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L6985F
38 V, 500 mA synchronous step-down switching regulator
with 30 µA quiescent current
Datasheet - production data
Description
HTSSOP16 (RTH = 40 °C/W)
Features
 0.5 A DC output current
 4 V to 38 V operating input voltage
 Low consumption mode or low noise mode
 30 µA IQ at light-load (LCM VOUT = 3.3 V)
 8 µA IQ-SHTDWN
 Output voltage adjustable from 0.85 V to VIN
 Adjustable fSW (250 kHz - 2 MHz)
 Embedded output voltage supervisor
 Synchronization
 Adjustable soft-start time
 Internal current limiting
 Overvoltage protection
The L6985F is a step-down monolithic switching
regulator able to deliver up to 0.5 A DC. The
output voltage adjustability ranges from 0.85V to
VIN. Thanks to the P-channel MOSFET high-side
power element, the device features 100% of the
duty cycle operation. The wide input voltage
range meets the specification for the 5 V, 12 V
and 24 V power supplies. The “Low Consumption
Mode” (LCM) is designed for applications active
during the idle mode, so it maximizes the
efficiency at the light-load with the controlled
output voltage ripple. The “Low Noise Mode”
(LNM) makes the switching frequency constant
overload current range, meeting the low noise
application specification. The output voltage
supervisor manages the reset phase for any
digital load (µC, FPGA). The RST open collector
output can also implement output voltage
sequencing during the power-up phase. The
synchronous rectification, designed for high
efficiency at the medium - heavy load, and the
high switching frequency capability make the size
of the application compact. Pulse-by-pulse
current sensing on both power elements
implements effective constant current protection.
 Output voltage sequencing
 Peak current mode architecture
 RDSON HS = 360 m, RDSON LS = 150 m
 Thermal shutdown
Applications
 Designed for 12 V and 24 V buses
 Programmable logic controllers (PLCs)
 Decentralized intelligent nodes
 Sensors and low noise applications (LNM)
February 2016
This is information on a product in full production.
DocID027836 Rev 2
1/65
www.st.com
Contents
L6985F
Contents
1
Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2
Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1
Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2
Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3
Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4
Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.5
ESD protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4
Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1
Power supply and voltage reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Switchover feature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2
Voltages monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.3
Soft-start and inhibit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3.1
Ratiometric startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.3.2
Output voltage sequencing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.4
Error amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.5
Light-load operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.6
4.7
4.5.1
Low noise mode (LNM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.5.2
Low consumption mode (LCM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Switchover feature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.6.1
LCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.6.2
LNM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Overcurrent protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
OCP and switchover feature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2/65
4.8
Overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.9
Thermal shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
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L6985F
5
6
Contents
Closing the loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
5.1
GCO(s) control to output transfer function . . . . . . . . . . . . . . . . . . . . . . . . 36
5.2
Error amplifier compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
5.3
Voltage divider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.4
Total loop gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
5.5
Compensation network design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Application notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
6.1
Output voltage adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
6.2
Switching frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
6.3
MLF pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6.4
Voltage supervisor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6.5
Synchronization (LNM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
6.6
Design of the power components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
6.6.1
Input capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
6.6.2
Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
6.6.3
Output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
7
Application board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
8
Efficiency curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
9
Package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
9.1
HTSSOP16 package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
10
Order codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
11
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
DocID027836 Rev 2
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65
Application schematic
1
L6985F
Application schematic
Figure 1. L6985F application schematic
4/65
DocID027836 Rev 2
L6985F
Pin settings
2
Pin settings
2.1
Pin connection
Figure 2. Pin connection (top view)
2.2
Pin description
Table 1. Pin description
No.
Pin
Description
1
RST
The RST open collector output is driven low when the output voltage is out of regulation. The RST
is released after an adjustable time DELAY once the output voltage is over the active delay
threshold.
2
VCC
Connect a ceramic capacitor (≥ 470 nF) to filter internal voltage reference. This pin supplies the
embedded analog circuitry.
3
SS/INH
An open collector stage can disable the device clamping this pin to GND (INH mode). An internal
current generator (4 A typ.) charges the external capacitor to implement the soft-start.
4
SYNCH/
ISKP
The pin features Master / Slave synchronization in LNM (see Section 6.5 on page 45) and skip
current level selection in LCM (see Section 4.5.2 on page 24).
5
FSW
A pull-up resistor (E24 series only) to VCC or pull-down to GND selects the switching frequency.
Pinstrapping is active only before the soft-start phase to minimize the IC consumption.
6
MLF
A pull-up resistor (E24 series only) to VCC or pull-down to GND selects the low noise mode/low
consumption mode and the active RST threshold. Pinstrapping is active only before the soft-start
phase to minimize the IC consumption.
7
COMP
Output of the error amplifier. The designed compensation network is connected at this pin.
8
DELAY
An external capacitor connected at this pin sets the time DELAY to assert the rising edge of the
RST o. c. after the output voltage is over the reset threshold. If this pin is left floating, RST is like
a Power Good.
9
FB
10
SGND
Signal GND
11
PGND
Power GND
Inverting input of the error amplifier
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65
Pin settings
L6985F
Table 1. Pin description (continued)
No.
Pin
12
PGND
13
LX
Switching node
14
LX
Switching node
15
VIN
DC input voltage
16
VBIAS
-
E. p.
2.3
Description
Power GND
Typically connected to the regulated output voltage. An external voltage reference can be used to
supply a part of the analog circuitry to increase the efficiency at the light-load. Connect to GND if
not used.
Exposed pad must be connected to SGND, PGND.
Maximum ratings
Stressing the device above the rating listed in Table 2: Absolute maximum ratings may
cause permanent damage to the device. These are stress ratings only and operation of the
device at these or any other conditions above those indicated in the operating sections of
this specification is not implied. Exposure to absolute maximum rating conditions may affect
device reliability.
Table 2. Absolute maximum ratings
Symbol
Description
Min.
Max.
Unit
VIN
-0.3
40
V
DELAY
-0.3
VCC + 0.3
V
PGND
SGND - 0.3
SGND + 0.3
V
SGND
V
VCC
-0.3
(VIN + 0.3) or (max. 4)
V
SS / INH
-0.3
VIN + 0.3
V
-0.3
VCC + 0.3
V
-0.3
VCC + 0.3
V
VOUT
-0.3
10
V
FSW
-0.3
VCC + 0.3
V
SYNCH
-0.3
VIN + 0.3
V
VBIAS
-0.3
(VIN + 0.3) or (max. 6)
V
RST
-0.3
VIN + 0.3
V
LX
-0.3
VIN + 0.3
V
-40
150
°C
MLF
COMP
See Table 1
TJ
Operating temperature range
TSTG
Storage temperature range
-65 to 150
°C
TLEAD
Lead temperature (soldering 10 sec.)
260
°C
IHS, ILS
High-side / low-side switch current
2
A
6/65
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L6985F
2.4
Pin settings
Thermal data
Table 3. Thermal data
Symbol
2.5
Parameter
Value
Unit
Rth JA
Thermal resistance junction ambient (device soldered on the
STMicroelectronics® demonstration board)
40
°C/W
Rth JC
Thermal resistance junction to exposed pad for board design
(not suggested to estimate TJ from power losses).
5
C/W
Value
Unit
HBM
2
kV
MM
200
V
CDM
500
V
ESD protection
Table 4. ESD protection
Symbol
ESD
Test condition
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65
Electrical characteristics
3
L6985F
Electrical characteristics
TJ = 25 °C, VIN = 12 V unless otherwise specified.
Table 5. Electrical characteristics
Symbol
Parameter
VIN
Operating input voltage
range
VINH
VINL
IPK
IVY
ISKIP
IVY_SNK
Test condition
Note Min.
Typ.
Max.
4
38
VCC UVLO rising threshold
2.7
3.5
VCC UVLO falling threshold
2.4
3.5
Peak current limit
Duty cycle < 20%
0.8
Duty cycle = 100%
closed loop operation
0.65
Valley current limit
Skip current limit
Reverse current limit
0.9
LCM, VSYNCH = GND
(1)
LCM, VSYNCH = VCC
(2)
LNM or VOUT overvoltage
0.15
0.35
0.5
0.1
0.5
1
2
ISW = 0.5 A
0.36
0.72
RDSON LS Low-side RDSON
ISW = 0.5 A
0.15
0.30
fSW
Selected switching
frequency
FSW pinstrapping before SS
IFSW
FSW biasing current
SS ended
IMLF
D
TON MIN
MLF biasing current
V
A
RDSON HS High-side RDSON
Low noise mode /
LCM/LNM Low consumption mode
selection
Unit
See Table 6: fSW selection
0
MLF pinstrapping before SS
500
nA
See Table 7 on page 11
SS ended
0
(2)
Duty cycle

0
Minimum On time
500
nA
100
%
80
ns
VCC regulator
VCC
LDO output voltage
SWO
VBIAS threshold
(3 V< VBIAS < 5.5 V)
8/65
VBIAS = GND (no switchover)
2.9
3.3
3.6
VBIAS = 5 V (switchover)
2.9
3.3
3.6
Switch internal supply from VIN to
VBIAS
2.85
3.2
Switch internal supply from VBIAS to
VIN
2.78
3.15
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V
L6985F
Electrical characteristics
Table 5. Electrical characteristics (continued)
Symbol
Parameter
Test condition
Note Min.
Typ.
Max.
Unit
4
8
15
A
4
10
15
Power consumption
ISHTDWN
Shutdown current from VIN
VSS/INH = GND
LCM - SWO
VREF < VFB < VOVP (SLEEP)
VBIAS = 3.3 V
IQ OPVIN
IQ OPVBIAS
Quiescent current from VIN
Quiescent current from
VBIAS
LCM - NO SWO
VREF < VFB < VOVP (SLEEP)
VBIAS = GND
(3)
A
(3)
35
70
120
LNM - SWO
VFB = GND (NO SLEEP)
VBIAS = 3.3 V
0.5
1.5
5
LNM - NO SWO
VFB = GND (NO SLEEP)
VBIAS = GND
2
2.8
6
25
50
115
A
LNM - SWO
VFB = GND (NO SLEEP)
VBIAS = 3.3 V
0.5
1.2
5
mA
SS rising
200
460
700
100
140
LCM - SWO
VREF < VFB < VOVP (SLEEP)
VBIAS = 3.3 V
mA
(3)
Soft-start
VINH
VSS threshold
VINH HYST VSS hysteresis
ISS CH
CSS charging current
VSS < VINH OR
t < TSS SETUP OR
VEA+ > VFB
t > TSS SETUP AND
VEA+ < VFB
VSS START
SSGAIN
(2)
mV
1
A
(2)
Start of internal error
amplifier ramp
4
0.995
SS/INH to internal error
amplifier gain
1.1
1.150
V
0.85
0.859
V
50
500
nA
3
Error amplifier
VOUT
Voltage feedback
IVOUT
VOUT biasing current
AV
ICOMP
0.841
(2)
Error amplifier gain
EA output current capability
100
±6
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±12
dB
±25
A
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65
Electrical characteristics
L6985F
Table 5. Electrical characteristics (continued)
Symbol
Parameter
Test condition
Note Min.
Typ.
Max.
Unit
Inner current loop
gCS
V PP  g CS
Current sense
transconductance (VCOMP to IPK = 0.5 A
inductor current gain)
(2)
Slope compensation
(4)
1.67
A/V
0.2
0.3
0.4
Overvoltage trip
(VOVP/VREF)
1.15
1.2
1.25
Overvoltage hysteresis
0.5
2
5
A
Overvoltage protection
VOVP
VOVP
HYST
%
Synchronization (fan out: 6 slave devices typ.)
fSYN MIN
VSYN TH
ISYN
VSYN OUT
Synchronization frequency
LNM; fSW = VCC
266.5
SYNCH input threshold
LNM, SYNCH rising
0.70
SYNCH pulldown current
LNM, VSYN = 1.2 V
High level output
LNM, 5 mA sinking load
Low level output
LNM, 0.7 mA sourcing load
Selected RST threshold
MLF pinstrapping before SS
kHz
1.2
0.7
V
mA
1.40
0.6
V
Reset
VTHR
VTHR HYST RST hysteresis
VRST
RST open collector output
See Table 7 on page 11
(2)
2
%
VIN > VINH AND VFB < VTH
4 mA sinking load
0.4
2 < VIN < VINH
4 mA sinking load
0.8
V
Delay
VTHD
RST open collector released
VFB > VTHR
as soon as VDELAY > VTHD
ID CH
CDELAY charging current
1.19 1.234 1.258
VFB > VTHR
1
2
3
V
A
Thermal shutdown
TSHDWN
Thermal shutdown
temperature
(2)
165
THYS
Thermal shutdown
hysteresis
(2)
30
°C
1. Parameter tested in static condition during the testing phase. Parameter value may change over dynamic application
condition.
2. Not tested in production.
3. LCM enables SLEEP mode at the light-load.
4. Measured at fsw = 250 kHz.
10/65
DocID027836 Rev 2
L6985F
Electrical characteristics
TJ = 25 °C, VIN = 12 V unless otherwise specified.
Table 6. fSW selection
Symbol
fSW
RVCC (E24 series)
RGND (E24 series)
Tj
fSW min.
fSW typ.
fSW max.
0
NC
225
250
275
1.8 k
NC
3.3 k
NC
5.6 k
NC
380
10 k
NC
435
NC
0
18 k
NC
33 k
NC
56 k
NC
755
NC
1.8 k
870
NC
3.3 k
NC
5.6 k
NC
10 k
NC
18 k
NC
33 k
285
330
(1)
450
500
550
575
660
(1)
900
kHz
1000
1100
1150
(1)
1310
1500(2)
56 k
NC
Unit
1575
1750(2)
1925
1800
2000(2)
2200
1. Not tested in production.
2. No synchronization as slave in LNM.
TJ = 25 °C, VIN = 12 V unless otherwise specified.
Table 7. LNM / LCM selection table
Symbol
VRST
RVCC
RGND
(E24 1%)
(E24 1%)
0
NC
8.2 k
NC
18 k
NC
39 k
Operating VRST/VOUT
mode
(tgt. value)
VRST
VRST
VRST
min.
typ.
max.
93%
0.779
0.791
0.802
80%
0.670
0.680
0.690
87%
0.728
0.740
0.751
NC
96%
0.804
0.816
0.828
NC
0
93%
0.779
0.791
0.802
NC
8.2 k
80%
0.670
0.680
0.690
NC
18 k
87%
0.728
0.740
0.751
NC
39 k
96%
0.804
0.816
0.828
LCM
LNM
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Unit
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65
Functional description
4
L6985F
Functional description
The L6985F device is based on a “peak current mode”, constant frequency control. As
a consequence, the intersection between the error amplifier output and the sensed inductor
current generates the PWM control signal to drive the power switch.
The device features LNM (low noise mode) that is forced PWM control, or LCM (low
consumption mode) to increase the efficiency at the light-load.
The main internal blocks shown in the block diagram in Figure 3 are:
12/65

Embedded power elements. Thanks to the P-channel MOSFET as a high-side switch
the device features a low dropout operation

A fully integrated sawtooth oscillator with adjustable frequency

A transconductance error amplifier

The high-side current sense amplifier to sense the inductor current

A “Pulse Width Modulator” (PWM) comparator and the driving circuitry of the
embedded power elements

The soft-start blocks to ramp the error amplifier reference voltage and so decreases the
inrush current at power-up. The SS/INH pin inhibits the device when driven low.

The switchover capability of the internal regulator to supply a portion of the quiescent
current when the VBIAS pin is connected to an external output voltage

The synchronization circuitry to manage a master / slave operation and the
synchronization to an external clock

The current limitation circuit to implement the constant current protection, sensing
a pulse-by-pulse high-side / low-side switch current. In case of heavy short-circuit the
current protection is fold back to decrease the stress of the external components

A circuit to implement the thermal protection function

The OVP circuitry to discharge the output capacitor in case of an overvoltage event

The MLF pin strapping sets the LNM/LCM mode and the thresholds of the RST
comparator

FSW pinstrapping sets the switching frequency

The RST open collector output
DocID027836 Rev 2
L6985F
Functional description
Figure 3. Internal block diagram
4.1
Power supply and voltage reference
The internal regulator block consists of a start-up circuit, the voltage pre-regulator that
provides the current to all the blocks and the bandgap voltage reference. The starter
supplies the startup current when the input voltage goes high and the device is enabled
(SS/INH pin over the inhibits threshold).
The pre-regulator block supplies the bandgap cell and the rest of the circuitry with
a regulated voltage that has a very low supply voltage noise sensitivity.
Switchover feature
The switchover scheme of the pre-regulator block features to derive the main contribution of
the supply current for the internal circuitry from an external voltage (3 V < VBIAS < 5.5 V is
typically connected to the regulated output voltage). This helps to decrease the equivalent
quiescent current seen at VIN. (Please refer to Section 4.6: Switchover feature on page 28).
4.2
Voltages monitor
An internal block continuously senses the VCC, VBIAS and VBG. If the monitored voltages are
good, the regulator starts operating. There is also a hysteresis on the VCC (UVLO).
DocID027836 Rev 2
13/65
65
Functional description
L6985F
Figure 4. Internal circuit
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4.3
Soft-start and inhibit
The soft-start and inhibit features are multiplexed on the same pin. An internal current
source charges the external soft-start capacitor to implement a voltage ramp on the SS/INH
pin. The device is inhibited as long as the SS/INH pin voltage is lower than the VINH
threshold and the soft-start takes place when the SS/INH pin crosses VSS START. (See
Figure 5).
The internal current generator sources a 1 A typ. current when the voltage of the VCC pin
crosses the UVLO threshold. The current increases to 4 A typ. as soon as the SS/INH
voltage is higher than the VINH threshold. This feature helps to decrease the current
consumption in inhibit mode. An external open collector can be used to set the inhibit
operation clamping the SS/INH voltage below the VINH threshold.
The startup feature minimizes the inrush current and decreases the stress of the power
components during the power-up phase. The ramp implemented on the reference of the
error amplifier has a gain three times higher (SSGAIN) than the external ramp present at the
SS/INH pin.
14/65
DocID027836 Rev 2
L6985F
Functional description
Figure 5. Soft-start phase
The CSS is dimensioned accordingly with Equation 1:
Equation 1
I SSCH  T SS
4A  T SS
C SS = SS GAIN  -------------------------------- = 3  --------------------------V FB
0.85V
where TSS is the soft-start time, ISS CH the charging current and VFB the reference of the
error amplifier.
The soft-start block supports the precharged output capacitor.
DocID027836 Rev 2
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65
Functional description
L6985F
Figure 6. Soft-start phase with precharged COUT
During the normal operation a new soft-start cycle takes place in case of:

Thermal shutdown event

UVLO event

The device is driven in INH mode
The soft-start capacitor is discharged with a 0.6 mA typ. current capability for 1 msec time
max. For complete and proper capacitor discharge in case of fault condition, a maximum
CSS = 67 nF value is suggested.
The application example in Figure 7 shows how to enable the L6985F and perform the softstart phase driven by an external voltage step.
Figure 7. Enable the device with external voltage step
16/65
DocID027836 Rev 2
L6985F
Functional description
The maximum capacitor value has to be limited to guarantee the device can discharge it in
case of a thermal shutdown and UVLO events (see Section 4.3.1), so restart the switching
activity ramping the error amplifier reference voltage.
Equation 2
– 1 msec
C SS  ------------------------------------------------------------------------------------------V SS_FINAL – 0.9 V
R SS_EQ  ln  1 – ----------------------------------------------

600 A – R SS_EQ
where:
Equation 3
R UP  R DWN
R SS_EQ = --------------------------------R UP + R DWN
R DWN
V SS_FINAL =  V STEP – V DIODE   ---------------------------------R UP + R DWN
The optional diode prevents to disable the device if the external source drops to ground.
RUP value is selected in order to make the capacitor charge at first approximation
independent from the internal current generator (4 A typ. current capability, see Table 5 on
page 8), so:
Equation 4
V STEP – V DIODE – V SS END
----------------------------------------------------------------------- » I SS CHARGE  4 A
R UP
where:
Equation 5
V FB
V SS END = V SS START + --------------------SS GAIN
represents the SS/INH voltage correspondent to the end of the ramp on the error amplifier
(see Figure 5); refer to Table 5 for VSS START, VFB and SSGAIN parameters.
As a consequence the voltage across the soft-start capacitor can be written as:
Equation 6
1
v SS  t  = V SS_FINAL  ----------------------------------------t
1–e
– --------------------------------C SS  R SS_EQ
RSS_DOWN is selected to guarantee the device stays in inhibit mode when the internal
generator sources 1 A typ. out of the SS/INH pin and VSTEP is not present:
Equation 7
R DWN  I SS INHIBIT  R DWN  1 A « V INH  200 mV
so:
Equation 8
R DWN  100 k
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Functional description
L6985F
RUP and RDWN are selected to guarantee:
Equation 9
V SS_FINAL  2 V  V SS_END
The time to ramp the internal voltage reference can be calculated from Equation 10:
Equation 10
V SS_FINAL – V SS START
T SS = C SS  R SS_EQ  ln  -----------------------------------------------------------
 V SS_FINAL – V SS END 
that is the equivalent soft-start time to ramp the output voltage.
Figure 8 shows the soft-start phase with the following component selection: RUP = 180 k,
RDWN = 33 k, CSS = 200 nF, the 1N4148 is a small signal diode and VSTEP = 13 V.
Figure 8. External soft-start network VSTEP driven
The circuit in Figure 7 introduces a time delay between VSTEP and the switching activity that
can be calculated as:
Equation 11
V SS_FINAL
T SS DELAY = C SS  R SS_EQ  ln  -----------------------------------------------------------
 V SS_FINAL – V SS START
18/65
DocID027836 Rev 2
L6985F
Functional description
Figure 9 shows how the device discharges the soft-start capacitor after an UVLO or thermal
shutdown event in order to restart the switching activity ramping the error amplifier reference
voltage.
Figure 9. External soft-start after UVLO or thermal shutdown
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Functional description
4.3.1
L6985F
Ratiometric startup
The ratiometric startup is implemented sharing the same soft-start capacitor for a set of the
L6985F devices.
Figure 10. Ratiometric startup
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As a consequence all the internal current generators charge in parallel the external
capacitor. The capacitor value is dimensioned accordingly with Equation 12:
Equation 12
I SSCH  T SS
4A  T SS
C SS = n L6985F  SS GAIN  -------------------------------- = n L6985F  3  --------------------------0.85V
V FB
where nL6985F represents the number of devices connected in parallel.
For better tracking of the different output voltages the synchronization of the set of
regulators is suggested.
20/65
DocID027836 Rev 2
L6985F
Functional description
Figure 11. Ratiometric startup operation
DocID027836 Rev 2
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Functional description
4.3.2
L6985F
Output voltage sequencing
The L6985F device implements sequencing connecting the RST pin of the master device to
the SS/INH of the slave. The slave is inhibited as long as the master output voltage is
outside of the regulation so implementing the sequencing (see Figure 12).
Figure 12. Output voltage sequencing
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High flexibility is achieved thanks to the programmable RST thresholds (see Table 7 on
page 11) and programmable delay time. To minimize the component count the DELAY pin
capacitor can be also omitted so the pin works as a normal Power Good.
4.4
Error amplifier
The voltage error amplifier is the core of the loop regulation. It is a transconductance
operational amplifier whose non inverting input is connected to the internal voltage
reference (0.85 V), while the inverting input (FB) is connected to the external divider or
directly to the output voltage.
Table 8. Uncompensated error amplifier characteristics
Description
Values
Transconductance
155 µS
Low frequency gain
100 dB
The error amplifier output is compared with the inductor current sense information to
perform PWM control. The error amplifier also determines the burst operation at the lightload when the LCM is active.
22/65
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L6985F
4.5
Functional description
Light-load operation
The MLF pinstrapping during the power-up phase determines the light-load operation (refer
to Table 7 on page 11).
4.5.1
Low noise mode (LNM)
The low noise mode implements a forced PWM operation over the different loading
conditions. The LNM features a constant switching frequency to minimize the noise in the
final application and a constant voltage ripple at fixed VIN. The regulator in steady loading
condition never skip pulses and it operates in the continuous conduction mode (CCM) over
the different loading conditions thus making this operation mode ideal for noise sensitive
applications.
Figure 13. Low noise mode operation
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Functional description
4.5.2
L6985F
Low consumption mode (LCM)
The low consumption mode maximizes the efficiency at the light-load. The regulator
prevents the switching activity whenever the switch peak current request is lower than the
ISKIP threshold (350 mA typical). As a consequence the L6985F device works in bursts and
it minimizes the quiescent current request in the meantime between the switching operation.
In the LCM operation, the pin SYNCH/ISKIP level dynamically defines the ISKIP current
threshold (see Table 9).
Table 9. ISKIP current level
SYNCH / ISKIP (pin 4)
ISKIP current threshold
LOW
ISKIPH = 0.35 A typical
HIGH
ISKIPL = 0.1 A typical
The ISKIP programmability helps to optimize the performance in terms of the output voltage
ripple or efficiency at the light-load, that are parameters which disagree each other by
definition.
A lower skip current level minimizes the voltage ripple but increases the switching activity
(time between bursts gets closer) since less energy per burst is transfered to the output
voltage at the given load. On the other side, a higher skip level reduces the switching activity
and improves the efficiency at the light-load but worsen the voltage ripple.
No difference in terms of the voltage ripple and conversion efficiency for the medium and
high load current level, that is when the device operates in the discontinuous or continuous
mode (DCM vs. CCM).
Figure 14 and Figure 15 report the efficiency measurements with VOUT = 3.3 V to highlight
the ISKIPH and ISKIPL efficiency gap at the light-load also in comparison with the LNM
operation. The same efficiency at the medium / high load is confirmed at different ISKIP
levels.
Figure 14. Light-load efficiency comparison at different ISKIP - linear scale
24/65
DocID027836 Rev 2
L6985F
Functional description
Figure 15. Light-load efficiency comparison at different ISKIP - log scale
Figure 16 and Figure 17 show the LCM operation at the different ISKIP level.
Figure 16 shows the ISKIPH = 350 mA typ. and so 34 mV output voltage ripple.
Figure 17 shows the ISKIPL = 100 mA typ. and so 13 mV output voltage ripple.
Figure 16. LCM operation with ISKIPH = 350 mA typ. at zero load
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Functional description
L6985F
Figure 17. LCM operation with ISKIPH = 100 mA typ. at zero load
Given the energy stored in the inductor during a burst, the voltage ripple depends on the
capacitor value:
Equation 13
T
V OUT RIPPLE
26/65
BURST
Q IL
0  iL  t   dt 
= -------------- = -------------------------------------------C OUT
C OUT
DocID027836 Rev 2
L6985F
Functional description
Figure 18. LCM operation over loading condition (part 1)
Figure 19. LCM operation over loading condition (part 2)
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Functional description
L6985F
Figure 20. The regulator works in CCM
4.6
Switchover feature
The switchover maximizes the efficiency at the light-load that is crucial for LCM applications.
4.6.1
LCM
The LCM operation satisfies the high efficiency requirements of the battery powered
applications. In order to minimize the regulator quiescent current request from the input
voltage, the VBIAS pin can be connected to an external voltage source in the range
3 V < VBIAS < 5.5 V (see Section 4.1: Power supply and voltage reference on page 13).
In case the VBIAS pin is connected to the regulated output voltage (VOUT), the total current
drawn from the input voltage can be calculated as:
Equation 14
V BIAS
1
I QVIN = I QOPVIN + --------------------  ---------------  I QOPVBIAS
V IN
 L6985F
where IQ OP VIN, IQ OP VBIAS are defined in Table 5: Electrical characteristics on page 8 and
L6985F is the efficiency of the conversion in the working point.
4.6.2
LNM
Equation 14 is also valid when the device works in LNM and it can increase the efficiency at
the medium load since the regulator always operates in the continuous conduction mode.
28/65
DocID027836 Rev 2
L6985F
4.7
Functional description
Overcurrent protection
The current protection circuitry features constant current protection, so the device limits the
maximum peak current (see Table 5: Electrical characteristics on page 8) in overcurrent
condition.
The L6985F device implements a pulse-by-pulse current sensing on both power elements
(high-side and low-side switches) for effective current protection over the duty cycle range.
The high-side current sensing is called “peak”, the low-side sensing “valley”.
The internal noise generated during the switching activity makes the current sensing
circuitry ineffective for a minimum conduction time of the power element. This time is called
“masking time” because the information from the analog circuitry is masked by the logic to
prevent erroneous detection of the overcurrent event. As a consequence, the peak current
protection is disabled for a masking time after the high-side switch is turned on, the valley
for a masking time after the low-side switch is turned on. In other words, the peak current
protection can be ineffective at extremely low duty cycles, the valley current protection at
extremely high duty cycles.
The L6985F device assures an effective overcurrent protection sensing the current flowing
in both power elements. In case one of the two current sensing circuitry is ineffective
because of the masking time, the device is protected sensing the current on the opposite
switch. Thus, the combination of the “peak” and “valley” current limits assure the
effectiveness of the overcurrent protection even in extreme duty cycle conditions.
The valley current threshold is designed higher than the peak to guarantee a proper
operation. In case the current diverges because of the high-side masking time, the low-side
power element is turned on until the switch current level drops below the valley current
sense threshold. The low-side operation is able to prevent the high-side turn on, so the
device can skip pulses decreasing the switching frequency.
DocID027836 Rev 2
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Functional description
L6985F
Figure 21. Valley current sense operation in overcurrent condition
Figure 21 shows the switching frequency reduction during the valley current sense
operation in case of an extremely low duty cycle (VIN = 38 V, fSW = 2 MHz short-circuit
condition).
In a worst case scenario (like in Figure 21) of the overcurrent protection the switch current is
limited to:
Equation 15
V IN – V OUT
I MAX = I VALLEYTH + ------------------------------  T MASKHS
L
where IVALLEY_TH is the current threshold of the valley sensing circuitry (see Table 5:
Electrical characteristics on page 8) and TMASK_HS is the masking time of the high-side
switch 100 nsec. typ.).
In most of the overcurrent conditions the conduction time of the high-side switch is higher
than the masking time and so the peak current protection limits the switch current.
Equation 16
IMAX = IPEAK_TH
30/65
DocID027836 Rev 2
L6985F
Functional description
Figure 22. Peak current sense operation in overcurrent condition
The DC current flowing in the load in overcurrent condition is:
Equation 17
I RIPPLE  V OUT 
V IN – V OUT
I DCOC  V OUT  = I MAX – ---------------------------------------- = I MAX –  ------------------------------  T ON


2
2L
DocID027836 Rev 2
31/65
65
Functional description
L6985F
OCP and switchover feature
Output capacitor discharging the current flowing to ground during heavy short-circuit events
is only limited by parasitic elements like the output capacitor ESR and short-circuit
impedance.
Due to parasitic inductance of the short-circuit impedance, negative output voltage
oscillations can be generated with huge discharging current levels (see Figure 23).
Figure 23. Output voltage oscillations during heavy short-circuit
inductor current
short-circuit current
switching node
regulated output voltage
32/65
DocID027836 Rev 2
L6985F
Functional description
Figure 24. Zoomed waveform
inductor current
short-circuit current
switching node
regulated output voltage
The VBIAS pin absolute maximum ratings (see Table 2: Absolute maximum ratings on
page 6) must be satisfied over the different dynamic conditions.
If the VBIAS is connected to GND there are no issues (see Figure 23 and Figure 24).
A small resistor value (few ) in series with the VBIAS can help to limit the pin negative
voltage (see Figure 25) during heavy short-circuit events if it is connected to the regulated
output voltage.
DocID027836 Rev 2
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Functional description
L6985F
Figure 25. VBIAS in heavy short-circuit event
inductor current
switching node
VBIAS pin voltage
(cyan)
regulated output voltage
(purple)
4.8
Overvoltage protection
The overvoltage protection monitors the FB pin and enables the low-side MOSFET to
discharge the output capacitor if the output voltage is 20% over the nominal value.
This is a second level protection and should never be triggered in normal operating
conditions if the system is properly dimensioned. In other words, the selection of the
external power components and the dynamic performance determined by the compensation
network should guarantee an output voltage regulation within the overvoltage threshold
even during the worst case scenario in term of load transitions.
The protection is reliable and also able to operate even during normal load transitions for
a system whose dynamic performance is not in line with the load dynamic request. As
a consequence the output voltage regulation would be affected.
Figure 26 shows the overvoltage operation during a negative steep load transient for
a system designed with huge inductor value and a small output capacitor. The inductor
value limits the switch current slew rate and the extra charge flowing into the small capacitor
value generates an overvoltage event. This can be considered as an example for a system
with a dynamic performance not in line with the load request.
The L6985F device implements a 1 A typ. negative current limitation to limit the maximum
reversed switch current during the overvoltage operation.
34/65
DocID027836 Rev 2
L6985F
Functional description
Figure 26. Overvoltage operation
inductor current
switching node
VBIAS pin voltage
(cyan)
regulated output voltage
(purple)
4.9
Thermal shutdown
The shutdown block disables the switching activity if the junction temperature is higher than
a fixed internal threshold (165 °C typical). The thermal sensing element is close to the
power elements, ensuring fast and accurate temperature detection. A hysteresis of
approximately 30 °C prevents the device from turning ON and OFF continuously. When the
thermal protection runs away a new soft-start cycle will take place.
DocID027836 Rev 2
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Closing the loop
5
L6985F
Closing the loop
Figure 27. Block diagram of the loop
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5.1
GCO(s) control to output transfer function
The accurate control to output transfer function for a buck peak current mode converter can
be written as:
Equation 18
G CO  s 
s
 1 + ----
 z
1
= R LOAD  g cs  --------------------------------------------------------------------------------------------------------  ----------------------  F H  s 
R LOAD  T SW
s

------
1 + -----------------------------------   m C   1 – D  – 0.5   1 + 
L
p
where RLOAD represents the load resistance, gCS the equivalent sensing conductance of the
current sense circuitry, p the single pole introduced by the power stage and z the zero
given by the ESR of the output capacitor.
FH(s) accounts the sampling effect performed by the PWM comparator on the output of the
error amplifier that introduces a double pole at one half of the switching frequency.
36/65
DocID027836 Rev 2
L6985F
Closing the loop
Equation 19
1
 z = --------------------------------ESR  C OUT
Equation 20
m c   1 – D  – 0.5
1
 p = --------------------------------------- + ---------------------------------------------L  C OUT  f SW
R LOAD  C OUT
where:
Equation 21
Se

 m C = 1 + -----Sn

S = V  g  f
PP
CS SW
 e

V
–
V
IN
OUT
 S = --------------------------- n
L
Sn represents the on time slope of the sensed inductor current, Se the on time slope of the
external ramp (VPP peak-to-peak amplitude) that implements the slope compensation to
avoid sub-harmonic oscillations at the duty cycle over 50%.
Se can be calculated from the parameter VPP × gCS given in Table 5 on page 8.
The sampling effect contribution FH(s) is:
Equation 22
1
F H  s  = --------------------------------------------2
s
s
1 + -------------------- + --------2n  Qp 
n
where:
Equation 23
1
Q p = -----------------------------------------------------------   m c   1 – D  – 0.5 
DocID027836 Rev 2
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65
Closing the loop
5.2
L6985F
Error amplifier compensation network
The typical compensation network required to stabilize the system is shown in Figure 28.
Figure 28. Transconductance embedded error amplifier
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RC and CC introduce a pole and a zero in the open loop gain. CP does not significantly affect
system stability but it is useful to reduce the noise at the output of the error amplifier.
The transfer function of the error amplifier and its compensation network is:
Equation 24
A V0   1 + s  R c  C c 
A 0  s  = --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------2
s  R0   C0 + Cp   Rc  Cc + s   R0  Cc + R0   C0 + Cp  + Rc  Cc  + 1
where Avo = Gm · Ro
The poles of this transfer function are (if Cc >> C0 + CP):
Equation 25
1
f PLF = ------------------------------------2    R0  Cc
38/65
DocID027836 Rev 2
L6985F
Closing the loop
Equation 26
whereas the zero is defined as:
1
f PHF = -------------------------------------------------------2    R0   C0 + Cp 
Equation 27
1
f Z = ------------------------------------2    Rc  Cc
5.3
Voltage divider
The simple voltage divider contribution is:
Equation 28
R2
G DIV  s  = -------------------R1 + R2
A small signal capacitor in parallel to the upper resistor (see Figure 29) of the voltage divider
implements a leading network (fzero < fpole), sometimes necessary to improve the system
phase margin.
Figure 29. Leading network example
Laplace transformer of the leading network:
Equation 29
 1 + s  R 1  C R1 
R2
G DIV  s  = --------------------  ------------------------------------------------------------R1 + R2 
R1  R2
1 + s  --------------------  C R1


R1 + R2
DocID027836 Rev 2
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65
Closing the loop
L6985F
where:
Equation 30
1
f Z = ----------------------------------------2    R 1  C R1
1
f p = ------------------------------------------------------R1  R2
2    --------------------  C R1
R1 + R2
fZ  fp
5.4
Total loop gain
In summary, the open loop gain can be expressed as:
Equation 31
G  s  = G DIV  s   G CO  s   A 0  s 
Example 1
VIN = 12 V VOUT = 3.3 V and ROUT = 6.6 
Selecting L = 4.7µH, COUT = 10 µF and ESR = 1 m, RC= 110 k, CC= 68 pF,
CP = 1.2 pF (please refer to Example 2), the gain and phase bode diagrams are plotted
respectively in Figure 30 and Figure 31.
Equation 32
BW = 108kHz
phase margin = 64
40/65
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0
L6985F
Closing the loop
Figure 30. Module plot
Figure 31. Phase plot
The solid trace represents the transfer function including the sampling effect term (see
Equation 22 on page 37), the dotted trace neglects the contribution.
DocID027836 Rev 2
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65
Closing the loop
5.5
L6985F
Compensation network design
The maximum bandwidth of the system can be designed up to fSW/6 or up to 150 kHz to
guarantee a valid small signal model.
Equation 33
f SW
BW = min  --------- 150kHz
 6

Equation 34
2    BW  C OUT  V OUT
R C = ---------------------------------------------------------------0.85V  g CS  g m TYP
where gCS represents the current sense transconductance (see Table 5: Electrical
characteristics on page 8) and gm TYP the error amplifier transconductance.
Equation 35
5
C C = -------------------------------------2    R C  BW
Example 2
Considering L6985F, VIN = 12 V, VOUT = 3.3 V, L = 4.7 H, COUT = 10 F, fSW = 2 MHz.
Assuming to design the compensation network to achieve system bandwidth of 110 kHz:
Equation 36
··
f POLE = 2. 4kHz
Equation 37
V OUT
R LOAD = -------------- = 6.6
I OUT
so accordingly with Equation 34 and Equation 35:
Equation 38
R C = 107k  110k
Equation 39
C C = 66pF  68pF
42/65
DocID027836 Rev 2
L6985F
Application notes
6
Application notes
6.1
Output voltage adjustment
The internal reference is equal to 0.85 V typical, and the output voltage is adjusted
accordingly to Equation 40 (see Figure 32):
Equation 40
R1
R1
V OUT = V REF   1 + ------- = 0.85   1 + -------
R2
R2
where the Cr1 capacitor is sometimes useful to increase the small signal phase margin
(please refer to Section 5.5: Compensation network design).
Figure 32. L6985F application circuit
6.2
Switching frequency
A resistor connected to the FSW pin features the selection of the switching frequency. The
pinstrapping is performed at power-up, before the soft-start takes place. The FSW pin is
pinstrapped and then driven floating in order to minimize the quiescent current from VIN.
Please refer to Table 6: fSW selection on page 11 to identify the pull-up / pull-down resistor
value. fSW = 250 kHz / fSW = 500 kHz preferred codifications don't require any external
resistor.
DocID027836 Rev 2
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65
Application notes
6.3
L6985F
MLF pin
A resistor connected to the MLF pin features the selection of the between low noise mode /
low consumption mode and the different RST thresholds. The pinstrapping is performed at
power-up, before the soft-start takes place. The FSW pin is pinstrapped and then driven
floating in order to minimize the quiescent current from VIN.
Please refer to Table 7 on page 11 to identify the pull-up / pull-down resistor value. (LNM,
RST threshold 93%) / (LCM, RST threshold 93%) preferred codifications don't require any
external resistor.
6.4
Voltage supervisor
The embedded voltage supervisor (composed of the RST and the DELAY pins) monitors the
regulated output voltage and keeps the RST open collector output in low impedance as long
as the VOUT is out of regulation. In order to ensure a proper reset of digital devices with
a valid power supply, the device can delay the RST assertion with a programmable time.
Figure 33. Voltage supervisor operation
The comparator monitoring the FB voltage has four different programmable thresholds
(80%, 87%, 93%, 96% nominal output voltage) for high flexibility (see Section 6.3: MLF pin
and Table 7 on page 11).
When the RST comparator detects the output, voltage is in regulation, a 2 A internal
current source starts to charge an external capacitor to implement a voltage ramp on the
DELAY pin. The RST open collector is then released as soon as VDELAY = 1.234 V (see
Figure 33).
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Application notes
The CDELAY is dimensioned accordingly with Equation 41:
Equation 41
I SSCH  T DELAY
2A  T DELAY
C DELAY = ------------------------------------------ = ------------------------------------V DELAY
1.234V
The maximum suggested capacitor value is 270 nF.
6.5
Synchronization (LNM)
Beating frequency noise is an issue when multiple switching regulators populate the same
application board. The L6985F synchronization circuitry features the same switching
frequency for a set of regulators simply connecting their SYNCH pin together, so preventing
beating noise. The master device provides the synchronization signal to the others since the
SYNCH pin is I/O able to deliver or recognize a frequency signal.
For proper synchronization of multiple regulators, all of them have to be configured with the
same switching frequency (see Table 6 on page 11), so the same resistor connected at the
FSW pin.
In order to minimize the RMS current flowing through the input filter, the L6985F device
provides a phase shift of 180° between the master and the SLAVES. If more than two
devices are synchronized, all slaves will have a common 180° phase shift with respect to
the master.
Considering two synchronized L6985F devices which regulates the same output voltage
(i.e.: operating with the same duty cycle), the input filter RMS current is optimized and is
calculated as:
Equation 42
I RMS
I
OUT
 -----------  2D   1 – 2D 
 2
= 
 I OUT
-   2D – 1    2 – 2D 
 ---------- 2
if D < 0.5
if D > 0.5
The graphical representation of the input RMS current of the input filter in the case of two
devices with a 0° phase shift (synchronized to an external signal) or a 180° phase shift
(synchronized connecting their SYNCH pins) regulating the same output voltage is provided
in Figure 34. To dimension the proper input capacitor please refer to Section 6.6.1: Input
capacitor selection on page 50.
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Application notes
L6985F
Figure 34. Input RMS current
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Figure 35 shows two regulators not synchronized.
Figure 35. Two regulators not synchronized
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Application notes
Figure 36 shows the same regulators working synchronized. The MASTER regulator
(LX_5V trace) delivers the synchronization signal (SYNCH_3V3, SYNCH_5V pins are
connected together) to the SLAVE device (LX_3V3). The SLAVE regulator works in phase
with the synchronization signal which is out of phase with the MASTER switching operation.
Figure 36. Two regulators synchronized
Multiple L6985F devices can be synchronized to an external frequency signal fed to the
SYNCH pin. In this case the regulator set is phased to the reference and all the devices will
work with a 0° phase shift.
The frequency range of the synchronization signal is 275 kHz - 1.4 MHz and the minimum
pulse width is 100 nsec (see Figure 37).
Figure 37. Synchronization pulse definition
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Since the slope compensation contribution that is required to prevent subharmonic
oscillations in peak current mode architecture depends on the switching frequency, it is
important to select the same oscillator frequency for all regulators (all of them operate as
SLAVE) as close as possible to the frequency of the reference signal (please refer to
Table 6: fSW selection on page 11). As a consequence all the regulators have the same
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Application notes
L6985F
resistor value connected to the FSW pin, so the slope compensation is optimized
accordingly with the frequency of the synchronization signal. The slope compensation
contribution is latched at power-up and so fixed during the device operation.
The L6985F normally operates in MASTER mode, driving the SYNCH line at the selected
oscillator frequency as shown in Figure 35 and Figure 36.
In SLAVE mode the L6985F sets the internal oscillator at 250 kHz typ. (see Table 6 on
page 11 - first row) and drives the line accordingly.
Figure 38. L6985F synchronization driving capability
In order to safely guarantee that each regulator recognizes itself in SLAVE mode during the
normal operation, the external master must drive the SYNCH pin with a clock signal
frequency higher than the maximum oscillator spread (refer to Table 6) for at least 10
internal clock cycles.
Example 3
Selecting RFSW = 0  to GND.
Table 10. Example of oscillator frequency selection from Table 6
Symbol
RVCC (E24 series)
RGND (E24 series)
fSW min.
fSW typ.
fSW max.
fSW
NC
0
450
500
550
The device enters into the slave mode after 10 pulses at frequency higher than 550 kHz and
so it is able to synchronize to a clock signal in the range 275 kHz - 1.4 MHz (see Figure 37).
Anyway it is suggested to limit the frequency range within ± 20% fSW resistor nominal
frequency (see details in text below). If a not spread spectrum is required, all the regulators
synchronize to a frequency higher to the maximum oscillator spread (550 kHz in Example
3).
The device keeps operating in slave mode as far as the master is able to drive the SYNCH
pin faster than 275 kHz (maximum oscillator spread for 250 kHz oscillator), otherwise it goes
back into MASTER mode at the nominal oscillator frequency after successfully driving one
pulse at 250 kHz (see Figure 39) in the SYNCH line.
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Application notes
Figure 39. Slave to master mode transition
switching node
SLAVE mode
250kHz typ. standalone operation at nominal fsw
SYNCH signal
The external master can force a latched SLAVE mode driving the SYNCH pin low at powerup, before the soft-start starts the switching activity. So the oscillator frequency is 250 kHz
typ. fixed until a new UVLO event is triggered regardless FSW resistor value, that otherwise
counts to design the slope compensation. The same considerations above are also valid.
The master driving capability must be able to provide the proper signal levels at the SYNCH
pin (see Table 5 on page 8 - Synchronization section):
 Low level < VSYN THL= 0.7 V sinking 5 mA
 High level > VSYN THH = 1.2 V sourcing 0.7 mA
Figure 40. Master driving capability to synchronize the L6985F
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Application notes
L6985F
As anticipated above, in the SLAVE mode the internal oscillator operates at 250 kHz typ. but
the slope compensation is dimensioned accordingly with FSW resistors so, even if the
L6985F supports synchronization over the 275 kHz - 1.4 MHz frequency range, it is
important to limit the switching operation around a working point close to the selected
frequency (FSW resistor).
As a consequence, to guarantee the full output current capability and to prevent the
subharmonic oscillations the master must limit the driving frequency range within ± 20% of
the selected frequency.
A wider frequency range may generate subharmonic oscillation for duty > 50% or limit the
peak current capability (see IPK parameter in Table 5 on page 8) since the internal slope
compensation signal may be saturated.
In order to guarantee the synchronization as a slave over distribution, temperature and the
output load, the external clock frequency must be lower than 1.4 MHz.
6.6
Design of the power components
6.6.1
Input capacitor selection
The input capacitor voltage rating must be higher than the maximum input operating voltage
of the application. During the switching activity a pulsed current flows into the input capacitor
and so its RMS current capability must be selected accordingly with the application
conditions. Internal losses of the input filter depends on the ESR value so usually low ESR
capacitors (like multilayer ceramic capacitors) have a higher RMS current capability. On the
other hand, given the RMS current value, lower ESR input filter has lower losses and so
contributes to higher conversion efficiency.
The maximum RMS input current flowing through the capacitor can be calculated as:
Equation 43
D D
I RMS = I OUT   1 – ----  ---
 
Where IOUT is the maximum DC output current, D is the duty cycles, is the efficiency. This
function has a maximum at D = 0.5 and, considering  = 1, it is equal to IOUT/2.
In a specific application the range of possible duty cycles has to be considered in order to
find out the maximum RMS input current. The maximum and minimum duty cycles can be
calculated as:
Equation 44
V OUT + V LOWSIDE
D MAX = -----------------------------------------------------------------------------------------------V INMIN + V LOWSIDE – V HIGHSIDE
Equation 45
V OUT + V LOWSIDE
D MIN = -------------------------------------------------------------------------------------------------V INMAX + V LOWSIDE – V HIGHSIDE
Where VHIGH_SIDE and VLOW_SIDE are the voltage drops across the embedded switches.
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L6985F
Application notes
The peak-to-peak voltage across the input filter can be calculated as:
Equation 46
I OUT
D D
V PP = -------------------------   1 – ----  ---- + ESR   I OUT + I L 
C IN  f SW 
 
In case of negligible ESR (MLCC capacitor) the equation of CIN as a function of the target
VPP can be written as follows:
Equation 47
I OUT
D D
C IN = --------------------------   1 – ----  ---V PP  f SW 
 
Considering  = 1 this function has its maximum in D = 0.5:
Equation 48
I OUT
C INMIN = ---------------------------------------------4  V PPMAX  f SW
Typically CIN is dimensioned to keep the maximum peak-to-peak voltage across the input
filter in the order of 5% VIN_MAX.
Table 11. Input capacitors
Manufacturer
TDK
Taiyo Yuden
6.6.2
Series
Size
Cap value (F)
Rated voltage (V)
C3225X7S1H106M
1210
10
50
C3216X5R1H106M
1206
UMK325BJ106MM-T
1210
Inductor selection
The inductor current ripple flowing into the output capacitor determines the output voltage
ripple (please refer to Section 6.6.3). Usually the inductor value is selected in order to keep
the current ripple lower than 20% - 40% of the output current over the input voltage range.
The inductance value can be calculated by Equation 49:
Equation 49
V OUT
V IN – V OUT
I L = ------------------------------  T ON = --------------  T OFF
L
L
Where TON and TOFF are the on and off time of the internal power switch. The maximum
current ripple, at fixed VOUT, is obtained at maximum TOFF that is at a minimum duty cycle
(see Section 6.6.1: Input capacitor selection to calculate minimum duty).
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Application notes
L6985F
So fixing IL = 20% to 40% of the maximum output current, the minimum inductance value
can be calculated:
Equation 50
V OUT 1 – D MIN
L MIN = -------------------  ----------------------F SW
I LMAX
where fSW is the switching frequency 1/(TON + TOFF).
For example for VOUT = 3.3 V, VIN = 12 V, IOUT = 0.5 A and FSW = 2 MHz the minimum
inductance value to have IL = 30% of IOUT is about 8.2 µH.
The peak current through the inductor is given by:
Equation 51
I L
I L PK = I OUT + -------2
So if the inductor value decreases, the peak current (that has to be lower than the current
limit of the device) increases. The higher is the inductor value, the higher is the average
output current that can be delivered, without reaching the current limit.
In Table 12 some inductor part numbers are listed.
Table 12. Inductors
6.6.3
Manufacturer
Series
Inductor value (H)
Saturation current (A)
Coilcraft
XAL40xx
2.2 to 15
5.6 to 2.8
XAL50xx
2.2 to 22
9.2 to 3.6
XFL40xx
2.2 to 4.7
3.7 to 2.7
Output capacitor selection
The triangular shape current ripple (with zero average value) flowing into the output
capacitor gives the output voltage ripple, that depends on the capacitor value and the
equivalent resistive component (ESR). As a consequence the output capacitor has to be
selected in order to have a voltage ripple compliant with the application requirements.
The voltage ripple equation can be calculated as:
Equation 52
I LMAX
V OUT = ESR   I LMAX + --------------------------------------8  C OUT  f SW
Usually the resistive component of the ripple can be neglected if the selected output
capacitor is a multi layer ceramic capacitor (MLCC).
The output capacitor is important also for loop stability: it determines the main pole and the
zero due to its ESR. (See Section 5: Closing the loop on page 36 to consider its effect in the
system stability).
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L6985F
Application notes
For example with VOUT = 3.3 V, VIN = 12 V, IL = 0.25 A, fSW = 2 MHz (resulting by the
inductor value) and COUT = 10 F MLCC:
Equation 53
0 · 25
V OUT
I LMAX
 1

1 · 6mV
1
------------------  --------------  ------------------------------ =  --------  ---------------------------------------------- = ------------------- = 0.05%
V OUT V OUT C OUT  f SW
8

10F

2MHz
3.3
3.3


The output capacitor value has a key role to sustain the output voltage during a steep load
transient. When the load transient slew rate exceeds the system bandwidth, the output
capacitor provides the current to the load. In case the final application specifies a high slew
rate load transient, the system bandwidth must be maximized and the output capacitor has
to sustain the output voltage for time response shorter than the loop response time.
In Table 13 some capacitor series are listed.
Table 13. Output capacitors
Manufacturer
Series
Cap value (F)
Rated voltage (V)
ESR (m)
GRM32
22 to 100
6.3 to 25
<5
GRM31
10 to 47
6.3 to 25
<5
ECJ
10 to 22
6.3
<5
EEFCD
10 to 68
6.3
15 to 55
SANYO
TPA/B/C
100 to 470
4 to 16
40 to 80
TDK
C3225
22 to 100
6.3
<5
MURATA
PANASONIC
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Application board
7
L6985F
Application board
The reference evaluation board schematic is shown in Figure 41.
Figure 41. Evaluation board schematic (VOUT= 3.3 V)
51
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The additional input filter (C16, L3, C15, L2, C14) limits the conducted emission on the
power supply (refer to Figure 52 on page 61).
Table 14. Bill of material
Reference
Part number
C1, C10, C11
C2
Description
Manufacturer
Not mounted
C2012X7S2A105K
1 F - 0805 - 50 V - X7S - 10%
C3
470 nF - 10 V - 0603
C4
1.2 pF - 10 V - 0603
C5
100 nF - 10 V - 0603
C6
10 nF - 10 V - 0603
C7
2 pF - 10 V - 0603
C8
68 pF - 10 V - 0603
TDK
C9
CGA5L3X5R1H106K
10 F - 1206 - 50 V - X5R - 10%
TDK
C14, C15, C16
CGA5L3X5R1H475k
4.7 F - 1206 - 50 V - X7R - 10%
TDK
C13
UWD1H101MCQ1GS
100 F - 50 V - 20%
Nichicon
54/65
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R2
0  - 0603
R6
1 M - 1%- 0603
R4
56 k - 1% - 0603
DocID027836 Rev 2
(/%
L6985F
Application board
Table 14. Bill of material (continued)
Reference
Part number
Description
R7
240 k - 1% - 0603
R8
110 k - 1% - 0603
R9
82 k - 1% - 0603
R11
10  - 1% - 0603
R1, R3, R5, R10
Not mounted
Manufacturer
L1
XAL4030-472MEC
4.7 H
Coilcraft
L2
XAL4030-472MEC
4.7 H
Coilcraft
L3
MPZ2012S221A
EMC bead
TDK
J1
Open
J2
Open
J3
Closed
J4
Open
To adjust the ISKIP current level in LCM
operation. Leave open in LNM.
J5
U1
Switchover
L6985F
STMicroelectronics
DocID027836 Rev 2
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Application board
L6985F
The layout of the board is shown in Figure 42 and in Figure 43.
Figure 42. Top layer
Figure 43. Bottom layer
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Application board
Figure 44 and Figure 45 show the magnitude and phase margin Bode’s plots related to the
evaluation board presented in Figure 41 with the BOM of Table 14.
The small signal dynamic performance of the demonstration board is:
Equation 54
BW = 110kHz
phase margin = 64
0
Figure 44. Magnitude Bode’s plot of L6985F evaluation board
Figure 45. Phase margin Bode’s plot of L6985F evaluation board
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Efficiency curves
8
L6985F
Efficiency curves
Figure 46. L6985F efficiency curves: VIN = 13.5 V - VOUT = 3.3 V - fsw = 2 MHz
Figure 47. L6985F efficiency curves: VIN = 13.5 V - VOUT = 3.3 V - fsw = 2 MHz (log scale)
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Efficiency curves
Figure 48. L6985F efficiency curves: VIN = 24V - VOUT = 3.3 V - fsw = 2 MHz
Figure 49. L6985F efficiency curves: VIN = 24 V - VOUT = 3.3 V - fsw = 2 MHz (log scale)
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Efficiency curves
L6985F
Figure 50. L6985F efficiency curves: VIN = 13.5 V - VOUT = 5 V - fsw = 2 MHz
Figure 51. L6985F efficiency curves: VIN = 13.5 V - VOUT = 5 V - fsw = 2 MHz (log scale)
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Efficiency curves
Figure 52. L6985F efficiency curves: VIN = 24 V - VOUT = 5 V - fsw = 2 MHz
Figure 53. L6985F efficiency curves: VIN = 24 V - VOUT = 5 V - fsw = 2 MHz (log scale)
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Package information
9
L6985F
Package information
In order to meet environmental requirements, ST offers these devices in different grades of
ECOPACK® packages, depending on their level of environmental compliance. ECOPACK
specifications, grade definitions and product status are available at: www.st.com.
ECOPACK is an ST trademark.
9.1
HTSSOP16 package information
Figure 54. HTSSOP16 package outline
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Package information
.
Table 15. HTSSOP16 package mechanical data
Dimensions (mm)
Symbol
Min.
Typ.
Max.
A
1.20
A1
0.15
A2
0.80
b
0.19
0.30
c
0.09
0.20
D
4.90
5.00
5.10
D1
2.8
3
3.2
E
6.20
6.40
6.60
E1
4.30
4.40
4.50
E2
2.8
3
3.2
e
L
1.05
0.65
0.45
L1
k
1.00
0.60
0.75
1.00
0.00
aaa
8.00
0.10
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Order codes
10
L6985F
Order codes
Table 16. Order codes
Part numbers
Package
L6985F
Tube
HTSSOP16
L6985FTR
11
Packaging
Tape and reel
Revision history
Table 17. Document revision history
Date
Revision
05-May-2015
1
Initial release.
2
Updated Table 3: Thermal data on page 7 (added Rth JC).
Updated Table 6: fSW selection on page 11 (added note 2.
below table).
Updated Section 6.5: Synchronization (LNM) on page 45
(replaced value of “range” “2 MHz” by “1.4 MHz”, added text).
17-Feb-2016
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Changes
DocID027836 Rev 2
L6985F
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