MP2918
4V to 40V Input, Current Mode,
Synchronous, Step-Down Controller
DESCRIPTION
FEATURES
The MP2918 is a high-voltage, synchronous,
step-down, switching controller that can directly
step down voltages from up to 40V. The
MP2918 uses PWM current control architecture
with accurate cycle-by-cycle current limit and is
capable of driving dual N-channel MOSFET
switches.
Advanced Asynchronous Mode (AAM) enables
non-synchronous operation to optimize lightload efficiency.
The operating frequency of the MP2918 can be
programmed by an external resistor or
synchronized to an external clock for noisesensitive applications. Full protection features
include precision output over-voltage protection
(OVP), output over-current protection (OCP),
and thermal shutdown.
The MP2918 is available in TSSOP20-EP and
QFN-20 (3mmx4mm) packages.
APPLICATIONS
Automotive
Industrial Control Systems
Wide 4V to 40V Operating Input Range
Dual N-Channel MOSFET Driver
0.8V Voltage Reference with ±1.5%
Accuracy Over Temperature
Low Dropout Operation: Maximum Duty
Cycle at 99.5%
Programmable Frequency Range: 100kHz 1000kHz
External Sync Clock Range: 100kHz1000kHz
180°Out-of-Phase SYNCO Pin
Programmable Soft Start (SS)
Power Good (PG) Output Voltage Monitor
Selectable Cycle-by-Cycle Current Limit
Output Over-Voltage Protection (OVP)
Hiccup Over-Current Protection (OCP)
Internal LDO with External Power Supply
Option
Programmable Forced CCM and AAM
Available TSSOP20-EP and QFN-20
(3mmx4mm) Packages
All MPS parts are lead-free, halogen-free, and adhere to the RoHS
directive. For MPS green status, please visit the MPS website under
Quality Assurance. “MPS” and “The Future of Analog IC Technology” are
registered trademarks of Monolithic Power Systems, Inc.
TYPICAL APPLICATION
4-40V
VIN
VCC1
BST
IN
VCC1
FREQ
TG
VCC1
SW
VOUT
PG
VCC2
SGND
MP2918
EN/SYNC
BG
SENSE+
SENSE-
ILIM
CCM/AAM
PGND COMP
MP2918 Rev. 1.02
5/31/2017
FB
SYNCO
SS
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1
MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
ORDERING INFORMATION
Part Number*
Package
Top Marking
MP2918GF**
MP2918GL**
TSSOP20-EP
QFN-20 (3mm x 4mm)
See Below
See Below
*For Tape & Reel, add suffix –Z (e.g. MP2918GF–Z)
** Under qualification
TOP MARKING (TSSOP20-EP)
MPS: MPS prefix
YY: Year code
WW: Week code
MP2918: Part number
LLLLLLLLL: Lot number
TOP MARKING (QFN-20 (3mm x 4mm))
MP: MPS prefix:
Y: year code;
W: week code:
2918: first four digits of the part number;
LLL: lot number;
MP2918 Rev. 1.02
5/31/2017
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2
MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
PACKAGE REFERENCE
VCC2
3
18
SW
VCC1
4
17
BG
SGND
5
16
PGND
VCC2
1
16 SW
VCC1
2
15
SGND
3
MP2918
COMP
7
14
SENSE-
FB
8
13
SYNCO
CCM/AAM
9
12
ILIM
FREQ
10
11
PG
TSSOP-20 EP
ABSOLUTE MAXIMUM RATINGS (1)
Input supply voltage (VIN) ............................. 65V
BST supply voltage (VBST) ................. VIN + 6.5V
SW .................................................. -0.3V to 65V
EN/SYNC ..................................................... 55V
BST - SW .................................................... 6.5V
Supply voltage (VCC1) ............................... 6.5V
External supply voltage (VCC2) ................... 15V
SENSE +/- ................................................... 28V
Differential sense (SENSE+ to SENSE-) ............
.....................................................-0.7V to +0.7V
TG ............................... VSW - 0.3V to VBST + 0.3V
BG ................................... -0.3V to VCC1 + 0.3V
All other pins ................................-0.3V to +6.5V
(2)
Continuous power dissipation (TA = +25°C)
TSSOP-20 EP ............................................ 3.1W
QFN-20 (3mmx4mm) ................................. 2.6W
Junction temperature ................................150C
Lead temperature .....................................260C
Storage temperature ................ -65C to +175C
MP2918 Rev. 1.02
5/31/2017
SS
4
13 SENSE+
COMP
5
12 SENSE-
FB
6
11 SYNCO
ILIM
SENSE+
PG
15
FREQ
6
BG
14 PGND
MP2918
CCM/AAM
SS
17 TG
TG
10
19
BST
2
18
EN/SYNC
9
BST
IN
20
8
1
7
IN
19
TOP VIEW
20 EN/SYNC
TOP VIEW
QFN-20 (3mmx4mm)
Recommended Operating Conditions (3)
Supply voltage (VIN)… ........................ 4V to 40V
Output voltage (VOUT) ................................. ≤25V
Supply voltage for VCC2… .............. 4.7V to 12V
Operating junction temp. (TJ)….-40°C to +125°C
Thermal Resistance (4)
θJA
θJC
TSSOP-20 EP ........................ 40 ....... 8.... C/W
QFN-20 (3mmx4mm) .............. 48 ...... 10... C/W
NOTES:
1) Exceeding these ratings may damage the device.
2) The maximum allowable power dissipation is a function of the
maximum junction temperature TJ (MAX), the junction-toambient thermal resistance θJA, and the ambient temperature
TA. The maximum allowable continuous power dissipation at
any ambient temperature is calculated by PD (MAX) = (TJ
(MAX)-TA)/θJA. Exceeding the maximum allowable power
dissipation produces an excessive die temperature, causing
the regulator to go into thermal shutdown. Internal thermal
shutdown circuitry protects the device from permanent
damage.
3) Measured on JESD51-7, 4-layer PCB.
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3
MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
ELECTRICAL CHARACTERISTICS
VIN = 24V, TJ = -40°C to +125°C(4), EN/SYNC = 2V, VILIMIT = 75mV, unless otherwise noted.
Parameters
Symbol
Input Supply
VIN UVLO threshold (rising)
VIN UVLO threshold (falling)
VIN UVLO hysteresis
VIN supply current with VCC2
bias
VIN supply current without
VCC2 bias
Condition
Min
Typ
Max
Units
4
3.2
4.5
3.7
800
5
3.95
V
V
mV
25
40
μA
750
1000
μA
250
350
μA
0.5
5
μA
5
5.5
V
Load = 0 to 50mA, VCC2
floating or connected to SGND
1
3
%
VCC2 > 6V
5
INUV_RISING
INUV_FALLING
INUV_HYS
IQ_VCC2
IQ
VIN AAM current
IQ_AAM
VIN shutdown current
VCC Regulator
VCC1 regulator output voltage
from VIN
VCC1 regulator load
regulation from VIN
ISHDN
VCC1_VIN
VCC1 regulator output voltage
from VCC2
VCC1_VCC2
VCC2 = 12V, external bias,
VAAM = 5V, VFB = 0.84V,
SENSE+ = SENSE- = 0V,
no switching
VCC2 = 0V, VFB = 0.84V,
VAAM = 5V, SENSE+ = SENSE= 0V, no switching
VCC2 = 0V, VAAM = 0.6V,
VFB = 0.84V, SENSE+ =
SENSE- = 12V, no switching
VEN = 0V
VIN > 6V, load = 0 to 50mA
VCC1 regulator load
Load = 0 to 50mA, VCC2 = 12V
regulation from VCC2
VCC2 UVLO threshold (rising) VCC2_RISING
VCC2 UVLO threshold (falling) VCC2_FALLING
VCC2 threshold hysteresis
VCC2_HYS
VAAM = 5V, VFB = 0.84V,
SENSE+ = SENSE- = 12V,
VCC2 = 12V, no switching
VCC2 supply current
IVCC2
VAAM = 0.6V, VFB = 0.84V,
SENSE+ = SENSE- =12V,
VCC2 = 12V, no switching
Feedback (FB)
Feedback voltage
VFB
4V VIN 40V
Feedback current
IFB
VFB = 0.8V
Enable (EN/SYNC)
Enable threshold (rising)
VEN_RISING
Enable threshold (falling)
VEN_FALLING
Enable threshold hysteresis
VEN_TH
Enable input current
IEN
VEN/SYNC = 2V
Enable turn-off delay
tOFF
MP2918 Rev. 1.02
5/31/2017
4.5
V
1
3
%
4.7
4.45
250
4.92
4.75
V
V
mV
800
1100
μA
200
300
μA
0.788
0.800
10
0.812
V
nA
1.16
1.03
1.22
1.09
130
2
20
1.28
1.15
V
V
mV
μA
μs
4.3
4.05
10
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5
40
4
MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
ELECTRICAL CHARACTERISTICS (continued)
VIN = 24V, TJ = -40°C to +125°C(4), EN/SYNC = 2V, VILIMIT = 75mV, unless otherwise noted.
Parameters
Symbol
Condition
Min
Typ
Max
Units
RFREQ = 45.3kΩ
340
430
520
kHz
Oscillator and Sync
Operating frequency
Foldback operating frequency
Maximum programmable
frequency
Minimum programmable
frequency
EN/SYNC frequency range
EN/SYNC voltage rising
threshold
EN/SYNC voltage falling
threshold
Current Sense
Current sense common mode
voltage range
FSW
FSW_FOLDBACK VFB = 0.1V
FSWH
FSYNC
100
VSYNC_RISING
2
VSENSE+/-
Reverse current limit sense
voltage
VREV_ILIMIT
Valley current limit
VVAL_ILIMIT
Soft Start (SS)
Soft-start source current
Error Amplifier (EA)
Error amp transconductance (5)
Error amp open loop DC gain (5)
Error amp sink/source current
Protection
Over-voltage threshold
Over-voltage hysteresis
Thermal shutdown (5)
Thermal shutdown hysteresis (5)
MP2918 Rev. 1.02
5/31/2017
kHz
ISENSE
0
ILIM = SGND, VSENSE+ = 3.3V
ILIM = VCC1, VSENSE+ = 3.3V
ILIM = float, VSENSE+ = 3.3V
ILIM = SGND, VSENSE+ = 3.3V
ILIM = VCC1, VSENSE+ = 3.3V
ILIM = float, VSENSE+ = 3.3V
ILIM = SGND, VSENSE+ = 3.3V
ILIM = VCC1, VSENSE+ = 3.3V
ILIM = float, VSENSE+ = 3.3V
VSENSE+/-(CM) = 0V
VSENSE+/-(CM) = 3.3V
VSENSE+/-(CM) > 5V
ISS
SS = 0.5V
Gm
AO
IEA
∆V = 5mV
VOV
VOV_HYS
15
40
65
100
kHz
1000
kHz
V
VSYNC_FALLING
VILIMIT
FSW
1000
FSWL
Current limit sense voltage
Input current of sensor
50%
0.35
V
25
V
35
60
85
mV
mV
mV
-70
80
105
25
50
75
8
17
24
22.5
47.5
72.5
-45
115
150
-20
160
205
μA
μA
μA
2
4
6
μA
mV
mV
μS
dB
μA
500
70
±30
FB = 0.7/0.9V
110%
115%
10%
170
20
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120%
VFB
VFB
°C
°C
5
MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
ELECTRICAL CHARACTERISTICS (continued)
VIN = 24V, TJ = -40°C to +125°C(4), EN/SYNC = 2V, VILIMIT = 75mV, unless otherwise noted.
Parameters
Symbol
Gate Driver
TG pull-up resistor
TG pull-down resistor
BG pull-up resistor
BG pull-down resistor
Dead time
TG maximum duty cycle
TG minimum on time (5)
BG minimum on time
Power Good (PG)
Power good low
RTG_PULLUP
RTG_PULLDN
RBG_PULLUP
RBG_PULLDN
tdead
Dmax
tON_MIN_TG
tON_MIN_BG
VPG_Low
PG rising threshold
PGVth_RSING
PG falling threshold
PGVth_FALLING
PG threshold hysteresis
Power good leakage
Power good delay
AAM/CCM
AAM output current
CCM required AAM threshold
voltage
PGVth_HYS
IPG_LK
tPG_delay
IAAM
Condition
CLoad = 3.3nF
VFB = 0.7V
Iload = 4mA
VOUT rising
VOUT falling
VOUT falling
VOUT rising
Min
98
85%
101%
81%
105%
PG = 5V
Rising
Falling
2
1
3
1
60
99.5
92
175
0.1
90%
107%
87%
110%
3%
Max
Units
250
Ω
Ω
Ω
Ω
ns
%
ns
ns
0.3
96.5%
112.5%
92.5%
116.5%
2
RFREQ = 45.3kΩ
VCCM_TH
Typ
V
VFB
VFB
VFB
μA
37
28
μs
13.2
μA
2.3
V
NOTES:
4) Not tested in production, guaranteed by over-temperature correlation
5) Not tested in production, guaranteed by design and characterization.
MP2918 Rev. 1.02
5/31/2017
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MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
TYPICAL CHARACTERISTICS
VIN = 24V, TJ = -40°C to +125°C, unless otherwise noted.
MP2918 Rev. 1.02
5/31/2017
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7
MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
TYPICAL CHARACTERISTICS (continued)
VIN = 24V, TJ = -40°C to +125°C, unless otherwise noted.
MP2918 Rev. 1.02
5/31/2017
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MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
TYPICAL CHARACTERISTICS (continued)
VIN = 24V, TJ = -40°C to +125°C, unless otherwise noted.
MP2918 Rev. 1.02
5/31/2017
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MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
TYPICAL PERFORMANCE CHARACTERISTICS
VIN = 24V, VOUT = 5V, L = 4.7µH, AAM, FSW = 500kHz, TA = +25°C, unless otherwise noted.
MP2918 Rev. 1.02
5/31/2017
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MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN = 24V, VOUT = 5V, L = 4.7µH, AAM, FSW = 500kHz, TA = +25°C, unless otherwise noted.
MP2918 Rev. 1.02
5/31/2017
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MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN = 24V, VOUT = 5V, L = 4.7µH, AAM, FSW = 500kHz, TA = +25°C, unless otherwise noted.
MP2918 Rev. 1.02
5/31/2017
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MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN = 24V, VOUT = 5V, L = 4.7µH, AAM, FSW = 500kHz, TA = +25°C, unless otherwise noted.
MP2918 Rev. 1.02
5/31/2017
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13
MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
PIN FUNCTIONS
TSSOP-20
Pin #
QFN-20
Pin #
1
19
2
20
3
1
4
2
5
3
6
4
7
5
8
6
9
7
10
8
11
9
12
10
13
11
14
12
15
13
16
14
MP2918 Rev. 1.02
5/31/2017
Name
Description
Input supply. The MP2918 operates on a 4V to 40V input range. A
ceramic capacitor is needed to prevent large voltage spikes at the input.
Enable input. The EN/SYNC threshold is 1.22V with 130mV of
hysteresis. EN/SYNC is used to implement an input under-voltage lockout
EN/SYNC
(UVLO) function externally. If an external sync clock is applied to
EN/SYNC, the internal clock follows the sync frequency.
External power supply for the internal VCC1 regulator. VCC2 disables
the power from VIN for as long as VCC2 is higher than 4.7V. Do not
VCC2
connect a power supply greater than 12V to VCC2. Connect VCC2 to an
external power supply to reduce power dissipation and increase efficiency.
Internal bias supply. A ≥1µF decoupling capacitor is required between
VCC1
VCC1 and PGND.
Low-noise ground reference. SGND should be connected to the VOUT
SGND
side of the output capacitors.
Soft-start control input. SS is used to program the soft-start period with
SS
an external capacitor between SS and SGND.
Regulation control loop compensation. Connect an R-C network from
COMP
COMP to SGND to compensate for the regulation control loop.
Feedback. FB is the input of the error amplifier. An external resistive
FB
divider connected between the output and SGND is compared to the
internal +0.8V reference to set the regulation voltage.
Continuous conduction mode/advanced asynchronous mode.
Floating CCM/AAM or connecting CCM/AAM to VCC1 makes the part
CCM/AAM operate in CCM. Connecting an appropriate external resistor from
CCM/AAM to SGND (so AAM is at a low level) makes the part operate in
AAM. The AAM voltage should be no less than 480mV.
Frequency. Connect a resistor between FREQ and SGND to set the
FREQ
switching frequency.
PG
Power good output. The output of PG is an open drain.
Sense voltage limit set. The voltage at ILIM sets the nominal sense
ILIM
voltage at the maximum output current. There are three fixed options:
float, VCC1, and SGND.
Frequency synchronous out. SYNCO outputs a 180°out-of-phase clock
SYNCO
when the part works in CCM for dual-channel operation.
Negative input for the current sense. The sensed inductor current limit
SENSEthreshold is determined by the status of ILIM.
Positive input for the current sense. The sensed inductor current limit
SENSE+
threshold is determined by the status of ILIM.
High-current ground reference for the internal low-side switch driver
PGND
and the VCC1 regulator circuit. Connect PGND to the negative terminal
of the VCC1 decoupling capacitor directly.
IN
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14
MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
PIN FUNCTIONS (continued)
TSSOP-20
Pin #
QFN-20
Pin #
Name
17
15
BG
18
16
SW
19
17
TG
20
18
BST
Exposed
pad
MP2918 Rev. 1.02
5/31/2017
Description
Bottom gate driver output. Connect BG to the gate of the synchronous
N-channel MOSFET.
Switch node. SW is the reference for the VBST supply and high-current
returns for the bootstrapped switch.
Top gate drive. TG drives the gate of the top N-channel synchronous
MOSFET. The TG driver draws power from the BST capacitor and returns
to SW, providing a true floating drive to the top N-channel MOSFET.
Bootstrap. BST is the positive power supply for the internal, floating, highside MOSFET driver. Connect a bypass capacitor between BST and SW.
A diode from VCC1 to BST charges the BST capacitor when the low-side
switch is off.
Exposed pad. The exposed pad is on the bottom side of the device. It is
not connected to SGND or PGND electrically. Connect the exposed pad to
SGND and PGND during PCB layout for better thermal performance.
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MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
BLOCK DIAGRAM
SYNCO
ILIM
IN
4.7V
VCC
Regulator
VCC2
VCC1
VCC1
BST
FREQ
Oscillator
HS
Driver
VCC1
Current Limit
Comparator
EN/SYNC
Reference
Control
Vref Error Amplifier
LS
Driver
TG
SW
BG
SS
SS
PGND
FB
12X
PG
V PG
Current Sense
Amplifer
SENSE+
SENSE-
SGND
COMP
CCM/AAM
Figure 1: Functional Block Diagram
MP2918 Rev. 1.02
5/31/2017
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MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
TIME SEQUENCE
VIN
0
SW
0
EN
Threshold
EN
0
VCC1
Threshold
VCC1
110% REF
0
107% REF
105% REF
90%REF
62.5% REF
VO
0
15µs
115% REF
90% REF
SS
IL=ILi mit
IL=ILi mit
IL= Valley current limit
IL
0
IL = Reverse current limit
37µs
PG
28µ s
37µ s
28µs
Start-Up
N o r m al
OCP
N o r m al
28µs
37µ s
0
OV
N or mal
Shutdown
OC
Release
Figure 2: Time Sequence
MP2918 Rev. 1.02
5/31/2017
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17
MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
OPERATION
The MP2918 is a high-performance, step-down,
synchronous, DC/DC controller IC with a wide
input voltage range. It implements current-mode
control and programmable switching frequency
control architecture to regulate the output
voltage with external N-channel MOSFETs.
The MP2918 senses the voltage at FB. The
difference between the FB voltage (VFB) and an
internal 0.8V reference (VREF) is amplified to
generate an error voltage on COMP. This is
used as the threshold for the current-sense
comparator with a slope compensation ramp.
Under normal-load conditions, the controller
operates in full pulse-width modulation (PWM)
mode (see Figure 3). At the beginning of each
oscillator cycle, the top gate driver is enabled.
The top gate turns on for a period determined
by the duty cycle. When the top gate turns off,
the bottom gate turns on after a dead time and
remains on until the next clock cycle begins.
There is an optional power-save mode for lightload or no-load condition.
Advanced Asynchronous Mode (AAM)
The MP2918 employs advanced asynchronous
mode (AAM) functionality to optimize efficiency
during light-load or no-load condition (see
Figure 3). AAM is enabled when CCM/AAM is
at a low level by connecting an appropriate
resistor to SGND to ensure that the AAM
voltage (VAAM) is no less than 480mV. See
Equation (1):
VAAM (mV) = IAAM (μA) x RAAM (kΩ)
(1)
Where IAAM is the CCM/AAM output current.
AAM is disabled when CCM/AAM is floating or
connected to VCC1. Calculate the CCM/AAM
output current (IAAM) with Equation (2):
IAAM (μA) = 600 (mV) / RFREQ (kΩ)
(2)
If AAM is enabled, the MP2918 first enters nonsynchronous operation for as long as the
inductor current approaches zero at light load. If
the load decreases further to make the COMP
voltage (VCOMP) drop below the CCM/AAM
voltage (VAAM), the MP2918 enters AAM. In
AAM, the internal clock resets whenever VCOMP
crosses over VAAM. The crossover time is taken
MP2918 Rev. 1.02
5/31/2017
as the benchmark for the next clock cycle.
When the load increases and the DC value of
VCOMP is higher than VAAM, the operation mode
is discontinuous conduction mode (DCM) or
continuous conduction mode (CCM), which
have a constant switching frequency.
Inductor
Current
Inductor
Current
Forced CCM
AAM
t
t
Load
Load
Decreased
tDecreased
t
t
t
Figure 3: Forced CCM and AAM
Floating Driver and Bootstrap Charging
The floating top gate driver is powered by an
external bootstrap capacitor (CBST), which is
refreshed when the high-side MOSFET (HSFET) turns off, typically. This floating driver has
its own under-voltage lockout (UVLO)
protection. This UVLO’s rising threshold is
3.05V with a hysteresis of 170mV.
If the BST voltage is lower than the bootstrap
UVLO, the MP2918 enters constant-off-time
mode to ensure that the BST capacitor is high
enough to drive the HS-FET.
VCC1 Regulator and VCC2 Power Supply
Both the top and bottom MOSFET drivers and
most of the internal circuitries are powered by
the VCC1 regulator. An internal, low dropout,
linear regulator supplies VCC1 power from VIN.
Connect a ≥1μF ceramic capacitor from VCC1
to PGND.
If VCC2 is left open or connected to a voltage
less than 4.7V, an internal 5V regulator supplies
power to VCC1 from VIN. If VCC2 is greater
than 4.7V, the internal regulator that supplies
power to VCC1 from VCC2 is triggered. If
VCC2 is between 4.7V and 5V, the 5V regulator
is in dropout, and VCC1 approximately equals
VCC2. Using the VCC2 power supply allows the
VCC1 power to be derived from a highefficiency external source, such as one of the
MP2918’s switching regulator outputs.
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MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
Error Amplifier (EA)
The error amplifier (EA) compares VFB with the
internal 0.8V reference and outputs a current
proportional to the difference between the two
input voltages. This output current is used to
charge or discharge the external compensation
network to form VCOMP, which is used to control
the power MOSFET current. Adjusting the
compensation network from COMP to SGND
optimizes the control loop for good stability or
fast transient response.
Current Limit Function
The MP2918 has three fixed current limit
options: 25mV, when ILIM is connected to
SGND; 50mV, when ILIM is connected to VCC1;
and 75mV, when ILIM is floating.
When the peak value of the inductor current
exceeds the set current-limit threshold, the
output voltage begins dropping until FB is
37.5% below the reference. The MP2918 enters
hiccup mode to restart the part periodically. The
frequency is lowered when FB is below 0.4V.
This protection mode is especially useful when
the output is dead-shorted to ground. The
average short-circuit current is reduced greatly
to alleviate thermal issues. The MP2918 exits
hiccup mode once the over-current condition is
removed.
Low Dropout Operation
In low dropout mode, the MP2918 is designed
to operate in high-side fully on mode for as long
as the voltage difference across BST - SW is
greater than 3.05V, improving dropout. When
the voltage from BST to SW drops below 3.05V,
a UVLO circuit turns off the HS-FET. At the
same time, the low-side MOSFET (LS-FET)
turns on to refresh the charge on the BST
capacitor. After the BST capacitor voltage is recharged, the HS-FET turns on again to regulate
the output. Since the supply current sourced
from the BST capacitor is low, the HS-FET can
remain on for more switching cycles than are
required to refresh the BST capacitor,
increasing the effective duty cycle of the
switching regulator. Low dropout operation
makes the MP2918 suitable for automotive
cold-crank applications.
MP2918 Rev. 1.02
5/31/2017
Power Good (PG) Function
The MP2918 includes an open-drain power
good (PG) output that indicates whether the
regulator’s output is within ±10% of its nominal
value. When the output voltage falls outside of
this range, the PG output is pulled low. PG
should be connected to a voltage source no
more than 5V through a resistor (e.g.: 100kΩ).
The PG rising delay time is 37µs, and the PG
falling delay time is 28µs.
Soft Start (SS)
Soft start (SS) is implemented to prevent the
converter output voltage from overshooting
during start-up. When the chip starts up, the
internal circuitry generates a soft-start voltage
that ramps up from 0V to 1.2V. When it is lower
than REF, SS overrides REF, so the error
amplifier uses SS as the reference. When SS is
higher than REF, REF regains control.
An external capacitor connected from SS to
SGND is charged from an internal 4μA current
source, producing a ramped voltage. The softstart time (tSS) is set by the external SS
capacitor and can be calculated by Equation (3):
t SS ms
C SS nF VREF V
ISS μA
(3)
Where CSS is the external SS capacitor, VREF is
the internal reference voltage (0.8V), and ISS is
the 4μA SS charge current. There is no internal
SS capacitor. SS is reset when a fault
protection other than over-voltage protection
(OVP) occurs.
Output Over-Voltage Protection (OVP)
The output over-voltage is monitored by VFB. If
VFB is typically 15% higher than the reference,
the MP2918 enters discharge mode. The HSFET turns off, and the LS-FET turns on. The
LS-FET remains on until the reverse current
limit is triggered. The LS-FET then turns off,
and the inductor current increases to 0. The LSFET is turned on again after ZCD is triggered.
The MP2918 works in discharge mode until the
over-voltage condition is cleared.
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19
MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
EN/SYNC Control
The MP2918 has a dedicated enable
(EN/SYNC) control that uses a bandgapgenerated precision threshold of 1.22V. By
pulling EN/SYNC high or low, the IC can be
enabled or disabled. To disable the part,
EN/SYNC must be pulled low for at least 40µs.
Tie EN/SYNC to VIN through a resistor divider
(R16 and R17) to program the VIN start-up
threshold (see Figure 4). The EN/SYNC
threshold is 1.09V (falling edge), so the VIN
UVLO threshold is 1.09V x (1 + R16/R17).
Otherwise, if VIN ≤ 52V, EN/SYNC can be
connected to VIN directly. If VIN ≥ 52V, a ≥50kΩ
pull-up resistor is needed to prevent EN/SYNC
from breaking down.
VIN
R16
EN/
SYNC
R17
Figure 4: EN/SYNC Resistor Divider
Synchronize
The MP2918 can be synchronized to an
external clock ranging from 100kHz up to
1000kHz through EN/SYNC. The internal clock
rising edge is synchronized to the external clock
rising edge. The pulse width (both on and off) of
the external clock signal should be no less than
100ns.
Thermal Protection
Thermal protection prevents damage to the IC
from excessive temperatures. The die
temperature is monitored internally until the
thermal limit is reached. When the silicon die
temperature is higher than 170°C, the entire
chip shuts down. When the temperature is
lower than its lower threshold (typically 20°C),
the chip is enabled again.
Start-Up and Shutdown
If both VIN and EN/SYNC are higher than their
respective thresholds, the chip starts up. The
reference block starts first, generating stable
reference voltages and currents. The internal
regulator is then enabled. The regulator
provides a stable supply for the remaining
circuitries.
Three events can shut down the chip:
EN/SYNC low, VIN low, and thermal shutdown.
During the shutdown procedure, the signal path
is blocked first to avoid any fault triggering.
VCOMP and the internal supply rail are then
pulled down. The floating driver is not subject to
this shutdown command.
Pre-Bias Start-Up
If SS is less than FB at start-up, the output has
a pre-bias voltage, and neither TG nor BG is
turned on until SS is greater than FB.
Under-Voltage Lockout (UVLO)
Under-voltage lockout (UVLO) is implemented
to protect the chip from operating at an
insufficient input supply voltage. The MP2918
UVLO rising threshold is about 4.5V, while its
falling threshold is a consistent 3.7V.
MP2918 Rev. 1.02
5/31/2017
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20
MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
APPLICATION INFORMATION
Setting the Output Voltage
The external resistor divider is used to set the
output voltage (see Figure 5).
VOUT
R8
FB
R9
Figure 5: External Resistor Divider
If R8 is known, then R9 can be calculated with
Equation (4):
R9
R8
(4)
VOUT
1
0.8V
recommended value for RSENSE is between 7mΩ
and 50mΩ.
Programmable Switching Frequency
Consider different variables when choosing the
switching frequency. A high frequency
increases switching losses and gate charge
losses, while a low frequency requires more
inductance and capacitance, resulting in larger
real estate and higher cost. Setting the
switching frequency is a trade-off between
power loss and passive component size. In
noise-sensitive applications, the switching
frequency should be out of a sensitive
frequency band.
The MP2918’s frequency can be programmed
from 100kHz to 1000kHz with a resistor from
FREQ to SGND (see Table 2). The value of
RFREQ for a given operating frequency can be
calculated with Equation (6):
Table 1 lists the recommended feedback
resistor values for common output voltages.
Table 1: Resistor Selection for Common Output
Voltages
RFREQ (k)
20000
1
fs (kHz)
Table 2: Frequency vs. Resistor
VOUT (V)
R8 (kΩ)
R9 (kΩ)
Resistor (kΩ)
Frequency (kHz)
3.3
5
12
37.4 (1%)
63.4 (1%)
169 (1%)
12 (1%)
12 (1%)
12 (1%)
65
45.3
39
19
300
430
500
1000
Setting Current Sensing
The MP2918 has three fixed current limit
options: 25mV, when ILIM is connected to
SGND; 50mV, when ILIM is connected to VCC1;
and 75mV, when ILIM is floating. Ensure that
the application can deliver a full load of current
over the full operating temperature range when
setting ILIM.
(6)
VCC Regulator Connection
VCC1 can be powered from both VIN and VCC2.
If connecting VCC2 to an external power supply
to improve the overall efficiency, VCC2 should
be larger than 4.7V but smaller than 12V (see
Figure 6).
VIN
The current sense resistor (RSENSE) monitors the
inductor current. Its value is chosen based on
the current limit threshold. The relationship
between the peak inductor current (Ipk) and
RSENSE can be calculated with Equation (5):
V
R SENSE ILIMIT
Ipk
IN
VCC1
VCC2
LDO
C Vcc
MP2918
INTERNAL
Ext .
Power
Supply
(5)
A higher RSENSE value increases the power loss
across it. Considering the output current,
efficiency,
and
ILIM
threshold,
the
MP2918 Rev. 1.02
5/31/2017
C IN
4.5V
Figure 6: Internal Circuitry of VCC2
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21
MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
If VOUT is higher than 4.7V but less than 12V,
VCC2 can be connected to VOUT directly (see
Figure 7).
current capability. The RMS value of the ripple
current flowing through the input capacitor can
be calculated with Equation (9):
VIN
IL
Rsense
C IN
IRMS =ILOAD
VOUT
CO
IN
VCC1
C Vcc
IRMS = ILOAD/2
(10)
The input capacitor must be capable of
handling this ripple current.
4.5V
Figure 7: Configuration of VCC2 Connecting to
VOUT
Selecting the Inductor
An inductor with a DC current rating at least
25% higher than the maximum load current is
recommended for most applications. A largervalue inductor results in less ripple current and
a lower output ripple voltage. However, the
larger-value inductor also has a larger physical
size, higher series resistance, and lower
saturation current. Choose the inductor ripple
current to be approximately 30% of the
maximum load current. The inductance value
can then be calculated with Equation (7):
L
(9)
The worst-case condition occurs at VIN = 2VOUT,
shown in Equation (10):
VCC2
LDO
MP2918
INTERNAL
VOUT
V
(1- OUT )
VIN
VIN
VOUT (VIN - VOUT )
VIN ΔIL fS
(7)
Output Capacitor Selection
The output capacitor impedance should be low
at the switching frequency. The output voltage
ripple can be estimated with Equation (11):
ΔVOUT
(11)
VOUT VOUT
1
1
RESR
fS L
VIN
8 fS CO
Where CO is the output capacitance value, and
RESR is the equivalent series resistance (ESR)
value of the output capacitor.
For
tantalum
or
electrolytic
capacitor
applications,
the
ESR
dominates
the
impedance at the switching frequency. The
output voltage ripple can be approximated with
Equation (12):
ΔVOUT
VOUT VOUT
1
RESR
fS L
VIN
(12)
Where VOUT is the output voltage, VIN is the
input voltage, fS is the 300kHz switching
frequency, and ∆IL is the peak-to-peak inductor
ripple current.
Power MOSFET Selection
Two N-channel MOSFETs must be selected for
the controller: one for the high-side switch, and
one for the low-side switch.
The maximum inductor peak current can be
calculated with Equation (8):
The driver level of the HS-FET and LS-FET is
5V, so the threshold voltage (Vth) of the
selected MOSFETs must be no higher than this
value.
IL(MAX) =ILOAD +
ΔIL
2
(8)
Where ILOAD is the load current.
Selecting the Input Capacitor
Since the input capacitor absorbs the input
switching current, it requires an adequate ripple
current rating. The selection of the input
capacitor is based mainly on its maximum ripple
MP2918 Rev. 1.02
5/31/2017
The input voltage (VDS), continuous drain
current (ID), on resistance (RDS(ON)), total gate
charge (Qg), and thermal-related parameters
should be considered when choosing the power
MOSFETs.
VDS of the chosen MOSFETs should exceed the
maximum applied voltage between the drain
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MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
and source in the application, which is VIN(MAX)
plus additional rings on the switch node.
The MOSFET’s power dissipations can be
calculated with Equation (13) and Equation (14):
VCC1
RBST
External BST diode
IN4148
BST
VCC1
PHS =IOUT
2
V
RON-HS OUT +
VIN
CBST
1
VIN IOUT t r +t f fSW
2
+Qg-HS fSW Vdriver
V
PLS =IOUT 2 RON-LS 1 OUT
VIN
Qg-LS fSW Vdriver +
L
VOUT
SW
COUT
(13)
Figure 8: External Bootstrap Diode and Resistor
The recommended external BST diode is
IN4148, and the recommended BST capacitor
value is 0.1µF to 1μF.
+
(14)
VDROP IOUT t dead1+t dead2 fSW
Where RON-HS and RON-LS are the on resistance
of the HS-FET and LS-FET, tr and tf are the
rising and falling time of the switch, Qg-HS and
Qg-LS are the total gate charge of the HS-FET
and LS-FET, Vdriver is the gate driver voltage
(which is provided by VCC1), VDROP is the LSFET body diode forward voltage, tdead1 is the
dead time between the HS-FET turning off and
the LS-FET turning on, and tdead2 is the dead
time between the LS-FET turning off and the
HS-FET turning on.
Ensure that the thermal caused by the power
loss on the MOSFETs does not exceed the
allowed maximum thermal of the selected
MOSFETs.
Schottky Selection
The diode between SW and PGND (shown as
D2 in Figure 12) is used to absorb spikes, store
charges during dead time, and protect the body
diode of the LS-FET. Considering the size and
power loss during the dead time, a 1 - 3A
Schottky diode is recommended.
BST Charge Diode and Resistor Selection
An external BST diode can enhance the
efficiency of the regulator when the duty cycle is
high. A power supply between 2.5V and 5V can
be used to power the external bootstrap diode.
VCC1 or VOUT is recommended to be this power
supply in the circuit (see Figure 8).
A resistor in series with the BST capacitor (RBST)
can reduce the SW rising rate and voltage
spikes. This helps enhance EMI performance
and reduce voltage stress at a high VIN. A
higher resistance is better for SW spike
reduction but compromises efficiency. To make
a tradeoff between EMI and efficiency, a ≤20Ω
RBST is recommended.
Compensation Components
The MP2918 employs current-mode control for
easy compensation and fast transient response.
COMP is the output of the internal error
amplifier and controls system stability and
transient response. A series resistor-capacitor
(R-C) combination sets a pole zero combination
to control the control system’s characteristics
(see Figure 9). The DC gain of the voltage
feedback loop can be calculated with Equation
(15):
A VDC RLOAD GCS A O
VFB
VOUT
(15)
Where AO is the error amplifier voltage gain
(3000V/V), GCS is the current sense
transconductance 1/(12xRSENSE) (A/V), and
RLOAD is the load resistor value.
COMP
C6
C7
R5
Figure 9: Compensation Network
MP2918 Rev. 1.02
5/31/2017
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MP2918—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER
The system has two important poles: one from
the compensation capacitor (C6) and the output
resistor of the error amplifier, and the other from
the output capacitor and the load resistor (see
Figure 9). These poles can be calculated with
Equation (16) and Equation (17):
Gm
fP1
2π C6 A O
fP2
1
2π Co R LOAD
(16)
(17)
The system has one important zero due to the
compensation capacitor and the compensation
resistor (R5), which can be calculated with
Equation (18):
1
2π C6 R5
(18)
The system may have another significant zero if
the output capacitor has a large capacitance or
high ESR value and can be calculated with
Equation (19):
fESR
1
2π Co R ESR
1
2π C7 R5
to
design
the
1. Choose R5 to set the desired crossover
frequency with Equation (21):
R5
2π Co fC VOUT
Gm GCS
VFB
(21)
2. Choose C6 to achieve the desired phase
margin. For applications with typical
inductor values, set the compensation zero
(fZ1)
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