MAXIM MAX15020ATP

19-0811; Rev 1; 5/11
KIT
ATION
EVALU
LE
B
A
IL
A
AV
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
The MAX15020 high-voltage step-down DC-DC converter operates over an input voltage range of 7.5V to
40V. The device integrates a 0.2Ω high-side switch and
is capable of delivering 2A load current with excellent
load and line regulation. The output is dynamically
adjustable from 0.5V to 36V through the use of an external reference input (REFIN). The MAX15020 consumes
only 6μA in shutdown mode.
The device utilizes feed-forward voltage-mode architecture for good noise immunity in the high-voltage switching environment and offers external compensation for
maximum flexibility. The switching frequency is selectable to 300kHz or 500kHz and can be synchronized to
an external clock signal of 100kHz to 500kHz by using
the SYNC input. The IC features a maximum duty cycle
of 95% (typ) at 300kHz.
The device includes configurable undervoltage lockout
(UVLO) and soft-start. Protection features include
cycle-by-cycle current limit, hiccup-mode for output
short-circuit protection, and thermal shutdown. The
MAX15020 is available in a 20-pin TQFN 5mm x 5mm
package and is rated for operation over the
-40°C to +125°C temperature range.
Applications
Features
♦ Wide 7.5V to 40V Input Voltage Range
♦ 2A Output Current, Up to 96% Efficiency
♦ Dynamic Programmable Output Voltage (0.5V to
36V)
♦ Maximum Duty Cycle of 95% (typ) at 300kHz
♦ 100kHz to 500kHz Synchronizable SYNC
Frequency Range
♦ Configurable UVLO and Soft-Start
♦ Low-Noise, Voltage-Mode Step-Down Converter
♦ Programmable Output-Voltage Slew Rate
♦ Lossless Constant Current Limit with Fixed
Timeout to Hiccup Mode
♦ Extremely Low-Power Consumption (< 6µA typ) in
Shutdown Mode
♦ 20-Pin (5mm x 5mm) Thin QFN Package
Ordering Information
PART
MAX15020ATP+
TEMP RANGE
PIN-PACKAGE
-40°C to +125°C
20 TQFN-EP*
+Denotes a lead(Pb)-free/RoHS-compliant package.
*EP = Exposed pad.
Printer Head Driver Power Supply
Automotive Power Supply
Industrial Power Supply
Pin Configuration appears at end of data sheet.
Step-Down Power Supply
Typical Operating Circuit
VIN
(7.5V TO 40V)
ON/OFF
IN
REG
DVREG
BST
VOUT
(0.5V TO 36V)
LX
REFOUT
MAX15020
FB
PWM
INPUT
REFIN
EP
SS
SYNC GND FSEL PGND COMP
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
1
MAX15020
General Description
MAX15020
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
ABSOLUTE MAXIMUM RATINGS
Continuous Power Dissipation (TA = +70°C)
Thin QFN, single-layer board (5mm x 5mm)
(derate 21.3mW/°C above +70°C) ...........................1702.1mW
Thin QFN, multilayer board (5mm x 5mm)
(derate 34.5mW/°C above +70°C) ...........................2758.6mW
Maximum Junction Temperature .....................................+150°C
Storage Temperature Range ............................-60°C to +150°C
Lead Temperature (soldering, 10s) ................................+300°C
Soldering Temperature (reflow) .......................................+260°C
IN, ON/OFF to GND........…. ...................................-0.3V to +45V
LX to GND................................................-0.715V to (VIN + 0.3V)
BST to GND ..................................................-0.3V to (VIN + 12V)
BST to LX................................................................-0.3V to +12V
PGND, EP to GND .................................................-0.3V to +0.3V
REG, DVREG, SYNC to GND .................................-0.3V to +12V
FB, COMP, FSEL, REFIN, REFOUT,
SS to GND .............................................-0.3V to (VREG + 0.3V)
Continuous Current through Internal Power MOSFET
TJ = +125°C..........................................................................4A
TJ = +150°C.......................................................................2.7A
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
(VIN = 36V, VREG = VDVREG, VPGND = VGND = VEP = 0V, VSYNC = 0V, CREFOUT = 0.1μF, TA = TJ = -40°C to +125°C, FSEL = REG,
unless otherwise noted. Typical values are at TA = +25°C.) (Note 1)
PARAMETER
Input Voltage Range
SYMBOL
CONDITIONS
MIN
VIN
7.5
UVLO Rising Threshold
UVLORISING
6.80
UVLO Falling Threshold
UVLOFALLING
6.0
UVLO Hysteresis
UVLOHYST
TYP
MAX
UNITS
40.0
V
7.20
7.45
V
6.5
7.0
V
2.8
mA
0.7
Quiescent Supply Current
VIN = 40V, VFB = 1.3V
1.6
Switching Supply Current
VIN = 40V, VFB = 0V
14.5
Shutdown Current
ISHDN
VON/OFF = 0.2V, VIN = 40V
V
mA
6
15
μA
1.225
1.270
V
ON/OFF CONTROL
Input-Voltage Threshold
VON/OFF
VON/OFF rising
1.200
Input-Voltage Threshold
Hysteresis
120
Input Bias Current
Shutdown Threshold Voltage
VON/OFF = 0V to VIN
-250
VSD
mV
+250
nA
0.2
V
V
INTERNAL VOLTAGE REGULATOR (REG)
Output Voltage
IREG = 0 to 20mA
7.1
8.3
VFSEL = 0V
450
550
VFSEL = VREG
270
330
VFSEL = 0V
85
VFSEL = VREG
90
OSCILLATOR
Frequency
Maximum Duty Cycle
fSW
DMAX
SYNC/FSEL High-Level Voltage
SYNC Frequency Range
2
%
2
SYNC/FSEL Low-Level Voltage
fSYNC
VFSEL = VREG
100
_______________________________________________________________________________________
kHz
V
0.8
V
550
kHz
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
(VIN = 36V, VREG = VDVREG, VPGND = VGND = VEP = 0V, VSYNC = 0V, CREFOUT = 0.1μF, TA = TJ = -40°C to +125°C, FSEL = REG,
unless otherwise noted. Typical values are at TA = +25°C.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
8
15
26
μA
0.97
0.98
1.01
V
3.6
V
SOFT-START/REFIN/REFOUT/FB
Soft-Start Current
ISS
REFOUT Output Voltage
REFIN Input Range
0
REFIN = REFOUT
FB Accuracy
FB = COMP, VREFIN = 0.2V to 3.6V
FB Input Current
VSS = 0.2V, VFB = 0V
0.97
0.98
1.01
V
VREFIN
- 5mV
VREFIN
VREFIN
+ 5mV
mV
+250
nA
-250
Open-Loop Gain
80
dB
Unity-Gain Bandwidth
1.8
MHz
PWM Modulator Gain (VIN /
VRAMP)
fSYNC = 100kHz, VIN = 7.5V
9.4
fSYNC = 500kHz, VIN = 40V
8.9
V/V
CURRENT-LIMIT COMPARATOR
Cycle-by-Cycle Switch Current
Limit
2.5
IILIM
Number of ILIM Events to Hiccup
Hiccup Timeout
3.5
4.5
A
4
—
512
Clock
periods
POWER SWITCH
0.35
Ω
VBST = VLX = VIN = 40V
10
μA
VIN = 40V, VLX = VBST = 0V
10
Switch On-Resistance
VBST - VLX = 6V
BST Leakage Current
Switch Leakage Current
Switch Gate Charge
VBST - VLX = 6V
0.18
10
μA
nC
THERMAL SHUTDOWN
Thermal Shutdown Temperature
Thermal Shutdown Hysteresis
TSHDN
+160
°C
20
°C
Note 1: Limits are 100% production tested at TA = TJ = +25°C. Limits at -40°C and +125°C are guaranteed by design.
_______________________________________________________________________________________
3
MAX15020
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(VIN = 36V, Circuit of Figure 2, TA = +25°C, unless otherwise noted.)
0.7
0.6
0.5
0.4
0.3
0.2
0.15
0.10
6
0.05
0
-15
10
35
60
85
135
110
4
3
2
0
-40
-15
10
35
60
85
110
135
20
30
TEMPERATURE (°C)
INPUT VOLTAGE (V)
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE
OPERATING FREQUENCY
vs. TEMPERATURE
MAXIMUM DUTY CYCLE
vs. INPUT VOLTAGE
12
10
8
6
4
100
304
302
300
298
296
98
96
94
92
90
88
294
86
292
84
2
290
82
0
288
20
30
80
-40
40
-15
10
35
60
85
135
110
144
3.4
30
108
3.3
20
72
3.2
10
36
PHASE
GAIN (dB)
GAIN
0
0
LOAD CURRENT (A)
3.5
PHASE (DEGREES)
180
40
2.9
-20
-72
2.8
-30
-108
2.7
-144
2.6
-180
1000
2.5
0.1
1
10
FREQUENCY (kHz)
100
TA = +25°C
3.0
-36
VIN = 37V, VOUT = 15V,
IOUT = 2.02A
TA = -45°C
3.1
-10
-50
30
MAXIMUM LOAD CURRENT
vs. INPUT VOLTAGE
MAX15020 toc07
50
20
INPUT VOLTAGE (V)
LOOP GAIN/PHASE
vs. FREQUENCY
-40
10
0
TEMPERATURE (°C)
INPUT VOLTAGE (V)
MAX15020 toc08
10
40
MAX15020 toc06
306
DUTY CYCLE (%)
14
308
MAX15020 toc05
MAX15020 toc04
16
0
10
0
TEMPERATURE (°C)
OPERATING FREQUENCY (kHz)
-40
5
1
0.1
0
4
7
SUPPLY CURRENT (μA)
0.8
0.20
MAX15020 toc02
0.9
ON/OFF THRESHOLD HYSTERESIS (V)
MAX15020 toc01
UNDERVOLTAGE LOCKOUT HYSTERESIS
1.0
SHUTDOWN SUPPLY CURRENT
vs. INPUT VOLTAGE
ON/OFF THRESHOLD HYSTERESIS
vs. TEMPERATURE
MAX15020 toc03
UNDERVOLTAGE LOCKOUT HYSTERESIS
vs. TEMPERATURE
SUPPLY CURRENT (mA)
MAX15020
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
TA = +85°C
0
5
10
15
20
25
30
35
INPUT VOLTAGE (V)
_______________________________________________________________________________________
40
40
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
TURN-ON/TURN-OFF WAVEFORM
MAX15020 toc10
5V/div
VON/OFF
VOUT
1.03
0V
REFOUT VOLTAGE (V)
0V
1.05
VOUT
1V/div
1V/div
0V
0V
MAX15020 toc11
5V/div
VON/OFF
REFOUT VOLTAGE vs. TEMPERATURE
TURN-ON/TURN-OFF WAVEFORM
MAX15020 toc09
1.01
0.99
0.97
VIN = 12V, RLOAD = 27Ω
VIN = 40V, RLOAD = 27Ω
10ms/div
0.95
-40
10ms/div
-15
10
35
60
85
110
135
TEMPERATURE (°C)
EFFICIENCY vs. LOAD CURRENT
EFFICIENCY vs. LOAD CURRENT
VIN = 12V
70
90
80
EFFICIENCY (%)
EFFICIENCY (%)
80
60
50
40
30
MAX15020 toc13
VIN = 7.5V
90
100
MAX15020 toc12
100
70
60
50
40
30
VIN = 24V
20
20
fS = 500kHz
VOUT = 3.3V
VIN = 40V
10
fS = 500kHz
VIN = 40V
VOUT = 30V
10
0
0
0.01
0.1
1
10
0.01
OUTPUT CURRENT (A)
0.1
1
LOAD TRANSIENT
LOAD TRANSIENT
MAX15020 toc14
MAX15020 toc15
50mV/div
AC-COUPLED
VOUT
10
OUTPUT CURRENT (A)
IOUT
50mV/div
AC-COUPLED
VOUT
1.2A
2A
IOUT
1A
0.2A
VIN = 12V, VOUT = 3.3V
200μs/div
VIN = 12V, VOUT = 3.3V
200μs/div
_______________________________________________________________________________________
5
MAX15020
Typical Operating Characteristics (continued)
(VIN = 36V, Circuit of Figure 2, TA = +25°C, unless otherwise noted.)
Typical Operating Characteristics (continued)
(VIN = 36V, Circuit of Figure 2, TA = +25°C, unless otherwise noted.)
LOAD TRANSIENT
LOAD TRANSIENT
MAX15020 toc16
MAX15020 toc17
50mV/div
AC-COUPLED
VOUT
50mV/div
AC-COUPLED
VOUT
2A
1A
IOUT
1.1A
IOUT
0.25A
VIN = 40V, VOUT = 30V
VIN = 40V, VOUT = 30V
200μs/div
200μs/div
LIGHT-LOAD SWITCHING WAVEFORMS
SWITCHING WAVEFORMS
MAX15020 toc18
VLX
MAX15020 toc19
VLX
20V/div
20V/div
0V
0V
1A/div
ILX
1A/div
ILX
0A
ILOAD = 40mA
0A
ILOAD = 500mA
1μs/div
1μs/div
HEAVY-LOAD
SWITCHING WAVEFORMS
FEEDBACK VOLTAGE
vs. REFIN INPUT VOLTAGE
MAX15020 toc20
MAX15020 toc21
4.0
3.5
VLX
20V/div
0V
ILX
1A/div
0A
ILOAD = 2A
FEEDBACK VOLTAGE (V)
MAX15020
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
3.0
2.5
2.0
1.5
1.0
0.5
0
1μs/div
0
1
2
3
REFIN INPUT VOLTAGE (V)
6
_______________________________________________________________________________________
4
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
SOFT-START VOLTAGE RISE
vs. REFIN VOLTAGE RISE
VSS AND VOUT
RESPONSE TO REFIN PWM
MAX15020 toc23
MAX15020 toc22
10
CSS = 0.1μF
20V/div
VOUT
VSS dv/dt (V/ms)
0V
1
1V
VSS
0.5V
1V/div
VREFIN
0.1
10kΩ and 0.1μF RC ON REFIN 0V
CSS = 0.01μF
VPWM
1V/div
0V
D = 70% TO 100%
0.01
0.01
0.1
1
10
2ms/div
VREFIN dv/dt (V/ms)
MODULATOR GAIN
vs. INPUT VOLTAGE
SOFT-START CHARGE CURRENT
vs. TEMPERATURE
MODULATOR GAIN (V/V)
10.0
9.5
9.0
8.5
MAX15020 toc25
10.5
15.5
SOFT-START CHARGE CURRENT (μA)
MAX15020 toc24
11.0
15.4
15.3
15.2
15.1
15.0
14.9
14.8
14.7
14.6
8.0
14.5
5
10
15
20
25
30
INPUT VOLTAGE (V)
35
40
-40
-15
10
35
60
85
110
135
TEMPERATURE (°C)
_______________________________________________________________________________________
7
MAX15020
Typical Operating Characteristics (continued)
(VIN = 36V, Circuit of Figure 2, TA = +25°C, unless otherwise noted.)
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
MAX15020
Pin Description
PIN
NAME
1
COMP
2
FB
FUNCTION
Voltage-Error-Amplifier Output. Connect COMP to the necessary compensation feedback network.
Feedback Regulation Point. Connect to the center tap of an external resistor-divider connected between
the output and GND to set the output voltage. The FB voltage regulates to the voltage applied to REFIN.
3
ON/OFF
ON/OFF Control. The ON/OFF rising threshold is set to approximately 1.225V. Connect to the center tap of
a resistive divider connected between IN and GND to set the turn-on (rising) threshold. Connect ON/OFF to
GND to shut down the IC. Connect ON/OFF to IN for always-on operation given that VIN has risen above the
UVLO threshold. ON/OFF can be used for power-supply sequencing.
4
REFOUT
0.98V Reference Voltage Output. Bypass REFOUT to GND with a 0.1μF ceramic capacitor. REFOUT is to
be used only with REFIN. It is not to be used to power any other external circuitry.
5
SS
Soft-Start. Connect a 0.01μF or greater ceramic capacitor from SS to GND. See the Soft-Start (SS) section.
6
REFIN
External Reference Input. Connect to an external reference. VFB regulates to the voltage applied to REFIN.
Connect REFIN to REFOUT to use the internal 1V reference. See the Reference Input and Output (REFIN,
REFOUT) section.
7
FSEL
Internal Switching Frequency Selection Input. Connect FSEL to REG to select fSW = 300kHz. Connect FSEL
to GND to select fSW = 500kHz. When an external clock is connected to SYNC connect FSEL to REG.
8
SYNC
Oscillator Synchronization Input. SYNC can be driven by an external 100kHz to 500kHz clock to
synchronize the MAX15020’s switching frequency. Connect SYNC to GND to disable the synchronization
function. When using SYNC, connect FSEL to REG.
9
DVREG
Power Supply for Internal Digital Circuitry. Connect a 10Ω resistor from REG to DVREG. Connect DVREG to
the anode of the boost diode, D2 in Figure 2. Bypass DVREG to GND with at least a 1μF ceramic capacitor.
10
PGND
Power-Ground Connection. Connect the input filter capacitor’s negative terminal, the anode of the
freewheeling diode, and the output filter capacitor’s return to PGND. Connect externally to GND at a single
point near the input bypass capacitor’s return terminal.
11
N.C.
No Connection. Leave unconnected or connect to GND
12
BST
High-Side Gate Driver Supply. Connect BST to the cathode of the boost diode and to the positive terminal
of the boost capacitor.
13, 14, 15
LX
Source Connection of Internal High-Side Switch. Connect the inductor and rectifier diode’s cathode to LX.
16, 17, 18
IN
Supply Input Connection. Connect to an external voltage source from 7.5V to 40V.
19
REG
8V Internal Regulator Output. Bypass to GND with at least a 1μF ceramic capacitor. Do not use REG to
power external circuitry.
20
GND
Ground Connection. Solder the exposed pad to a large GND plane. Connect GND and PGND together at
one point near the input bypass capacitor return terminal.
—
EP
8
Exposed Pad. Connect EP to GND. Connecting EP does not remove the requirement for proper ground
connections to the appropriate pins. See the PCB Layout and Routing section.
_______________________________________________________________________________________
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
MAX15020
ON/OFF
IN
MAX15020
LDO
REF
REG
REF
EN
REF
REFOUT
THERMAL
SHDN
REF
OK
VPOK
ENABLE
SWITCHING
REGOK
ICSS
IN
CLK
OVERLOAD
MANAGEMENT
SSA
HIGH-SIDE
CURRENT
SENSE
ILIM
REFIN
REF_ILIM
SS
BST
E/A
FB
LOGIC
COMP
LX
IN
DVREG
CPWM
EN
SYNC
RAMP
OSC
PGND
FSEL
CLK
GND
Figure 1. Functional Diagram
_______________________________________________________________________________________
9
MAX15020
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
VIN
7.5V TO 40V
C3
0.1μF
C1
560μF
D2
R1
97.5kΩ
ON/OFF
IN
DVREG
R2
4.02kΩ
PWM
INPUT
MAX15020
C11
0.027μF
C2
1μF
FB
C9
0.1μF
SS
R7
10kΩ
C7
0.1μF
R8
340Ω
COMP
C8
0.22μF
VOUT
C13
330pF
REFIN
0Ω
R6
10kΩ
D1
REG
R3
10kΩ
L1
22μH
LX
R5
10Ω
C10
1μF
C4
1μF
BST
REFOUT SYNC GND FSEL PGND
C5
0.1μF
C6
560μF
EP
C12
0.1μF
R9
15.8kΩ
Figure 2. Typical Application Circuit
Detailed Description
The MAX15020 voltage-mode step-down converter
contains an internal 0.2Ω power MOSFET switch. The
MAX15020 input voltage range is 7.5V to 40V. The
internal low RDS(ON) switch allows for up to 2A of output current. The external compensation, voltage feedforward, and automatically adjustable maximum ramp
amplitude simplify the loop compensation design allowing for a variety of L and C filter components. In shutdown, the supply current is typically 6μA. The output
voltage is dynamically adjustable from 0.5V to 36V.
Additional features include an externally programmable
UVLO through the ON/OFF pin, a programmable softstart, cycle-by-cycle current limit, hiccup-mode output
short-circuit protection, and thermal shutdown.
Internal Linear Regulator (REG)
REG is the output terminal of the 8V LDO powered from
IN and provides power to the IC. Connect REG externally to DVREG to provide power for the internal digital
circuitry. Place a 1μF ceramic bypass capacitor, C2,
next to the IC from REG to GND. During normal opera10
tion, REG is intended for powering up only the internal
circuitry and should not be used to supply power to the
external loads.
UVLO/ON/OFF Threshold
The MAX15020 provides a fixed 7V UVLO function
which monitors the input voltage (VIN). The device is
held off until VIN rises above the UVLO threshold.
ON/OFF provides additional turn-on/turn-off control.
Program the ON/OFF threshold by connecting a resistive divider from IN to ON/OFF to GND. The device
turns on when VON/OFF rises above the ON/OFF threshold (1.225V), given that VIN has risen above the UVLO
threshold.
Driving ON/OFF to ground places the IC in shutdown.
When in shutdown the internal power MOSFET turns
off, all internal circuitry shuts down, and the quiescent
supply current reduces to 6μA (typ.). Connect an RC
network from ON/OFF to GND to set a turn-on delay
that can be used to sequence the output voltages of
multiple devices.
______________________________________________________________________________________
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
t SS =
VREFIN × C SS
15μ A
where tSS is in seconds and CSS is in Farads.
500kHz based upon the selection of FSEL. For external
synchronization, drive SYNC with an external clock from
100kHz to 500kHz and connect FSEL to REG. When driven with an external clock, the device synchronizes to
the rising edge of SYNC.
PWM Comparator/Voltage Feed-Forward
An internal ramp generator is compared against the
output of the error amplifier to generate the PWM signal. The maximum amplitude of the ramp (VRAMP) automatically adjusts to compensate for input voltage and
oscillator frequency changes. This causes the VIN /
VRAMP to be a constant 9V/V across the input voltage
range of 7.5V to 40V and the SYNC frequency range of
100kHz to 500kHz. This simplifies loop compensation
design by allowing large input voltage ranges and
large frequency range selection.
Reference Input and
Output (REFIN, REFOUT)
Output Short-Circuit
Protection (Hiccup Mode)
The MAX15020 features a reference input for the internal error amplifier. The IC regulates FB to the SS voltage
which is driven by the DC voltage applied to REFIN.
Connect REFIN to REFOUT to use the internal 0.98V reference. Connect REFIN to a variable DC voltage source
to dynamically control the output voltage. Alternatively,
REFIN can also be driven by a duty-cycle control PWM
source through a lowpass RC filter (Figure 2).
The MAX15020 protects against an output short circuit
by utilizing hiccup-mode protection. In hiccup mode, a
series of sequential cycle-by-cycle current-limit events
cause the part to shut down and restart with a soft-start
sequence. This allows the device to operate with a continuous output short circuit.
During normal operation, the switch current is measured
cycle-by-cycle. When the current limit is exceeded, the
internal power MOSFET turns off until the next on-cycle
and the hiccup counter increments. If the counter
counts four consecutive overcurrent limit events, the
device discharges the soft-start capacitor and shuts
down for 512 clock periods before restarting with a softstart sequence. Each time the power MOSFET turns on
and the device does not exceed the current limit, the
counter is reset.
Internal Digital Power Supply (DVREG)
DVREG is the supply input for the internal digital power
supply. The power for DVREG is derived from the output of the internal regulator (REG). Connect a 10Ω
resistor from REG to DVREG. Bypass DVREG to GND
with at least a 1μF ceramic capacitor.
Error Amplifier
The output of the internal error amplifier (COMP) is
available for frequency compensation (see the
Compensation Design section). The inverting input is
FB, the noninverting input is SS, and the output is
COMP. The error amplifier has an 80dB open-loop gain
and a 1.8MHz GBW product. When an external clock is
used, connect FSEL to REG.
Oscillator/Synchronization Input (SYNC)
With SYNC connected to GND, the IC uses the internal
oscillator and switches at a fixed frequency of 300kHz or
Thermal-Overload Protection
The MAX15020 features an integrated thermal-overload protection. Thermal-overload protection limits the
total power dissipation in the device and protects it in
the event of an extended thermal fault condition. When
the die temperature exceeds +160°C, an internal thermal sensor shuts down the part, turning off the power
MOSFET and allowing the IC to cool. After the temperature falls by 20°C, the part restarts beginning with the
soft-start sequence.
______________________________________________________________________________________
11
MAX15020
Soft-Start (SS)
At startup, after VIN is applied and the UVLO threshold
is reached, a 15μA (typ) current is sourced into the
capacitor (CSS) connected from SS to GND forcing the
VSS voltage to ramp up slowly. If VREFIN is set to a DC
voltage or has risen faster than the CSS charge rate,
then V SS will stop rising once it reaches V REFIN . If
VREFIN rises at a slower rate, VSS will follow the VREFIN
voltage rise rate. VOUT rises at the same rate as VSS
since VFB follows VSS.
Set the soft-start time (tSS) using following equation:
MAX15020
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
Applications Information
Setting the ON/OFF Threshold
When the voltage at ON/OFF rises above 1.225V, the
MAX15020 turns on. Connect a resistive divider from IN
to ON/OFF to GND to set the turn-on voltage (see
Figure 2). First select the ON/OFF to the GND resistor
(R2), then calculate the resistor from IN to ON/OFF (R1)
using the following equation:
⎡ V
IN
R1 = R2 × ⎢
⎢ VON/ OFF
⎣
⎤
− 1⎥
⎥
⎦
where VIN is the input voltage at which the converter
turns on, VON/OFF = 1.225V and R2 is chosen to be
less than 600kΩ.
If ON/OFF is connected to IN directly, the UVLO feature
monitors the supply voltage at IN and allows operation
to start when VIN rises above 7.2V.
Setting the Output Voltage
Connect a resistor-divider from OUT to FB to GND to
set the output voltage (see Figure 2). First calculate the
resistor (R7) from OUT to FB using the guidelines in the
Compensation Design section. Once R7 is known, calculate R8 using the following equation:
R8 =
R7
⎡ VOUT
⎢
⎣ VFB
Inductor Selection
Three key inductor parameters must be specified for
operation with the MAX15020: inductance value (L),
peak inductor current (IPEAK), and inductor saturation
current (ISAT). The minimum required inductance is a
function of operating frequency, input-to-output voltage
differential, and the peak-to-peak inductor current
(ΔIL). Higher ΔIL allows for a lower inductor value while
a lower ΔIL requires a higher inductor value. A lower
inductor value minimizes size and cost and improves
large-signal and transient response, but reduces efficiency due to higher peak currents and higher peak-topeak output voltage ripple for the same output
capacitor. Higher inductance increases efficiency by
reducing the ripple current. Resistive losses due to
extra wire turns can exceed the benefit gained from
lower ripple current levels especially when the inductance is increased without also allowing for larger
inductor dimensions. A good compromise is to choose
ΔIP-P equal to 40% of the full load current.
Calculate the inductor using the following equation:
⎤
− 1⎥
⎦
where VFB = REFIN and REFIN = 0 to 3.6V.
Setting the Output-Voltage Slew Rate
The output-voltage rising slew rate tracks the VSS slew
rate, given that the control loop is relatively fast compared with the V SS slew rate. The maximum V SS
upswing slew rate is controlled by the soft-start current
charging the capacitor connected from SS to GND
according to the formula below:
dVOUT R 7 + R 8 dVSS R 7 + R 8 I SS
=
×
=
dt
R8
dt
R8
C SS
when driving VSS with a slow-rising voltage source at
REFIN, VOUT will slowly rise according to the VREFIN
slew rate.
12
The output-voltage falling slew rate is limited to the discharge rate of CSS assuming there is enough load current to discharge the output capacitor at this rate. The
CSS discharge current is 15μA. If there is no load, then
the output voltage falls at a slower rate based upon
leakage and additional current drain from COUT.
L=
(VIN − VOUT ) × VOUT
VIN × fSW × Δ IL
VIN and VOUT are typical values so that efficiency is
optimum for typical conditions. The switching frequency (fSW) is fixed at 300kHz or 500kHz and can vary
between 100kHz and 500kHz when synchronized to an
external clock (see the Oscillator/Synchronization Input
(SYNC) section). The peak-to-peak inductor current,
which reflects the peak-to-peak output ripple, is worst
at the maximum input voltage. See the Output
Capacitor Selection section to verify that the worst-case
output ripple is acceptable. The inductor saturating
current (ISAT) is also important to avoid runaway current during continuous output short circuit. Select an
inductor with an ISAT specification higher than the maximum peak current limit of 4.5A.
______________________________________________________________________________________
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
ESR =
C IN =
Δ VESR
Δ IL ⎞
⎛
⎜⎝ IOUT _ MAX + 2 ⎟⎠
IOUT _ MAX × D(1 − D)
Δ VQ × fSW
where:
ΔIL =
D=
(VIN − VOUT ) × VOUT
VIN × fSW × L
VOUT
VIN
IOUT_MAX is the maximum output current, D is the duty
cycle, and fSW is the switching frequency.
The MAX15020 includes internal and external UVLO
hysteresis and soft-start to avoid possible unintentional
chattering during turn-on. However, use a bulk capacitor if the input source impedance is high. Use enough
input capacitance at lower input voltages to avoid possible undershoot below the UVLO threshold during
transient loading.
Output Capacitor Selection
The allowable output-voltage ripple and the maximum
deviation of the output voltage during load steps determine the output capacitance and its ESR. The output
ripple is mainly composed of ΔV Q (caused by the
capacitor discharge) and ΔVESR (caused by the voltage drop across the ESR of the output capacitor). The
equations for calculating the peak-to-peak output voltage ripple are:
Δ VQ =
Δ IL
16 × C OUT × fSW
Δ VESR = ESR × Δ IL
Normally, a good approximation of the output-voltage
ripple is ΔVRIPPLE ≈ ΔVESR + ΔVQ. If using ceramic
capacitors, assume the contribution to the output-voltage ripple from ESR and the capacitor discharge to be
equal to 20% and 80%, respectively. ΔIL is the peak-topeak inductor current (see the Input Capacitor
Selection section) and fSW is the converter’s switching
frequency.
The allowable deviation of the output voltage during
fast load transients also determines the output capacitance, its ESR, and its equivalent series inductance
(ESL). The output capacitor supplies the load current
during a load step until the controller responds with a
greater duty cycle. The response time (t RESPONSE)
depends on the closed-loop bandwidth of the converter
(see the Compensation Design section). The resistive
drop across the output capacitor’s ESR, the drop
across the capacitor’s ESL (ΔVESL), and the capacitor
discharge cause a voltage droop during the load step.
Use a combination of low-ESR tantalum/aluminum electrolytic and ceramic capacitors for better transient load
and voltage ripple performance. Surface-mount capacitors and capacitors in parallel help reduce the ESL.
Keep the maximum output-voltage deviations below the
tolerable limits of the electronics powered. Use the following equations to calculate the required ESR, ESL,
and capacitance value during a load step:
ESR =
VESR
I STEP
I
×t
C OUT = STEP RESPONSE
Δ VQ
ESL =
Δ VESL × t STEP
I STEP
where ISTEP is the load step, tSTEP is the rise time of the
load step, and tRESPONSE is the response time of the
controller.
______________________________________________________________________________________
13
MAX15020
Input Capacitor Selection
The discontinuous input current of the buck converter
causes large input ripple currents and therefore the
input capacitor must be carefully chosen to keep the
input-voltage ripple within design requirements. The
input-voltage ripple is comprised of ΔVQ (caused by the
capacitor discharge) and ΔVESR (caused by the ESR
(equivalent series resistance) of the input capacitor).
The total voltage ripple is the sum of ΔVQ and ΔVESR.
Calculate the input capacitance and ESR required for a
specified ripple using the following equations:
MAX15020
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
Compensation Design
The MAX15020 uses a voltage-mode control scheme
that regulates the output voltage by comparing the
error-amplifier output (COMP) with an internal ramp to
produce the required duty cycle. The output lowpass
LC filter creates a double pole at the resonant frequency, which has a gain drop of -40dB/decade. The error
amplifier must compensate for this gain drop and
phase shift to achieve a stable closed-loop system.
The basic regulator loop consists of a power modulator,
an output feedback divider, and a voltage error amplifier. The power modulator has a DC gain set by VIN /
VRAMP, with a double pole and a single zero set by the
output inductance (L), the output capacitance (COUT)
(C6 in the Figure 2) and its ESR. The power modulator
incorporates a voltage feed-forward feature, which automatically adjusts for variations in the input voltage
resulting in a DC gain of 9. The following equations
define the power modulator:
GMOD(DC) =
VIN
VRAMP
=9
1
fLC =
2π L × C
1
fESR =
2π × C OUT × ESR
The switching frequency is internally set at 300kHz or
500kHz, or can vary from 100kHz to 500kHz when driven
with an external SYNC signal. The crossover frequency
(fC), which is the frequency when the closed-loop gain is
equal to unity, should be set as fSW / 2π or lower.
The error amplifier must provide a gain and phase
bump to compensate for the rapid gain and phase loss
from the LC double pole. This is accomplished by utilizing a Type 3 compensator that introduces two zeros
and three poles into the control loop. The error amplifier
has a low-frequency pole (fP1) near the origin.
In reference to Figures 3 and 4, the two zeros are at:
1
1
f Z1 =
and f Z2 =
2π × R9 × C12
2π × (R6 + R7) × C11
And the higher frequency poles are at:
1
1
fP2 =
and fP3 =
2π × R6 × C11
⎛ C12 × C13 ⎞
2π × R9 × ⎜
⎝ C12 + C1 3 ⎟⎠
14
Compensation When fC < fESR
Figure 3 shows the error-amplifier feedback as well as
its gain response for circuits that use low-ESR output
capacitors (ceramic). In this case fZESR occurs after fC.
fZ1 is set to 0.8 x fLC(MOD) and fZ2 is set to fLC to compensate for the gain and phase loss due to the double
pole. Choose the inductor (L) and output capacitor
(C OUT ) as described in the Inductor Selection and
Output Capacitor Selection sections.
Choose a value for the feedback resistor R9 in Figure 3
(values between 1kΩ and 10kΩ are adequate).
C12 is then calculated as:
C12 =
1
2π × 0. 8 × fLC × R9
fC occurs between fZ2 and fP2. The error-amplifier gain
(GEA) at fC is due primarily to C11 and R9.
Therefore, GEA(fC) = 2π x fC x C11 x R9 and the modulator gain at fC is:
GMOD(fC) =
GMOD(DC)
2
(2π) × L × C OUT × fC 2
Since GEA(fC) x GMOD(fC) = 1, C11 is calculated by:
f × L × C OUT × 2π
C11 = C
R9 × GMOD(DC)
fP2 is set at 1/2 the switching frequency (fSW). R6 is
then calculated by:
R6 =
1
2π × C11 × 0. 5 × fSW
Since R7 >> R6, R7 + R6 can be approximated as R7.
R7 is then calculated as:
R7 =
1
2π × fLC × C11
fP3 is set at 5 x fC. Therefore, C13 is calculated as:
C13 =
C12
2π × C12 × R9 × fP3 − 1
______________________________________________________________________________________
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
C13
R9
C11
R9
C11
R6
R6
C12
VOUT
C12
R7
VOUT
FB
R8
ERROR
AMPLIFIER
R7
FB
COMP
ERROR
AMPLIFIER
R8
SS
SS
CLOSED-LOOP
GAIN
CLOSED-LOOP
GAIN
ERRORAMPLIFIER
GAIN
GAIN
(dB)
fZ1
fZ2
COMP
fC
fP2
fP3
FREQUENCY (Hz)
Figure 3. Error-Amplifier Compensation Circuit (Closed-Loop
and Error-Amplifier Gain Plot) for Ceramic Capacitors
Compensation when fC > fZESR
For larger ESR capacitors such as tantalum and aluminum electrolytics, fZESR can occur before fC. If fZESR
< fC, then fC occurs between fP2 and fP3. fZ1 and fZ2
remain the same as before, however, fP2 is now set
equal to fZESR. The output capacitor’s ESR zero frequency is higher than fLC but lower than the closedloop crossover frequency. The equations that define
the error amplifier’s poles and zeros (fZ1, fZ2, fP1, fP2,
and fP3) are the same as before. However, fP2 is now
lower than the closed-loop crossover frequency. Figure
4 shows the error-amplifier feedback as well as its gain
response for circuits that use higher-ESR output capacitors (tantalum or aluminum electrolytic).
Pick a value for the feedback resistor R9 in Figure 4
(values between 1kΩ and 10kΩ are adequate).
C12 is then calculated as:
1
C12 =
2π × 0. 8 × fLC × R9
The error-amplifier gain between fP2 and fP3 is approximately equal to R9 / R6 (given that R6 << R7). R6 can
then be calculated as:
ERRORAMPLIFIER
GAIN
GAIN
(dB)
fZ1
fZ2
fP2
fC
fP3 FREQUENCY (Hz)
Figure 4. Error-Amplifier Compensation Circuit (Closed-Loop
and Error-Amplifier Gain Plot) for Higher ESR Output Capacitors
R6 =
R9 × 10 × fLC 2
fC 2
C11 is then calculated as:
C
× ESR
C11 = OUT
R6
Since R7 >> R6, R7 + R6 can be approximated as R7.
R7 is then calculated as:
R7 =
1
2π × fLC × C11
fP3 is set at 5 x fC. Therefore, C13 is calculated as:
C13 =
C12
2π × C12 × R9 × fP3 − 1
Based on the calculations above, the following compensation values are recommended when the switching frequency of DC-DC converter ranges from 100kHz
to 500kHz. (Note: The compensation parameters in
Figure 2 are strongly recommended if the switching
frequency is from 300kHz to 500kHz.)
______________________________________________________________________________________
15
MAX15020
C13
MAX15020
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
Power Dissipation
The MAX15020 is available in a thermally enhanced
package and can dissipate up to 2.7W at TA = +70°C.
When the die temperature reaches +160°C, the part
shuts down and is allowed to cool. After the parts cool
by 20°C, the device restarts with a soft-start.
The power dissipated in the device is the sum of the
power dissipated from supply current (PQ), transition
losses due to switching the internal power MOSFET
(PSW), and the power dissipated due to the RMS current through the internal power MOSFET (PMOSFET).
The total power dissipated in the package must be limited such that the junction temperature does not
exceed its absolute maximum rating of +150°C at maximum ambient temperature. Calculate the power lost in
the MAX15020 using the following equations:
The power loss through the switch:
The power loss due to the switching supply current
(ISW):
PQ = VIN x ISW
The total power dissipated in the device is:
PTOTAL = PMOSFET + PSW + PQ
PCB Layout and Routing
Use the following guidelines to layout the switching
voltage regulator:
1)
Place the IN and DVREG bypass capacitors close
to the MAX15020 PGND pin. Place the REG
bypass capacitor close to the GND pin.
2)
Minimize the area and length of the high-current
loops from the input capacitor, switching MOSFET,
inductor, and output capacitor back to the input
capacitor negative terminal.
Keep short the current loop formed by the switching MOSFET, Schottky diode, and input capacitor.
Keep GND and PGND isolated and connect them
at one single point close to the negative terminal
of the input filter capacitor.
Place the bank of output capacitors close to the
load.
3)
PMOSFET = IRMS _ MOSFET 2 × R ON
IRMS _ MOSFET =
⎡I 2 PK + + (IPK + × IPK − ) + IPK − ⎤ × D
⎣
⎦ 3
ΔI
IPK + = IOUT + L
2
Δ IL
IPK − = IO UT −
2
RON is the on-resistance of the internal power MOSFET
(see the Electrical Characteristics table).
4)
5)
6)
7)
The power loss due to switching the internal MOSFET:
V ×I
× (t R × t F ) × fSW
PSW = IN OUT
4
8)
where tR and tF are the rise and fall times of the internal
power MOSFET measured at LX.
9)
16
Distribute the power components evenly across
the board for proper heat dissipation.
Provide enough copper area at and around the
MAX15020 and the inductor to aid in thermal dissipation.
Use 2oz copper to keep the trace inductance and
resistance to a minimum. Thin copper PCBs can
compromise efficiency since high currents are
involved in the application. Also, thicker copper
conducts heat more effectively, thereby reducing
thermal impedance.
Place enough vias in the pad for the EP of the
MAX15020 so that the heat generated inside can
be effectively dissipated by PCB copper.
______________________________________________________________________________________
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
Chip Information
PROCESS: BiCMOS
LX
LX
LX
BST
N.C.
TOP VIEW
Package Information
15
14
13
12
11
For the latest package outline information and land patterns
(footprints), go to www.maxim-ic.com/packages. Note that a
“+”, “#”, or “-” in the package code indicates RoHS status only.
Package drawings may show a different suffix character, but
the drawing pertains to the package regardless of RoHS status.
IN 16
10
PGND
IN 17
9
DVREG
8
SYNC
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
7
FSEL
20 TQFN-EP
T2055+5
21-0140
90-0010
6
REFIN
IN 18
MAX15020
REG 19
2
3
4
5
ON/OFF
REFOUT
SS
1
FB
+
COMP
GND 20
THIN QFN
(5mm x 5mm)
______________________________________________________________________________________
17
MAX15020
Pin Configuration
MAX15020
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
Revision History
PAGES
CHANGED
REVISION
NUMBER
REVISION
DATE
0
4/07
Initial release
—
1
5/11
Corrected the feedback resistor reference from R6 to R9 in the Compensation When
fC < fESR section
14
DESCRIPTION
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.
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