also a
synchronous rectifier current limit of -0.85A, to protect
the device from current flowing into LX. If this negative
current limit is exceeded, the synchronous rectifier
turns off, and the inductor current continues to flow
through the high-side MOSFET body diode back to the
input until the beginning of the next cycle, or until the
inductor current drops to zero.
The current-mode feedback system regulates the peak
inductor current as a function of the output voltage error
signal. Because the average inductor current is nearly
the same as the peak inductor current (assuming that
the inductor value is relatively high to minimize ripple
current), the circuit acts as a switch-mode transconduc-
tance amplifier. It pushes the output LC filter pole, nor-
mally found in a voltage-mode PWM, to a higher
frequency. To preserve inner loop stability and eliminate
inductor staircasing, an internal slope-compensation
ramp is summed into the main PWM comparator.
Soft-Start
To reduce the supply inrush current, soft-start circuitry
ramps up the output voltage during startup by charging
the SS capacitor with a 20µA current source. When SS
reaches its nominal value, the output is in full regula-
tion. The soft-start time (t ) is determined from:
SS
V
SS
During the second half of the switching cycle (off-time),
the internal high-side MOSFET turns off and the internal
low-side N-channel MOSFET turns on. The inductor
releases the stored energy as its current ramps down
while still providing current to the output. The output
capacitor stores charge when the inductor current
exceeds the load current and discharges when the
inductor current is lower, smoothing the voltage across
t
=
×C
SS
SS
I
SS
where V is the soft-start (reference) voltage (1.25V for
SS
the MAX1973; 0.75V for the MAX1974), I
is 20µA,
SS
and C is the value of the capacitor connected to SS.
SS
Soft-start occurs when power is first applied and when
the device exits shutdown. The part also goes through
_______________________________________________________________________________________
9
Smallest 1A, 1.4MHz Step-Down Regulators
MAX1974
ONLY
VOLTAGE
CLAMP
POK
COMP
OUT
IN
PMOS
CURRENT SENSE
FEEDBACK
SELECT
FB
CTL1
LX
VOLTAGE
MARGINING
ERROR
AMP
PWM
LOGIC BLOCK
MAX1973
ONLY
CTL2
ON
PWM
COMPARATOR
SOFT-START
PGND
Σ
NMOS
CURRENT LIMIT
MAX1974
ONLY
SS
REFERENCE
GND
1.4MHz
OSCILLATOR
SLOPE
COMP
MAX1973
MAX1974
REFERENCE
READY
CHIP
ENABLE
BIAS
IN
UNDERVOLTAGE
LOCK OUT
Figure 1. Functional Diagram
soft-start when coming out of undervoltage lockout
(UVLO) or thermal-overload protection.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipa-
tion and protects the IC from damage in case of an
overload or short-circuit condition. When the IC junction
Undervoltage Lockout (UVLO)
If V drops below 2.35V (typ), the MAX1973/MAX1974
IN
assume that the supply voltage is too low to provide a
valid output voltage, and the UVLO circuit inhibits
switching. Once V rises above 2.4V, UVLO is dis-
IN
abled and the soft-start sequence begins.
temperature (T ) exceeds +170°C, the device shuts
J
down. The part turns on again after the junction temper-
ature cools by 20°C. This results in a pulsed output dur-
ing continuous thermal-overload conditions.
10 ______________________________________________________________________________________
Smallest 1A, 1.4MHz Step-Down Regulators
Voltage Margining and Shutdown
Table 1. CTL_ Input Functions (MAX1973)
A voltage-margining feature is provided on the
CTL1
CTL2
FUNCTION
MAX1973 to shift the output voltage up or down by 4%.
Voltage margining is useful for the automatic testing of
systems at high and low supply conditions to find
potential failures. See Table 1 for the MAX1973 voltage
margining and shutdown truth table.
GND
GND
Shutdown
Positive voltage margining,
regulation voltage increased 4% from
normal operation
GND
IN
A shutdown feature is included on both the MAX1973
and the MAX1974. Shutdown turns off the IC and
reduces the supply current about 0.1µA. For the
MAX1974, drive ON high for normal operation, or low
for shutdown. For the MAX1973, drive both CTL1 and
CTL2 high for normal operation, or drive both low for
shutdown. For a simple enable/shutdown function with
no voltage margining on the MAX1973, connect CTL1
to CTL2 and drive as one input.
Negative voltage margining,
regulation voltage lowered 4% from
normal operation
IN
IN
GND
IN
Normal operation
Table 2. Preset Output Voltages
OUTPUT VOLTAGE
FBSEL
Power-OK Output (POK)
A power-OK output (POK) is provided on the MAX1974.
This is an open-drain output indicating when the output
voltage is in regulation. If the output voltage falls below
90% of its nominal value, POK goes low. POK remains
low until the output voltage rises to 92.5% of its nominal
value. At that point, POK goes high impedance. To use
POK as a logic output, connect a 10kΩ to 100kΩ pullup
resistor from POK to the power supply of the logic
receiving the POK signal. POK continues to function in
shutdown or UVLO. Note that a minimum voltage of 1V
at IN is required to ensure that POK provides a valid
MAX1973
1.8V
MAX1974
1V
GND
IN
2.5V
1.5V
Adjustable down Adjustable down to
to 1.25V 0.75V
Not Connected
For the MAX1973, V = 1.25V, allowing its output to be
FB
set down to 1.25V. For the MAX1974, V
allowing its output to be set down to 0.75V
= 0.75V,
FB
The MAX1973/MAX1974 PWM circuitry is capable of a
stable minimum duty cycle of 17%. This limits the mini-
mum output voltage that can be generated to 0.17 ✕
output. When V drops to zero, POK is high imped-
IN
ance. See the Typical Operating Characteristics.
V . Instability may result for V /V
ratios below 0.17.
IN
IN OUT
Applications Information
Inductor Selection
Output Voltage Selection
The output voltage can be set to one of two preset val-
ues, or can be set by an external resistor-divider. For
preset output voltages, connect FB to the output as
shown in Figures 2 and 3. Connect FBSEL to GND or IN
to select the desired preset output voltage (see Table 2).
A 2.2µH to 4.7µH inductor with a saturation current of at
least 1.25A is recommended for full-load (1mA) applica-
tions. For lower load currents, the inductor current rating
can be reduced. For most applications, use an inductor
with a current rating 1.25 times the maximum required
output current. For best efficiency, the inductor’s DC
resistance should be as small as possible. See Table 3
for recommended inductors and manufacturers.
To set the output voltage to a value other than the preset
values, FBSEL is not connected, and FB is connected to
a voltage-divider as shown in Figures 4 and 5. Select a
value for R2 in the 1kΩ to 22kΩ range, and then calcu-
late the value of R1 from the following equation:
For most designs, the inductor value (L
derived from the following equation:
) can be
INIT
V
OUT
V
V − V
(
)
OUT IN OUT
R1=R2×
−1
L
=
INIT
V
FB
V
× LIR × I
× f
IN
OUT(MAX) SW
______________________________________________________________________________________ 11
Smallest 1A, 1.4MHz Step-Down Regulators
3.3µH
3.3µH
1.25V TO V
V
= 2.6V TO 5.5V
V
= 2.6V TO 5.5V
IN
IN
IN
1.8V
IN
LX
FB
IN
LX
FB
4.7µF
4.7µF
R
36kΩ
470pF
C
4.7µF
4.7µF
R1
R2
COMP
FBSEL
SS
COMP
FBSEL
SS
C
C
MAX1973
MAX1973
CTL1
CTL2
CTL1
CTL2
0.1µF
0.1µF
GND
PGND
GND
PGND
Figure 2. MAX1973 with 1.8V Preset Output
Figure 4. MAX1973 with Adjustable Output Voltage Set by R1
and R2
3.3µH
3.3µH
0.75V TO V
V
= 2.6V TO 5.5V
V
IN
= 2.6V TO 5.5V
IN
1.5V
IN
IN
LX
IN
LX
FB
4.7µF
4.7µF
43kΩ
R
C
4.7µF
4.7µF
R1
R2
COMP
FBSEL
SS
FB
COMP
FBSEL
SS
IN
330pF
IN
C
C
ON
IN
MAX1974
MAX1974
ON
IN
100kΩ
POK
100kΩ
POK
POK
0.1µF
0.1µF
PGND
POK
GND
GND
PGND
Figure 3. MAX1974 with Preset 1.5V Output
Figure 5. MAX1974 with Adjustable Output Voltage Set by R1
and R2
where f
is the switching frequency (1.4✕106 Hz), and
capacitor must meet the ripple current requirement
SW
LIR is the inductor ripple current as a percentage of the
maximum load current. Keep LIR between 20% and
40% for best compromise of cost, size, and perfor-
mance. The peak inductor current is approximately:
(I
) imposed by the switching currents defined by
RMS
the following equation:
I
OUT
I
=
V
(V − V
)
RMS
OUT IN
OUT
V
IN
LIR
2
I
= 1+
×I
OUT(MAX)
L(PEAK)
Choose a capacitor that exhibits less than 10°C tem-
perature rise at the maximum operating RMS current for
optimum long-term reliability.
Input Capacitor
A 4.7µF ceramic input capacitor is recommended for
most applications because of its low equivalent series
resistance (ESR), equivalent series inductance (ESL),
and cost. To ensure stability over a wide temperature
range, an X5R or X7R dielectric is recommended.
Output Capacitor
A 4.7µF ceramic output capacitor is recommended for
most applications because of its low ESR, ESL, and
lower cost. To ensure stability over a wide temperature
range, an X5R or X7R dielectric is recommended.
The input capacitor reduces peak currents drawn from
the power source and reduces noise and voltage ripple
on the input caused by the circuit’s switching. The input
Key selection parameters for a ceramic output capacitor
are capacitance, ESR, and voltage rating. These affect
the overall stability, output ripple voltage, and transient
12 ______________________________________________________________________________________
Smallest 1A, 1.4MHz Step-Down Regulators
Table 3. Recommended Inductors
SATURATION
CURRENT (A)
DIMENSIONS
L ✕ W ✕ H (mm)
MANUFACTURER
PART
INDUCTANCE (µH)
ESR (mΩ)
Coilcraft
Sumida
Toko
LPO1704-32M
CDRD3D16-R3
A682AY-3R3M
3.3
3.3
3.3
160
85
1.3
1.1
5.5 ✕ 6.6 ✕ 1
4 ✕ 4 ✕ 1.8
134
0.97
4.4 ✕ 4.4 ✕ 3.1
response of the DC-DC converter. With ceramic capaci-
tors, the voltage ripple from ESL is negligible.
capacitor between COMP and GND to form a pole-zero
pair. The external inductor, output capacitor, compen-
sation resistor, and compensation capacitor determine
the loop bandwidth and stability. The inductor and out-
put capacitor are chosen based on performance, size,
and cost. Additionally, the compensation resistor and
capacitor are selected to optimize the control loop.
Table 4 and Table 5 list typical component values. The
rest of this section is a more detailed discussion on cal-
culating compensation components.
Output ripple is generated by variations in the charge
stored in the output capacitance, and the voltage drop
across the capacitor ESR.
V
= V
+ V
RIPPLE
RIPPLE(C) RIPPLE(ESR)
The output voltage ripple due to the output capacitance is:
I
P−P
V
=
RIPPLE(C)
The controller uses a current-mode control scheme that
regulates the output voltage by forcing the required
current through the external inductor. The voltage
across the internal high-side MOSFET’s on-resistance
is used to sense inductor current. Current-mode control
eliminates the double pole caused by the inductor and
output capacitor found in other control schemes.
8×C
× f
OUT SW
The output voltage ripple due to capacitor ESR is:
=I ×ESR
V
RIPPLE(ESR) P−P
I
is the peak-to-peak inductor current:
P-P
Simple Type 1 compensation with a single resistor (R )
C
and capacitor (C ) is all that is needed to provide a
C
V
− V
V
OUT
IN
OUT
I
=
×
P−P
stable and high-bandwidth loop.
f
×L
V
IN
SW
Use the formula below to calculate the value of C ,
C
then use the nearest standard value:
These equations are suitable for initial capacitor selec-
tion, but final values should be set by testing a proto-
type or evaluation circuit. As a rule, a smaller ripple
current results in less output voltage ripple. Because
the inductor ripple current is inversely proportional to
inductor value, output voltage ripple decreases with
larger inductance.
V
1
1
FB
C
=
×
×g ×
m
C
0.5×I
R
2π × f
C
OUT(MAX)
CS
where V is 1.25V for the MAX1973 and 0.75V for the
FB
MAX1974, the current-sense transresistance (R ) is
CS
Load transient response depends on the selected output
capacitor. During a load transient, the output voltage
0.26Ω (typ), and the transconductance from FB to
COMP (g ) is 50µS (typ). For best stability and
m
instantly changes by ESR ✕ ∆I . Before the controller
LOAD
response performance, the closed-loop unity-gain fre-
can respond, the output deviates further, depending on
the inductor and output capacitor values. After a short
time (see the Typical Operating Characteristics), the
controller responds by regulating the output voltage
back to its nominal state. The controller response time
depends on the closed-loop bandwidth. With a higher
bandwidth the response time is faster. However, to main-
tain stable operation, the bandwidth should not be set
quency (f ) should be approximately 140kHz (one-
C
tenth the switching frequency).
Use the following equation to calculate R :
C
C
V
OUT
OUT
R
=
×
C
C
0.5×I
OUT(MAX)
C
Below is a numerical example of calculating compen-
sation values for a circuit using the MAX1973 with 2.5V
output and maximum output current of 1A:
above f /10.
SW
Compensation Components
An internal transconductance error amplifier compen-
sates the control loop. Connect a series resistor and
MAX1973
______________________________________________________________________________________ 13
Smallest 1A, 1.4MHz Step-Down Regulators
V
= 2.5V
PC Board Layout
A properly designed PC board layout is important in
any switching regulator. The switching power stage
requires particular attention. Follow these guidelines for
good PC board layout:
OUT
I
= 1A
OUT(MAX)
C
= 4.7µF
OUT
V
= 1.25V
1) Place decoupling capacitors as close to IC pins as
possible. Keep the power ground plane (connected
to PGND) and signal ground plane (connected to
GND) separate. Connect the two ground planes with
a single connection from PGND to GND.
FB
R
= 0.26Ω
= 50µS
CS
g
m
f = 140kHz
C
2) Input and output capacitors are connected to the
power ground plane; all other capacitors are con-
nected to the signal ground plane.
V
1
1
FB
C
=
×
×g ×
m
C
0.5×I
R
2π × f
C
OUT(MAX)
1
CS
3) Keep the high-current paths as short and wide as
possible.
1.25
1
−6
=
×
× 50×10
×
= 547pF
0.5×1 0.26
2π ×140000
4) If possible, connect IN, LX, and PGND separately to
a large land area to help cool the IC to further
improve efficiency and long-term reliability.
Select the nearest standard value: C = 560pF
C
C
V
OUT
OUT
R
=
×
=
5) Ensure all feedback connections are short and
direct. Place feedback resistors (if used) as close to
the IC as possible.
C
C
0.5×I
C
OUT(MAX)
−6
4.7×10
2.5
×
= 41.9kΩ
6) Route high-speed switching nodes (LX) away from
sensitive analog areas (FB, COMP, SS).
−12
0.5×1
560×10
Select the nearest standard value: R = 43kΩ
C
Table 4. Recommended Components for the MAX1973
V
(V)
C
(µF)
C
(µF)
C
(pF)
R (kΩ)
C
OUT
IN
OUT
C
2.5
4.7
4.7
4.7
4.7
560
560
43
30
1.8
Table 5. Recommended Components for the MAX1974
V
(V)
C
(µF)
C
(µF)
C
(pF)
R (kΩ)
C
OUT
IN
OUT
C
1.5
4.7
4.7
4.7
4.7
330
330
43
27
1.0
Selector Guide
Chip Information
TRANSISTOR COUNT: 1998
PART
FEATURES
OUTPUT PRESET
PROCESS: BiCMOS
MAX1973EUB
MAX1974EUB
Voltage Margining
Power-OK Output
1.8V or 2.5V
1V or 1.5V
14 ______________________________________________________________________________________
Smallest 1A, 1.4MHz Step-Down Regulators
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 15
© 2002 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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