STMICROELECTRONICS PM6644

PM6644
350 mA adjustable step-down regulator
Datasheet − production data
Features
■
4.5 V to 25 V input voltage range
■
Output voltage VOUT: fixed 3.47 V or adjustable
0.9 V to 8 V
■
350 mA valley current limit
■
Constant-on-time control
■
Programmable switching frequency
■
Pulse skipping mode (skip mode) at light loads
■
Independent EN signals
■
Latched OVP and UVP
DFN10
switching regulator can be programmed to
regulate a fixed value of 3.47 V or it can deliver an
adjustable voltage, depending on the FB pin setup.
Applications
■
Networking power supply
■
Portable applications
■
Microcontroller supply
■
Industrial supply
Description
The PM6644 is a 350 mA valley current limit stepdown regulator capable of delivering an
adjustable output voltage in the range between
0.9 V and 8 V. A fixed value of output voltage is
also available (3.47 V), saving the external
resistor divider. It is housed in a small DFN10 3x3
package. The switching regulator is based on
COT (constant-on-time) architecture, that assures
fast load transient response; the embedded
voltage feed-forward provides nearly constant
switching frequency operation. The pulse skipping
technique increases efficiency at very light load.
The switching frequency can be adjusted from
200 kHz to 600 kHz through a simple resistor. The
Table 1.
Device summary
Part number
Package
Packing
PM6644
DFN10
Tube
PM6644TR
DFN10
Tape and reel
June 2012
This is information on a product in full production.
Doc ID 023203 Rev 1
1/35
www.st.com
35
Contents
PM6644
Contents
1
2
Simplified application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1
Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2
Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3
Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.4
Recommended operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.5
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.6
Typical operating characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Device description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1
3
Output voltage set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1.2
Constant-on-time control (COT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1.3
PWM control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.1.4
Skip mode management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.1.5
Current sensing and current limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.1.6
Soft-start and soft-end . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1.7
Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1.8
Overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1.9
Undervoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1.10
VCC undervoltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1.11
VCC and BYP power management . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.1.12
3.3 V linear regulator section (REF3) . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.1.13
General fault management: thermal protection . . . . . . . . . . . . . . . . . . . 20
External component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.1.1
Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.1.2
Input capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.1.3
Output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.1.4
Maximum RMS output current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Typical application configuration VOUT = 8 V . . . . . . . . . . . . . . . . . . . . 25
4.1
2/35
2.1.1
Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.1
4
Switching regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Test set configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Doc ID 023203 Rev 1
PM6644
Contents
4.2
Characterization report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.3
Efficiency vs. load (VIN = 12 V, VOUT = 8 V) . . . . . . . . . . . . . . . . . . . . . . . 27
5
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
6
Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
7
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
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List of tables
PM6644
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.
Table 9.
Table 10.
Table 11.
Table 12.
Table 13.
Table 14.
4/35
Device summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Recommended operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Frequency configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Fault management summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
VCC and BYP management (EN pin > 2 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
Inductor part number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Input capacitor part numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Output capacitor part number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
DFN10 (3x3 mm) mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34
Doc ID 023203 Rev 1
PM6644
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Figure 34.
Figure 35.
VOUT = 3.47 V fixed configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Adjustable VOUT configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Pinout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Efficiency vs. load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Switching frequency vs. load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
VOUT vs. load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
VREF3 vs. output load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Shutdown VIN current vs. VIN (EN=0 V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
No load VIN current vs. VIN (EN=VIN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Power-up sequence, no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Power-up sequence, 69 Ω load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Soft-end, no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Soft-end, 69 Ω load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Load transient 0-200 mA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
VREF3 load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
VREF3 line regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
VREF3 line transient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Simplified block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
On-time generator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Constant-on-time controller architecture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Inductor current in skip mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Current waveforms in current limit conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Schematic and bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
No load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Load = 50 mA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Load = 100 mA. Switching frequency = 370 kHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Load = 300 mA. Switching frequency = 410 kHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Load step = 0 to 300 mA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Recommended layout - top layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Recommended layout - inner layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Recommended layout - bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
DFN10 (3x3 mm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
DFN10 (3x3 mm) footprint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
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Simplified application schematic
1
PM6644
Simplified application schematic
Figure 1.
VOUT = 3.47 V fixed configuration
6).
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Figure 2.
Adjustable VOUT configuration
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6/35
Doc ID 023203 Rev 1
PM6644
Simplified application schematic
Figure 3.
Pinout
1.1
Pin description
Table 2.
Pin description
Pin
Name
1
REF
Description
1.216 V internal reference voltage. Do not connect this pin to any external component.
2
FB
Feedback input for the switching section:
If this pin is connected to VCC, OUT operates at 3.47 V (Fixed mode).
If this pin is connected to a resistive divider from OUT to GND, OUT can be adjusted
from 0.9 V to 8 V.
3
TON
Switching frequency setting. Connect to VIN with a resistor to properly set the switching
frequency.
4
EN
ENABLE (EN) pin. The EN pin is used to enable both the switching regulator and
internal reference. Tie to ground to shut down the device. Apply 2.1 V or more for normal
operation. If the EN pin is not used for power sequencing, tie this pin to the VIN pin.
5
GND
6
SW
The SW pin is the switching node of the switching regulator with integrated power
MOSFETs. Connect this pin to the inductor.
7
VIN
Input voltage for the switching regulator. Bypass to GND with a 1-2.2 µF MLCC
capacitor. The VIN pin supplies current to the internal switching regulator and to the
integrated voltage generator that supplies VCC if BYP < 2.4 V.
8
VCC
Output of a regulator that supplies the main switching controller. Bypass to GND with a 1
µF capacitor. An integrated voltage generator regulates at 3.8 V (when VIN> = 6 V) if
BYP< 2.4 V. When BYP > 3.2 V, the integrated voltage generator shuts down and VCC is
connected to BYP through a MOSFET switch (see Figure 18 ).
9
BYP
VCC BYPASS pin (BYP). If BYP > 3.2 V, VCC is supplied by BYP through a MOSFET
switch. Bypass to GND with a 10-100 nF capacitor.
10
REF3
Integrated 3.3 V high accuracy reference voltage. Bypass to GND with a 100 nF
capacitor. VREF3 is the voltage at REF3 pin.
EXP PAD
EXP PAD
Power and signal ground connection.
Exposed pad. Connect to signal ground.
Doc ID 023203 Rev 1
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Simplified application schematic
PM6644
1.2
Absolute maximum ratings
Table 3.
Absolute maximum ratings
Parameter
Value
Unit
-0.3 to 35
V
-0.3 to VIN +0.3
V
-0.3 to 6
V
REF, FB to GND
-0.3 to VCC +0.3
V
REF3 to GND
-0.3 to BYP +0.3
V
2.25
W
±1000
V
VIN to GND
EN, SW, TON to GND
VCC, BYP
Power dissipation at Tamb = 25 °C
Maximum withstanding voltage range test condition: CDF-AECQ100002- “human body model” acceptance criteria: “normal performance”.
1.3
Thermal data
Table 4.
Thermal data
Symbol
Parameter
Value
Unit
Rthj-a
Thermal resistance junction to ambient
45
°C/W
Tj
Junction operating temperature range
-40 to 125
°C
1.4
Recommended operating conditions
Table 5.
Recommended operating conditions
Value
Symbol
Parameter
Unit
Min.
VIN
Input voltage range
EN
BYP
Typ.
Max.
4.5
25
V
EN voltage range
0
25
V
BYP operative voltage range
0
5
V
300
mA
300
mA
Switching regulator
embedded high-side
MOSFET(1)
RMS current capability
Switching regulator
embedded low-side
MOSFET
1. Refer to Section 3.1.4: Maximum RMS output current.
8/35
Doc ID 023203 Rev 1
PM6644
1.5
Simplified application schematic
Electrical characteristics
VIN=12 V, no load on REF3, EN=VIN, FB=VCC, R=1 MΩ between TON and VIN, Tj=25 °C
unless otherwise specified.
Table 6.
Electrical characteristics
Symbol
Parameter
Test condition
Min.
Typ.
Max.
Unit
3.40
3.47
3.54
V
0.92
V
Switching controller output accuracy
BYP
FB
Fixed output voltage valley
regulation
FB=VCC, no load
No load, BYP=3.47 V;
Adjustable output voltage
valley regulation (FB=ADJ) Tj=0 °C - 70 °C(1)
0.88
Current limit and zero crossing comparator
Valley current limit
350
380
SW voltage ramp slew rate = 40
V/ms
14
22
30
mA
REF3 output voltage
(VREF3 voltage)
BYP=3.47 V external voltage, no
load
3.2
3.3
3.4
V
Line regulation
BYP=3.47 V to 5 V, no load
2.3
6.3
mV/V
Load regulation
BYP=3.47 V external voltage, 0
mA < lload < 2 mA
500
VCC voltage
BYP< BYP falling threshold,
VIN> 6
Zero crossing current
threshold
mA
3.3 V voltage reference
REF3
µV
VCC supply
VCC
BYP falling threshold/
REF3 turn-off threshold
BYP
3.4
3.8
2.4
2.7
BYP rising threshold (i.e.
VCC=BYP)/ REF3 turn-on
threshold
4.2
V
V
2.9
3.2
V
Regulator bias currents
VIN
BYP
VIN shutdown current
EN=0 V
13
21
µA
VIN quiescent current with
BYP > BYP falling
threshold
BYP=3.47 V (not switching),
REF3@no load
26
35
µA
BYP quiescent current with
BYP=3.47 V (not switching),
BYP > BYP falling
REF3@no load
threshold
190
230
2.7
3
µA
Fault management
Rising edge of PVCC
VCC
V
VCC UVLO threshold
Falling edge of PVCC
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2.6
V
9/35
Simplified application schematic
Table 6.
PM6644
Electrical characteristics (continued)
Symbol
Parameter
Test condition
Min.
Typ.
Max.
Unit
FB
Overvoltage trip threshold
Referred to FB nominal
regulation point, BYP= 3.47 V
+14
+20
%
FB
Undervoltage threshold
Referred to FB nominal
regulation point, BYP= 3.47 V
62
73
%
Inputs and outputs
FB
FB logic level
EN
EN level
FB logic level to be in fixed
mode
VCC-0.8
V
All circuitry OFF
1.3
All circuitry ON
V
2.4
V
1. In the range Tj = 0 °C-70 °C limits are guaranteed by design and statistical analysis, not production tested. Production test
at Tj=25 °C.
1.6
Typical operating characteristics
FB = VCC, RTON = 1 MΩ, VIN = 12 V, EN = VIN, BYP connected to the switching regulator
output, no load unless specified. Measurements performed on the evaluation kit
(PM6644_DFN).
Figure 4.
Efficiency vs. load
Figure 5.
3WITCHINGFREQUENCY24/.-
3WITCHINGFREQUENCY24/.
&SW;+(Z=
%FFICIENCY ;=
Switching frequency vs. load
%FFICIENCY6).6
%FFICIENCY6).6
%FFICIENCY6).6
%FFICIENCY6).6
,/!$;!=
10/35
!-V
Doc ID 023203 Rev 1
,/!$;!=
!-V
PM6644
Simplified application schematic
Figure 6.
VOUT vs. load
Figure 7.
VREF3 vs. output load
62%&;6=
6OUT;6=
6OUT,OADREGULATION6).6
62%&636OUTLOAD6).6
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Figure 8.
,/!$;!=
!-V
Shutdown VIN current vs. VIN
(EN=0 V)
!-V
,/!$;!=
Figure 9.
No load VIN current vs. VIN
(EN=VIN)
6).OPERATINGCURRENT
%
%
%
%
"906OUT
"90'.$6OUT6
6).CURRENT;U!=
6).CURRENT;U!=
%
%
6).SHUTDOWNCURRENT
%
%
%
%
%
%
6).;6=
%
!-V
Figure 10. Power-up sequence, no load
6).;6=
!-V
Figure 11. Power-up sequence, 69 Ω load
Doc ID 023203 Rev 1
11/35
Simplified application schematic
PM6644
Figure 12. Soft-end, no load
Figure 13. Soft-end, 69 Ω load
Figure 14. Load transient 0-200 mA
Figure 15. VREF3 load regulation
2%&,OAD2EGULATION
62%&;6=
,OAD;!=
!-V
Figure 16. VREF3 line regulation
Figure 17. VREF3 line transient
2%&;6=
2%&,INE2EGULATION
"90;6=
!-V
12/35
Doc ID 023203 Rev 1
PM6644
Simplified application schematic
Figure 18. Simplified block diagram
BYP
VCC
3.8 V
generator
VIN
+
-
800mV
EN_ok
HS command
1.216V reference
VCC
REF
generator
EN
+
VCC_OK
2.1V (falling thr.)
Bias
reference
generator
EN
BIAS
+
EN
BIAS_OK
FB_REF=0.9V
Ith_bias
REF_OK
+
REF_OK
GND
-
LS gate
driver
3.2V (rising thr.)
Boot_fuses_OK
900mV
SW
BYP_ON
REF
-
Anticross conduction
HS gate
driver
LS mos
Not (BYP_ON)
BYP_ON
HS mos
REF3
3.3V reference
Vth_ref
LS command
SW-GND
GND
GND
@
EN
-
+
FB
Ton_end
TON
SMPS_enable
LS mos on
Ton_pulse
+
320mV
SS0, SS1
-
ToffMIN=500ns
EN
GND
SMPS_enable
Ton_start
0mV
EN
S
Q
R
Q
Ton_pulse
HS command
SMPS_enable
GND
1100mV
+
EN
-
2.4V (rising thr.)
Ton_end
+
Α*SW
@LS mos on
-
BYP
PWM
COMPARATOR
EN_ok
FB
FB_REF
EN
+
S
Q
R
Q
LS command
SMPS_enable
0mV
No Fault
SMPS_enable
120% of
FB _REF
SS0, SS1
FB
SMPS_enable
OVP
FB D
EN
UVP
OVERTEMP
BOOT_FUSES_OK
SS_end
EN_ok
EN
S1
Fault Management
No Fault ENB C
FB=0.9V
EN
FB
VCC-1V
+
+
EN
OVERTEMP
VCC_OK
-
-
150°C
Not(Boot_fuses_OK)
Die temperature
GND
UVP
GND
Multiplexer
S2
OVP
-
Sink mos
Ton_start
Boot_fuses_OK
+
SW
62% of
FB_REF
-
FB
EN
SW-GND
@LS mos on
+
Start up
VCC_OK
managment
+
FB=VCC
BOOT _FUSES _ OK
Boot_fuses_OK
AM11778v1
Doc ID 023203 Rev 1
13/35
Device description
2
PM6644
Device description
The PM6644 combines a 350 mA valley current limit step-down regulator with a high
accuracy 3.3 V voltage reference in a small DFN10 3x3 package.
The switching regulator is based on constant-on-time (COT) architecture. This type of
control offers a very fast load transient response with a minimum external component count.
The switching regulator can regulate 3.47 V in Fixed mode (the FB pin tied at VCC) or it can
deliver an adjustable voltage between 0.9 V and 8 V (the FB pin connected to the output
voltage rail through an external resistor divider).
The switching frequency can be adjusted from 200 kHz to 600 kHz by a resistor between
TON and the VIN pin.
The embedded input and output voltage feed-forward provides nearly constant switching
frequency operation.
A pulse skipping technique allows increasing efficiency at very light load.
The switching regulator has protection against overvoltage, undervoltage and overcurrent.
The power MOSFET and switching controller of the switching regulator are supplied by VCC
voltage. An integrated voltage generator from VIN > 6 V provides 3.8 V at the VCC pin when
the BYP pin < 2.4 V if BYP > 3.2 V, the integrated voltage generator is turned off and VCC is
connected to BYP through a MOSFET switch (switch-over function).
An integrated 3.3 V linear regulator (supplied by VCC) provides an accurate 3.3 V output
(REF3).
The PM6644 also provides protection against overtemperature, turning off both switching
regulator and 3.3 V reference.
2.1
Switching regulator
2.1.1
Output voltage set-up
The switching sections can be configured in several ways.
Output voltage is configured with the FB pin. If the FB pin is tied to VCC, the PM6644
regulates 3.47 V. Using an external resistor divider the output can be adjusted following this
equation:
Equation 1
Vout = 0.9V ⋅ ⎛ R1
-------- + 1⎞
⎝ R2
⎠
where R1, R2 are the resistors of the FB pin divider. REF is a voltage reference used to
internally generate the 0.9 V threshold used to set the output voltage of the switching
regulator.
2.1.2
Constant-on-time control (COT)
The PM6644 implements a pseudo-fixed frequency algorithm using the COT architecture.
14/35
Doc ID 023203 Rev 1
PM6644
Device description
The COT architecture bases its algorithm on the output ripple derived across the output
capacitor’s ESR. The controller has an internal on-time (TON) generator triggered on the
output voltage valley: when VOUT reaches the regulation value a new TON starts. The TON
duration is given by the following equation:
Equation 2
0.9V ⋅ R TON ⋅ C
T ON = --------------------------------------------V IN
where TON is the on-time duration, C is an integrated capacitance (9.3 pF typ.), RTON is the
resistor between the VIN and TON pins, VOUT is the sensed output voltage and VIN is the
input voltage (sensed at the VIN pin). Figure 19 shows how the on-time is generated.
Figure 19. On-time generator
TON
BYP
α=0.263
t
+
FB
TON
RTON
-
VIN
TON
C=9.3pF
t
On time start
AM11779v1
The duty cycle in a buck converter is:
Equation 3:
TON
VOUT
----------= D = -------------T SW
V IN
The switching frequency in continuous current mode (CCM) is:
Equation 4
V OUT
VOUT
D
fsw = ----------- = ----------------------------------------------- = --------------------------------------------V IN
0.9V ⋅ R TON ⋅ C
TON
---------------------------------------------0.9V ⋅ R TON ⋅ C
---------------------------------------------V IN
In order to reduce noise in TON generation, a further capacitance C1 may be added between
the TON pin and the GND pin. In this case the switching frequency is:
Doc ID 023203 Rev 1
15/35
Device description
PM6644
Equation 5
V OUT
V OUT
D - = ---------------------------------------------------------------- = ---------------------------------------------------------------fsw = ---------V IN
TON
0.9V ⋅ R TON ⋅ ( C + C 1 )
---------------------------------------------------------------0.9V ⋅ R TON ⋅ ( C + C1 )
----------------------------------------------------------------V IN
The switching frequency is theoretically constant, but in a real application it depends on
parasitic voltage drops that occur during the charging path (high-side switch resistance,
inductor resistance (DCR)) and discharging path (low-side switch resistance, DCR). As a
result, the switching frequency increases as a function of the load current. The following
table shows some examples of switching frequencies that can be selected through the TON
pin (C1 not mounted):
Table 7.
2.1.3
Frequency configurations
TON resistor
VOUT = 3.47 V frequency
VOUT = 0. 9 V frequency
R_TON
load = 200 mA (PWM mode)
load = 200 mA (PWM mode)
2M
245 kHz
1M
470 kHz
500 K
260 kHz
250 K
495 kHz
PWM control
Figure 20 shows the simplified schematic of the constant-on-time controller. The COT
architecture uses a minimum OFF-time (TOFFMIN = 500 ns typ.) to allow inductor valley
current sensing on the synchronous switch. A minimum on-time is also introduced to assure
the correct startup sequence.
An adaptive anti-cross conduction algorithm avoids current paths between VIN and GND
during switching transition.
16/35
Doc ID 023203 Rev 1
PM6644
Device description
Figure 20. Constant-on-time controller architecture
ToffMIN
VIN
Vout sense
PHASE - GND
+
320mV
-
S
Q
R
Q
PHASE
Α*Rdson*ILS
+
R1
Vout sense
ILS
- PWM
COMPARATOR
FB
Α*Rdson*ILS
R2
+
0.725
VREF
GND
0.9V
Ton generator
AM11793v1
The PM6644 has a one-shot generator that turns on the high-side MOSFET when the
following conditions are satisfied simultaneously:
●
The PWM comparator is high
●
The inductor valley current is below the current limit threshold
●
The minimum OFF-time has timed out
A slope proportional to the low-side MOSFET current (A * RDS(on) * ILS) is added at the input
of the PWM comparator in order to ensure stability. The slope determines a load line on the
output voltage of about 0.16 Ω when the controller works in PWM mode.
2.1.4
Skip mode management
To improve efficiency at light load, the PM6644 implements pulse skip operation mode. The
inductor current is sensed and, if it is equal to zero, the synchronous MOSFET is turned off.
As a consequence, the output capacitor is left floating and the discharge depends only on
the current sourced by the load. The new TON starts when the output reaches the voltage
regulation. As a consequence, at light load conditions the switching frequency decreases,
improving the total efficiency of the converter. Working in discontinuous current mode, the
switching and the conduction losses are reduced by skipping some cycles.
If the output load is high enough to make the system work in CCM (continuous conduction
mode), Skip mode is automatically changed into PWM mode.
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17/35
Device description
PM6644
Figure 21. Inductor current in skip mode
IL
Load> ∆IL/2
Load< ∆IL/2
IL
∆IL
∆IL
t
t
TSWDCM
TSWCCM
AM11781v1
2.1.5
Current sensing and current limit
The PM6644 implements a positive valley current limit to protect the application from an
overcurrent fault. The inductor current is sensed during the OFF-time TOFF by measuring
the voltage drop across the integrated low-side MOSFET using the MOSFET RDS(on) as a
lossless sensing element. The voltage drop is then compared with a fixed voltage threshold
so that the inductor (or low-side MOSFET) trip current of the comparator is about 350 mA
(minimum value).
A new switching cycle cannot start until the inductor current goes lower than the 350 mA
current limit threshold. As a result, the device can work with a maximum inductor RMS
current ILRMS (max) equal to:
Equation 6
∆I
I LRMS ( max ) = 350mA + -------L2
where ∆IL is the inductor current ripple.
Figure 22. Current waveforms in current limit conditions
Current
Inductor current
maximum DC current
350mA valley current
limit threshold
Time
AM11780v1
18/35
Doc ID 023203 Rev 1
PM6644
2.1.6
Device description
Soft-start and soft-end
The switching section has an EN pin. A non-programmable soft-start procedure takes place
when the EN pin rises above 2.1 V.
To prevent high input inrush currents, the current limit is increased from 25% to 100% of the
current limit threshold with steps of 25%.
The procedure is not programmable and ends typically in 2.8 ms. The overvoltage protection
is always active while the undervoltage protection is enabled at the end of the 2.8 ms.
Driving one EN pin below 1.8 V makes the section perform a soft-end: gate driving signals
are pulled low and the output is discharged through an internal MOSFET with RDS(on) of
50 Ω typ.
2.1.7
Monitoring
The PM6644 controls its switching output to prevent any damage or uncontrolled working
condition.
2.1.8
Overvoltage protection
The PM6644 provides a latched overvoltage protection (OVP). If the output voltage rises
above 120% of the nominal value, a latched OVP protection is activated. The controller turns
on the low-side MOSFET keeping the output voltage at 0 V.
The protection is latched and this fault is cleared by cycling VCC < 2.1 V and then > 3 V.
2.1.9
Undervoltage protection
If, during regulation, the output voltage drops under 62% of the nominal value, an
undervoltage latched fault is detected. The controller performs a soft-end procedure (see
Section 2.1.6 ). The undervoltage fault is reset by toggling the EN pin or by cycling VCC <
2.1 V and then > 3 V.
2.1.10
VCC undervoltage
The device monitors the voltage at the VCC pin. The switching section can start operating
only if the voltage at the PVCC pin is above 3 V. If PVCC falls below 2.1 V, the switching
section is turned off until PVCC voltage goes over 3 V.
Table 8.
Fault management summary
Fault
Condition
Device behavior
Overvoltage
VOUT > +120%
The low-side MOSFET is turned on keeping the
output voltage at 0 V. Latched fault, cleared
toggling EN or cycling VCC < 2.1 V and then > 3
V.
Undervoltage
VOUT < 62%
The controller performs a soft-end. Latched fault
cleared toggling EN or cycling VCC < 2.1 V and
then > 3 V.
VCC undervoltage
VCC < 2.1 V
The controller turns off the switching section until
PVCC voltage goes over 3 V. Not latched fault.
Doc ID 023203 Rev 1
19/35
Device description
2.1.11
PM6644
VCC and BYP power management
VCC supplies both the controller and the drivers of the integrated high-side and low-side
MOSFETs. An integrated 3.8 V generator from the VIN pin provides the voltage to the VCC
pin.
The PM6644 provides a switch-over function that allows the turning-off of the 3.8 V
generator when a voltage is applied at the BYP pin. If the voltage at the BYP pin is higher
than 3.2 V, the internal generator is turned off and the VCC pin is connected with an internal
switch (16 Ω typ.) to the BYP pin. This feature decreases the power dissipation of the device.
If BYP < 2.4 V, the internal switch is turned off and the VCC output is supplied with the 3.8 V
generator.
Table 9.
2.1.12
VCC and BYP management (EN pin > 2 V)
BYP
VIN
VCC
5 V generator
< 2.4 V
<6V
VIN- 1 V
Enabled
< 2.4 V
3.8 V
Enabled
> 3.2 V
BYP
Disabled
Switch-over
resistance
16 Ω
3.3 V linear regulator section (REF3)
The PM6644 has an integrated linear regulator (REF3) that can provide a maximum RMS
current of 5 mA. The input of the linear regulator is the BYP pin. The linear regulator is
turned on when BYP > 3.2 V. Connect pin REF3 with a 100 nF ceramic capacitor to GND.
2.1.13
General fault management: thermal protection
If the internal temperature of the device exceeds typically +150 °C, the controller shuts down
immediately all the internal circuitry. The switching section performs the soft-end
management. Toggling EN or cycling VCC < 2.1 V and then VCC > 3 V, resets the latched
fault.
20/35
Doc ID 023203 Rev 1
PM6644
Application information
3
Application information
3.1
External component selection
3.1.1
Inductor selection
Once the switching frequency is defined, inductor selection depends on the desired inductor
ripple current and load transient performance.
Low inductance means greater ripple current and may generate greater output noise. On
the other hand, low inductor values involve fast load transient response.
A good compromise between the transient response time, the efficiency, the cost and the
size, is to choose the inductor value in order to maintain the inductor current ripple ∆IL
between 20% and 50% of the maximum output current ILOAD(max.). The maximum ∆IL
occurs at the maximum input voltage. With these considerations, the inductor value can be
calculated with the following relationship:
Equation 7
V IN – V OUT V OUT
L = ------------------------------ ⋅ -------------f sw ⋅ ∆I L
V IN
where fSW is the switching frequency, VIN is the input voltage, VOUT is the output voltage and
∆IL is the selected inductor current ripple.
In order to prevent overtemperature working conditions, the inductor must be able to provide
an RMS current greater than the maximum RMS inductor current ILRMS:
Equation 8
2
I LRMS =
2 ( ∆I L ( max ) )
( I LOAD ( max ) ) + -------------------------------12
where ∆Ι L (max.) is the maximum current ripple:
Equation 9
VINmax – VOUT
V OUT
∆IL ( max ) = --------------------------------------- ⋅ ------------------f sw ⋅ L
V INmax
If hard saturation inductors are used, the inductor saturation current should be much greater
than the maximum inductor peak current Ipeak:
Equation 10
∆I L ( max )
Ipeak = I LOAD ( max ) + ------------------------2
Using soft saturation inductors it is possible to choose inductors with a saturation current
limit at nearly Ipeak. In Table 10 there is a list of some inductor part numbers.
Doc ID 023203 Rev 1
21/35
Application information
.
Table 10.
PM6644
Inductor part number
Part number
Inductance
(µH)
DCR (Ω)
RMS current
(A)(1)
Saturation
current (A)(2)
Coilcraft
EPL3015-333ML
33
0.989
0.59
0.32
Coilcraft
LPS3314-333ML
33
0.92
0.58
0.38
Coilcraft
MSS5121-333ML
33
0.48
0.76
0.64
Manufacturer
3.1.2
1.
40 °C temperature rise.
2.
20% inductance drop.
Input capacitor selection
In a buck topology converter the current that flows into the input capacitor is a pulsed current
with zero average value. The input RMS current of the switching regulator can be roughly
estimated as follows:
Equation 11
I CinRMS = I LOAD ( max ) ⋅
D ⋅ (1 – D)
where D is the duty cycles and ILOAD(max.) is the maximum load current of the switching
regulator. The input capacitor should be chosen with an RMS rated current higher than the
maximum RMS current given by the formula.
Tantalum capacitors are good in terms of low ESR and small size, but they can occasionally
burn out if subjected to very high current during the charge.
Ceramic capacitors usually have a higher RMS current rating with smaller size and they
remain the best choice. In battery-powered applications, a 1-2.2 µF input ceramic capacitor
can be enough.
Table 11 shows an example of ceramic capacitor part numbers.
Table 11.
3.1.3
Input capacitor part numbers
Manufacturer
Part number
Capacitor value (µF)
Rated voltage
TAYIO YUDEN
TMK212BJ225MG-T
2.2
25
Output capacitor selection
The controller can work with ceramic or tantalum output capacitors.
The selection of the output capacitor impacts on the stability of the controller:
Equation 12
18 ⋅ α
C OUT > ---------------------------------------2 ⋅ Π ⋅ f sw ⋅ k
Equation 13
0.9V
α = -------------V OUT
22/35
Doc ID 023203 Rev 1
PM6644
Application information
K is a constant (< 0.1) that defines the ratio between the controller bandwidth and the
switching frequency.
The output capacitor must store the inductor energy generating an output ripple within the
output voltage ripple requirements.
If an output tantalum capacitor is used, in CCM the voltage ripple VRIPPLEout is given by:
Equation 14
V RIPPLEout = R out ⋅ ∆I L
A low ESR capacitor is required to reduce the output voltage ripple.
If an output ceramic capacitor is used, in CCM the voltage ripple VRIPPLEout is given by:
Equation 15
Tsw ⋅ ∆I L
V RIPPLEout = -----------------------8 ⋅ C OUT
Finally the output capacitor choice heavily impacts the load transient response.
Table 12 shows a list of some capacitor part numbers.
Table 12.
Output capacitor part number
Part number
Capacitor value
(µF)
Rated voltage (V)
ESR max. (MΩ)
TAYIO YUDEN
JMK212BJ226MG-T
22
6.3
70
SANYO
POSCAP 6TPC33M
33
6.3
7 to 15
Manufacturer
3.1.4
Maximum RMS output current
Both high-side and low-side embedded power MOSFETs of the switching regulator can
withstand a maximum RMS current of 300 mA.
The maximum sustainable RMS output current ILOADRMS of the switching regulator depends
on the application specifications of:
●
input voltage VIN
●
output voltage VOUT
●
inductor current ripple ∆IL (that depends on the switching frequency FSW and on the
inductor value L, according to Equation 7 ).
The maximum RMS currents of high-side (IRMS,HS) and low-side (IRMS,LS) MOSFETs are
given by:
Equation 16
2
I RMS,
HS
= D ⋅ I LRMS = D ⋅
2 ( ∆I L )
( I LOAD ) + --------------- = 300mA
12
Doc ID 023203 Rev 1
23/35
Application information
PM6644
Equation 17
2
I RMS,
LS
= I LRMS = ( 1 – D ) ⋅
2 ( ∆I L )
( ILOAD ) + --------------- = 300mA
12
where ILOAD is the RMS output current.
The minimum ILOAD between equation 16 and equation 17, combined with RMS load current
limitation due to valley current limit (Equation 6 ), determines the maximum RMS output
current ILOADRMS sustained by the switching regulator:
Equation 18
ILOADRMS
⎧
⎪
⎪
⎪
⎪
= MIN ⎨
⎪
⎪
⎪
⎪
⎩
2 ( ∆I ) 2 ⎫
L ⎪
⎛ 300mA
-------------------⎞ – -----------⎝ D ⎠
12 ⎪
⎪
2 ( ∆I ) 2 ⎪
L
⎛ 300mA
⎞
------------------- – ------------- ⎬
⎝ 1–D ⎠
12 ⎪
⎪
∆IL
⎪
350mA + -------⎪
2
⎭
Example 1
VIN = 5 V, FB = VCC (VOUT = 3.47 V), ∆IL= 68.5 mA (L = 33 uH, Fsw = 470 kHz).
High-side can withstand a load current of ILOADRMS = 432 mA.
Low-side can withstand a load current of ILOADRMS = 977 mA.
ILOADRMS due to valley current limit = 384 mA.
As a result, ILOADRMS = 384 mA (limitation determined by the valley current limit).
The PM6644 switching regulator can source 384 mA RMS. 384 mA is also the peak load
current.
Example 2
VIN = 25 V, FB = VCC (VOUT = 3.47 V), ∆IL = 192.5 mA (L = 33 uH, Fsw = 470 kHz).
High-side can withstand a load current of ILOADRMS = 2164 mA.
Low-side can withstand a load current of ILOADRMS = 344 mA.
ILOADRMS due to valley current limit = 446 mA.
As a result, ILOADRMS = 344 mA (limitation determined by the low-side RMS max. current).
The PM6644 switching regulator can source 344 mA RMS. The peak load current is 446
mA.
24/35
Doc ID 023203 Rev 1
PM6644
4
Typical application configuration VOUT = 8 V
Typical application configuration VOUT = 8 V
This section is intended as a guideline for all the measurements involving the PM6644
device with VIN = 12 V ±5%, VOUT = 8 V.
4.1
Test set configuration
The device-under-test (D.U.T.) has been mounted on the designed evaluation kit with the
following schematic and bill of materials.
Figure 23. Schematic and bill of materials
#/-0/.%.4&//402).43
222
##
#
#
,MMXMM
0-MMXMM
/54
K
2
340-
#
P
2
2%&
&"
4/.
%.
'.$
%0
K
2
%0
2%&
"90
6##
6).
37
"90
6##
6).
*
/54
,
#
U6
6).
6).
*
6).
6).
6/54
#
U6
*
6IN6
&SWK(Z
U
U
,#/),#2!&4,03-,#
###*2!#45
#
*
6/54
6OUT6
-AXLOADM!
4.2
6).
!-V
Characterization report
Steady-state waveforms
Figure 24. No load
Figure 25. Load = 50 mA
Doc ID 023203 Rev 1
25/35
Typical application configuration VOUT = 8 V
PM6644
Figure 26. Load = 100 mA. Switching frequency = Figure 27. Load = 300 mA. Switching frequency =
370 kHz
410 kHz
Figure 28. Load step = 0 to 300 mA
26/35
Doc ID 023203 Rev 1
PM6644
Efficiency vs. load (VIN = 12 V, VOUT = 8 V)
The output voltage varies from 8.096 V at light load to 7.916 V at heavy load. Load
regulation = 180 mV.
Figure 29. Efficiency
%FFICIENCY ;=
,/!$;!=
!-V
Figure 30. Load regulation
6OUT;6=
4.3
Typical application configuration VOUT = 8 V
,/!$;!=
Doc ID 023203 Rev 1
!-V
27/35
Typical application configuration VOUT = 8 V
PM6644
Bias current
28/35
VIN =12.158 V
VIN = 12.158 V
Vout = 8.150 V (externally forced)
Vout = 8.107 V
No switching
No load, pulse skipping (820 Hz, double
pulses)
BYP = GND
BYP = GND
IVIN = 126 µA
IVIN = 444 µA
Doc ID 023203 Rev 1
PM6644
5
Conclusion
Conclusion
The device regulates VOUT = 8 V properly with the designed schematic and bill of material.
The 12 V input voltage range should have an accuracy of ± 5%.
PCB design guidelines
The layout is very important in terms of efficiency, stability and system noise. It is possible to
refer to the PM6644 evaluation kit board for a complete layout example.
For good PC board layout, follow these guidelines:
●
Place all the power components (inductors, input and output capacitors) on the top
side. Refer them to a ground plan, GND in an inner layer. Connect the exposed pad of
the PM6644 to the GND plan with vias (design a GND pad on the top side with the
same size as the exposed pad). On the top side connect the GND pin with a short trace
to the exposed pad.
●
Place input capacitors close to the VIN pin, in order to minimize AC current drops
during high-side MOSFET turn-on. Add vias to the GND plan.
●
Place the output capacitor close to the GND pin, in order to minimize AC current drops
during high-side and low-side MOSFET turn-on. Add vias to the GND plan.
●
Place filtering capacitors close to pins REF3, BYP and VCC.
●
Place the resistor near the TON pin in order to minimize parasitic capacitance on the
TON pin.
Figure 31. Recommended layout - top layer
Figure 32. Recommended layout - inner layer
Doc ID 023203 Rev 1
29/35
Conclusion
PM6644
Figure 33. Recommended layout - bottom layer
30/35
Doc ID 023203 Rev 1
PM6644
6
Package mechanical data
Package mechanical data
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.
Table 13.
DFN10 (3x3 mm) mechanical data
mm
Dim.
A
Min.
Typ.
Max.
0.80
0.90
1.00
0.02
0.05
0.65
0.80
A1
A2
0.55
A3
0.20
b
0.18
0.25
0.30
D
2.85
3.00
3.15
D2
2.20
E
2.85
E2
1.40
e
L
2.70
3.00
3.15
1.75
0.50
0.30
ddd
0.40
0.50
0.08
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Package mechanical data
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Figure 34. DFN10 (3x3 mm)
7426335_G
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Package mechanical data
Figure 35. DFN10 (3x3 mm) footprint
7426335_G
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Revision history
7
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Revision history
Table 14.
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Document revision history
Date
Revision
19-Jun-2012
1
Changes
Initial release.
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