LTC3720
Single Phase VRM8.5
Current Mode Step-Down Controller
FEATURES
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DESCRIPTIO
5-Bit Programmable Output Voltage:
1.05V to 1.825V (VRM8.5)
No Sense Resistor Required
2% to 87% Duty Cycle at 200kHz
t
ON(MIN)
≤
100ns
Supports Active Voltage Positioning
True Current Mode Control
Stable with Ceramic C
OUT
Dual N-Channel MOSFET Synchronous Drive
Power Good Output Voltage Monitor
Wide V
IN
Range: 4V to 36V
±1%
0.8V Reference
Adjustable Current Limit
Adjustable Switching Frequency
Forced Continuous Control Pin
Programmable Soft-Start
Output Overvoltage Protection
Optional Short-Circuit Shutdown Timer
Micropower Shutdown: I
Q
< 30µA
Available in 28-Lead Narrow SSOP Package
The LTC
®
3720 is a synchronous step-down switching
regulator controller for CPU power. An output voltage
between 1.05V and 1.825V is selected by a 5-bit code
(Intel VRM8.5 VID specification). The controller uses a
valley current control architecture to deliver very low duty
cycles without requiring a sense resistor. Operating fre-
quency is selected by an external resistor and is compen-
sated for variations in V
IN
and V
OUT
.
Discontinuous mode operation provides high efficiency
operation at light loads. A forced continuous control pin
reduces noise and RF interference and can assist second-
ary winding regulation by disabling discontinuous mode
operation when the main output is lightly loaded.
Fault protection is provided by internal foldback current
limiting, an output overvoltage comparator and optional
short-circuit shutdown timer. Soft-start capability for sup-
ply sequencing is accomplished using an external timing
capacitor. The regulator current limit level is also user
programmable. Wide supply range allows operation from
4V to 36V at the input.
, LTC and LT are registered trademarks of Linear Technology Corporation.
No R
SENSE
is a trademark of Linear Technology Corporation.
Pentium is a registered trademark of Intel Corporation.
APPLICATIO S
s
s
Power Supplies for Pentium
®
Processors
Notebook Computers and Servers
TYPICAL APPLICATIO
LTC3720
PGOOD
I
ON
0.1µF
RUN/SS
470pF
20k
SGND
BOOST
V
CC
INTV
CC
VID4
VID3
5-BIT VID
VID2
VID1
VID0
SENSE
+
BG
PGND
SENSE¯
V
OSENSE
I
TH
V
IN
TG
SW
330k
IRF7811W
×2
0.33µF
CMDSH-3
L1
1µH
10µF
×5
V
IN
5V TO 24V
+
UPS840
+
C
OUT
270µF
2V
×4
EFFICIENCY (%)
V
OUT
1.05V TO 1.825V
20A
4.7µF
IRF7811W
×3
C
OUT
: CORNELL DUBILIER
ESRE271M02B
L1: SUMIDA CEP125-IROMC
3720 F01a
Figure 1. High Efficiency Step-Down Converter
U
Efficiency vs Output Current
100
V
OUT
= 1.45V
95 L1 = 1µH
90
85
80
75
70
65
60
55
0.01
1
10
0.1
OUTPUT CURRENT (A)
100
3720 F01b
U
U
V
IN
= 5V
V
IN
= 15V
3720f
1
LTC3720
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
RUN/SS
V
ON
PGOOD
V
RNG
FCB
I
TH
SGND
I
ON
V
FB
1
2
3
4
5
6
7
8
9
28 BOOST
27 TG
26 SW
25 SENSE
+
24 SENSE
–
23 PGND
22 BG
21 INTV
CC
20 V
IN
19 EXTV
CC
18 V
CC
17 VID4
16 VID3
15 VID2
Input Supply Voltage
V
IN
, I
ON
..................................................36V to – 0.3V
Boosted Topside Driver Supply Voltage
BOOST .................................................. 42V to – 0.3V
SW, SENSE
+
Voltages ................................. 36V to – 5V
EXTV
CC
, (BOOST – SW), RUN/SS, V
CC
VID0-VID4, PGOOD Voltages ..................... 7V to – 0.3V
FCB, V
ON
, V
RNG
Voltages .......... INTV
CC
+ 0.3V to – 0.3V
I
TH
, V
FB
, V
OSENSE
Voltages ....................... 2.7V to – 0.3V
TG, BG, INTV
CC
, EXTV
CC
Peak Currents .................... 2A
TG, BG, INTV
CC
, EXTV
CC
RMS Currents .............. 50mA
Operating Ambient Temperature Range
LTC3720EGN (Note 2) ........................ – 40°C to 85°C
Junction Temperature (Note 3) ............................ 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LTC3720EGN
SGND 10
V
FB
11
V
OSENSE
12
VID0 13
VID1 14
GN PACKAGE
28-LEAD NARROW PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 95°C/ W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL
I
Q
PARAMETER
Input DC Supply Current
Normal
Shutdown Supply Current
Feedback Reference Voltage
Feedback Voltage Line Regulation
Feedback Voltage Load Regulation
Feedback Pin Input Current
Error Amplifier Transconductance
Forced Continuous Threshold
Forced Continuous Pin Current
On-Time
Minimum On-Time
Minimum Off-Time
Maximum Current Sense Threshold
V
SENSE–
– V
SENSE+
Minimum Current Sense Threshold
V
SENSE–
– V
SENSE+
Output Overvoltage Fault Threshold
Output Undervoltage Fault Threshold
RUN Pin Start Threshold
Main Control Loop
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are T
A
= 25°C. V
IN
= 15V unless otherwise noted.
CONDITIONS
MIN
TYP
MAX
UNITS
900
15
I
TH
= 1.2V (Note 4)
V
IN
= 4V to 30V, I
TH
= 1.2V (Note 4)
I
TH
= 0.5V to 1.9V (Note 4)
I
TH
= 1.2V (Note 4)
q
q
q
2000
30
0.808
– 0.3
±50
2
0.84
–2
300
575
100
400
153
107
214
V
FB
∆V
FB(LINEREG)
∆V
FB(LOADREG)
I
FB
g
m(EA)
V
FCB
I
FCB
t
ON
t
ON(MIN)
t
OFF(MIN)
V
SENSE(MAX)
0.792
0.800
0.002
– 0.05
–5
1.4
0.76
200
425
1.7
0.8
–1
250
500
50
250
V
FCB
= 0.8V
I
ON
= 60µA, V
ON
= 1.5V
I
ON
= 30µA, V
ON
= 1.5V
I
ON
= 180µA, V
ON
= 0V
I
ON
= 60µA, V
ON
= 1.5V
V
RNG
= 1V, V
FB
= 0.76V
V
RNG
= 0V, V
FB
= 0.76V
V
RNG
= INTV
CC
, V
FB
= 0.76V
V
RNG
= 1V, V
FB
= 0.84V
V
RNG
= 0V, V
FB
= 0.84V
V
RNG
= INTV
CC
, V
FB
= 0.84V
5.5
520
q
q
q
q
113
79
158
133
93
186
– 67
– 47
– 93
7.5
600
1.5
V
SENSE(MIN)
∆V
FB(OV)
V
FB(UV)
V
RUN/SS(ON)
9.5
680
2
0.8
2
U
µA
µA
V
%/V
%
nA
mS
V
µA
ns
ns
ns
ns
mV
mV
mV
mV
mV
mV
%
mV
V
3720f
W
U
U
W W
W
LTC3720
ELECTRICAL CHARACTERISTICS
SYMBOL
V
RUN/SS(LE)
V
RUN/SS(LT)
I
RUN/SS(C)
I
RUN/SS(D)
V
IN
(UVLO)
TG R
UP
TG R
DOWN
BG R
UP
BG R
DOWN
TG t
r
TG t
f
BG t
r
BG t
f
V
INTVCC
∆V
LDO(LOADREG)
V
EXTVCC
∆V
EXTVCC
∆V
EXTVCC(HYS)
PGOOD Output
∆V
FBH
∆V
FBL
∆V
FB(HYS)
V
PGL
VID DAC
V
CC
V
VID(T)
V
VID(LEAK)
∆V
OSENSE
R
PULLUP
R
VID
I
VCC
Operating Supply Voltage Range
VID0-VID4 Logic Threshold Voltage
VID0-VID4 Leakage Current
DAC Output Accuracy
Pull-Up Resistance on VID
Resistance from V
OSENSE
to V
FB
Supply Current
PGOOD Upper Threshold
PGOOD Lower Threshold
PGOOD Hysteresis
PGOOD Low Voltage
PARAMETER
RUN Pin Latchoff Enable Threshold
RUN Pin Latchoff Threshold
Soft-Start Charge Current
Soft-Start Discharge Current
Undervoltage Lockout
TG Driver Pull-Up On Resistance
TG Driver Pull-Down On Resistance
BG Driver Pull-Up On Resistance
BG Driver Pull-Down On Resistance
TG Rise Time
TG Fall Time
BG Rise Time
BG Fall Time
Internal V
CC
Voltage
Internal V
CC
Load Regulation
EXTV
CC
Switchover Voltage
EXTV
CC
Switch Drop Voltage
EXTV
CC
Switchover Hysteresis
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are T
A
= 25°C. V
IN
= 15V unless otherwise noted.
CONDITIONS
RUN/SS Pin Rising
RUN/SS Pin Falling
– 0.5
0.8
V
IN
Falling
V
IN
Rising
TG High
TG Low
BG High
BG Low
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF
6V < V
IN
< 30V, V
EXTVCC
= 4V
I
CC
= 0mA to 20mA, V
EXTVCC
= 4V
I
CC
= 20mA, V
EXTVCC
Rising
I
CC
= 20mA, V
EXTVCC
= 5V
q
q
q
q
MIN
TYP
4
3.5
– 1.2
1.8
3.4
3.5
2
2
3
1
20
20
20
20
MAX
4.5
–3
3
3.9
4.0
3
3
4
2
UNITS
V
V
µA
µA
V
V
Ω
Ω
Ω
Ω
ns
ns
ns
ns
Internal V
CC
Regulator
4.7
4.5
5
– 0.1
4.7
150
200
V
FB
Rising
V
FB
Falling
V
FB
Returning
I
PGOOD
= 5mA
3.1
V
CC
= 3.3V
V
VID0
-V
VID4
= V
CC
V
OSENSE
Programmed from
1.05V to 1.825V (Note 5), V
CC
= 5V
V
DIODE
= 0.6V (Note 6)
(Note 7)
– 0.25
28
6
0.4
1.2
0.01
0
40
10
1
5.5
– 5.5
7.5
– 7.5
1
0.15
9.5
– 9.5
2
0.4
5.5
2
±1
0.25
56
14
10
300
5.3
±2
V
%
V
mV
mV
%
%
%
V
V
V
µA
%
kΩ
kΩ
µA
Note 1:
Absolute Maximum Ratings are those values beyond which the life of
a device may be impaired.
Note 2:
The LTC3720E is guaranteed to meet performance specifications from
0°C to 70°C. Specifications over the –40°C to 85°C operating temperature
range are assured by design, characterization and correlation with statistical
process controls.
Note 3:
T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
as follows:
LTC3720EGN: T
J
= T
A
+ (P
D
• 95°C/W)
Note 4:
The LTC3720 is tested in a feedback loop that adjusts V
FB
to achieve
a specified error amplifier output voltage (I
TH
).
Note 5:
The LTC3720 VID DAC is tested in a feedback loop that adjusts
V
OSENSE
to achieve a specified feedback voltage (V
FB
= 0.8V) for each DAC VID
code.
Note 6:
Each built-in pull-up resistor attached to VID inputs also has a series
diode connected to V
CC
to allow input voltages higher than the V
CC
supply
without damage or clamping. (See Operation section for further details.)
Note 7:
Supply current is specified with all VID inputs floating. Due to the
internal pull-ups on the VID pins, the supply current will increase depending
on the number of grounded VID lines. Each grounded VID line will draw
approximately (V
CC
– 0.6V)/40k mA. (See Operation section for further
details.)
3720f
3
LTC3720
TYPICAL PERFOR A CE CHARACTERISTICS
Current Sense Threshold
vs I
TH
Voltage
300
V
RNG
=
2V
1.4V
1V
ON-TIME (ns)
1k
10k
CURRENT SENSE THRESHOLD (mV)
200
100
0.7V
0.5V
ON-TIME (ns)
0
–100
10
–200
0
0.5
1.0
1.5
2.0
I
TH
VOLTAGE (V)
On-Time vs Temperature
300
250
200
150
100
50
0
–50 –25
MAXIMUM CURRENT SENSE THRESHOLD (mV)
I
ION
= 30µA
V
VON
= 0V
V
RNG
= 1V
125
100
75
50
25
0
0
0.2
0.4
V
FB
(V)
0.6
0.8
3720 G05
MAXIMUM CURRENT SENSE THRESHOLD (mV)
ON-TIME (ns)
50
25
75
0
TEMPERATURE (°C)
Maximum Current Sense
Threshold vs RUN/SS Voltage
MAXIMUM CURRENT SENSE THRESHOLD (mV)
V
RNG
= 1V
MAXIMUM CURRENT SENSE THRESHOLD (mV)
150
125
100
75
50
25
0
140
FEEDBACK REFERENCE VOLTAGE (V)
1.5
2
2.5
3
RUN/SS VOLTAGE (V)
4
U W
2.5
3720 G01
On-Time vs I
ON
Current
V
VON
= 0V
1000
On-Time vs V
ON
Voltage
I
ION
= 30µA
800
600
400
100
200
3.0
1
10
I
ON
CURRENT (µA)
100
3720 G02
0
0
2
1
V
ON
VOLTAGE (V)
3
3720 G02
Current Limit Foldback
150
300
250
200
150
100
50
0
Maximum Current Sense
Threshold vs V
RNG
Voltage
100
125
0.5
0.75
1.0
1.25
1.5
V
RNG
VOLTAGE (V)
1.75
2.0
3720 G04
3720 G06
Maximum Current Sense
Threshold vs Temperature
150
V
RNG
= 1V
Feedback Reference Voltage
vs Temperature
0.82
0.81
130
0.80
120
0.79
110
3.5
3729 G07
100
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
0.78
–50 –25
75
0
25
50
TEMPERATURE (°C)
100
125
3720 G08
3720 G09
3720f
LTC3720
TYPICAL PERFOR A CE CHARACTERISTICS
Error Amplifier g
m
vs Temperature
2.0
1200
1000
EXTV
CC
OPEN
1.8
INPUT CURRENT (µA)
g
m
(mS)
1.6
∆INTV
CC
(%)
1.4
1.2
1.0
–50
–25
50
25
0
75
TEMPERATURE (°C)
EXTV
CC
Switch Resistance
vs Temperature
10
0
–0.25
FCB PIN CURRENT (µA)
EXTV
CC
SWITCH RESISTANCE (Ω)
8
FCB PIN CURRENT (µA)
6
4
2
0
–50
–25
50
25
0
75
TEMPERATURE (°C)
RUN/SS Latchoff Thresholds
vs Temperature
UNDERVOLTAGE LOCKOUT THRESHOLD (V)
5.0
4.0
RUN/SS THRESHOLD (V)
4.5
LATCHOFF ENABLE
4.0
3.5
LATCHOFF THRESHOLD
3.0
–50
–25
U W
100
3720 G10
Input and Shutdown Currents
vs Input Voltage
60
50
0
INTV
CC
Load Regulation
–0.1
SHUTDOWN CURRENT (µA)
800
SHUTDOWN
600
400
200
EXTV
CC
= 5V
0
40
30
20
10
0
0
5
20
15
25
10
INPUT VOLTAGE (V)
30
35
–0.2
–0.3
–0.4
125
–0.5
0
10
30
40
20
INTV
CC
LOAD CURRENT (mA)
50
3720 G12
3720 G11
FCB Pin Current vs Temperature
3
RUN/SS Pin Current
vs Temperature
2
PULL-DOWN CURRENT
1
–0.50
–0.75
–1.00
–1.25
–1.50
–50 –25
0
PULL-UP CURRENT
–1
100
125
50
25
75
0
TEMPERATURE (°C)
100
125
–2
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
3720 G13
3720 G14
3720 G15
Undervoltage Lockout Threshold
vs Temperature
3.5
3.0
2.5
75
0
25
50
TEMPERATURE (°C)
100
125
2.0
–50 –25
75
0
25
50
TEMPERATURE (C)
100
125
3720 G16
3720 G17
3720f
5
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