FEATURES
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LTC1709-85
2-Phase, 5-Bit VID,
Current Mode, High Efficiency,
Synchronous Step-Down Switching Regulator
DESCRIPTIO
The LTC
®
1709-85 is a 2-phase, VID programmable, syn-
chronous step-down switching regulator controller that
drives two all N-channel external power MOSFET stages in
a fixed frequency architecture. The 2-phase controller
drives its two output stages out of phase at frequencies up
to 300kHz to minimize the RMS ripple currents in both
input and output capacitors. The 2-phase technique effec-
tively multiplies the fundamental frequency by two, im-
proving transient response while operating each channel
at an optimum frequency for efficiency. Thermal design is
also simplified.
An operating mode select pin (FCB) can be used to select
among three modes including Burst Mode
®
operation for
highest efficiency. An internal differential amplifier provides
true remote sensing of the regulated supply’s positive and
negative output terminals as required in high current ap-
plications.
The RUN/SS pin provides soft-start and optional timed,
short-circuit shutdown. Current foldback limits MOSFET
dissipation during short-circuit conditions when the
overcurrent latchoff is disabled. OPTI-LOOP compensation
allows the transient response to be optimized for a wide
range of output capacitors and ESR values.
, LTC and LT are registered trademarks of Linear Technology Corporation.
OPTI-LOOP and Burst Mode are registered trademarks of Linear Technology Corporation.
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Output Stages Operate Antiphase Reducing Input
Capacitance Requirements and Power Supply
Induced Noise
Dual Input Supply Capability for Load Sharing
5-Bit VID Code (VRM 8.5): V
OUT
= 1.05V to 1.825V
True Remote Sensing Differential Amplifier
Power Good Output Indicator
±1%
Output Voltage Accuracy
Active Voltage Positioning Capable
Current Mode Control Ensures Current Sharing
OPTI-LOOP
®
Compensation Minimizes C
OUT
Three Operational Modes: PWM, Burst and Cycle Skip
Programmable Fixed Frequency: 150kHz to 300kHz
Wide V
IN
Range: 4V to 36V Operation
Adjustable Soft-Start Current Ramping
Internal Current Foldback and Short-Circuit Shutdown
Overvoltage Soft Latch Eliminates Nuisance Trips
Available in 36-Lead Narrow SSOP Package
APPLICATIO S
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Server/Desktop Computers
Internet Servers
Large Memory Arrays
DC Power Distribution Systems
TYPICAL APPLICATIO
0.1µF
220pF
I
TH
SGND
PGOOD
5 VID BITS
FCB
RUN/SS
3.3k
+
V
IN
TG1
BOOST1
SW1
S
S
10µF
35V
×4
1µH
0.002Ω
0.47µF
LTC1709-85
BG1
PGND
VID0–VID4
EAIN
ATTENOUT
ATTENIN
V
DIFFOUT
V
OS –
V
OS +
SENSE1
+
SENSE1
–
TG2
BOOST2
SW2
BG2
INTV
CC
SENSE2
+
SENSE2
–
0.47µF
1µH
0.002Ω
V
OUT
1.05V TO 1.825V
40A
10µF
+
Figure 1. High Current Dual Phase Step-Down Converter
170985f
U
U
U
V
IN
5V TO 28V
+
C
OUT
1000µF
4V
×2
170985 F01
1
LTC1709-85
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
RUNN/SS
SENSE1
+
SENSE1
–
EAIN
PLLFLTR
PLLIN
FCB
I
TH
SGND
1
2
3
4
5
6
7
8
9
36 NC
35 TG1
34 SW1
33 BOOST1
32 V
IN
31 BG1
30 EXTV
CC
29 INTV
CC
28 PGND
27 BG2
26 BOOST2
25 SW2
24 TG2
23 PGOOD
22 V
BIAS
21 VID3
20 VID2
19 VID1
Input Supply Voltage (V
IN
).........................36V to – 0.3V
Topside Driver Voltages (BOOST1,2) .........42V to – 0.3V
Switch Voltage (SW1, 2) .............................36V to – 5 V
SENSE1
+
, SENSE2
+
, SENSE1
–
,
SENSE2
–
Voltages ................... (1.1)INTV
CC
to – 0.3V
EAIN, V
OS+
, V
OS–
, EXTV
CC
, INTV
CC
, RUN/SS,
V
BIAS
, ATTENIN, ATTENOUT, PGOOD,
VID25mV–VID3 Voltages ........................7V to – 0.3V
Boosted Driver Voltage (BOOST-SW) ..........7V to – 0.3V
PLLFLTR, PLLIN, V
DIFFOUT
,
FCB Voltages ................................... INTV
CC
to – 0.3V
I
TH
Voltage ................................................2.7V to – 0.3V
Peak Output Current <1µs(TGL1,2, BG1,2) ................ 3A
INTV
CC
RMS Output Current ................................ 50mA
Operating Ambient Temperature Range
(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
LTC1709EG-85
V
DIFFOUT
10
V
OS–
11
V
OS
+
12
SENSE2
–
13
SENSE2
+
14
ATTENOUT 15
ATTENIN 16
VID25mV 17
VID0 18
G PACKAGE
36-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 85°C/W
Consult LTC Marketing for parts specified with wider operating temperature
ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL
V
EAIN
V
SENSEMAX
I
INEAIN
V
LOADREG
PARAMETER
Regulated Feedback Voltage
Maximum Current Sense Threshold
Feedback Current
Output Voltage Load Regulation
(Note 4)
Main Control Loop
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 15V, V
BIAS
= 3.3V, V
RUN/SS
= 5V unless otherwise noted.
CONDITIONS
I
TH
Voltage = 1.2V (Note 4)
q
q
MIN
0.792
62
TYP
0.800
75
–5
MAX
0.808
88
– 50
0.5
– 0.5
0.02
0.84
–1
4.8
0.88
4
UNITS
V
mV
nA
%
%
%/V
V
µA
V
V
V
mmho
V/mV
170985f
(Note 4)
Measured in Servo Loop,
∆I
TH
Voltage: 1.2V to 0.7V
Measured in Servo Loop,
∆I
TH
Voltage: 1.2V to 2V
V
IN
= 3.6V to 30V (Note 4)
q
q
0.1
– 0.1
0.002
0.76
0.8
– 0.17
4.3
V
REFLNREG
V
FCB
I
FCB
V
BINHIBIT
V
OVL
UVLO
g
m
g
mOL
Reference Voltage Line Regulation
Forced Continuous Threshold
Forced Continuous Current
Burst Inhibit (Constant Frequency)
Threshold
Output Overvoltage Threshold
Undervoltage Lockout
Transconductance Amplifier g
m
Transconductance Amplifier Gain
q
Measured at FCB pin
Measured at V
EAIN
V
IN
Ramping Down
I
TH
= 1.2V, Sink/Source 5µA (Note 4)
I
TH
= 1.2V, (g
m
• Z
L
; No Ext Load) (Note 4)
q
0.84
3
0.86
3.5
3
1.5
2
U
W
U
U
W W
W
LTC1709-85
ELECTRICAL CHARACTERISTICS
SYMBOL
I
Q
PARAMETER
Input DC Supply Current
Normal Mode
Shutdown
Soft-Start Charge Current
RUN/SS Pin ON Arming
RUN/SS Pin Latchoff Arming
RUN/SS Discharge Current
Shutdown Latch Disable Current
Total Sense Pins Source Current
Maximum Duty Factor
Top Gate Transition Time:
Rise Time
Fall Time
Bottom Gate Transition Time:
Rise Time
Fall Time
Top Gate Off to Bottom Gate On Delay
Synchronous Switch-On Delay Time
Bottom Gate Off to Top Gate On Delay
Top Switch-On Delay Time
Minimum On-Time
Internal V
CC
Voltage
INTV
CC
Load Regulation
EXTV
CC
Voltage Drop
EXTV
CC
Switchover Voltage
EXTV
CC
Switchover Hysteresis
Operating Supply Voltage Range
Resistance Between ATTENIN
and ATTENOUT Pins
Resistive Divider Error
VID25mV to VID3 Pull-Up Resistance
V
BIAS
= 3.3V
(Note 8)
2
1
190
120
280
220
140
310
50
f
PLLIN
< f
OSC
f
PLLIN
> f
OSC
– 15
15
180
250
160
360
q
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 15V, V
BIAS
= 3.3V, V
RUN/SS
= 5V unless otherwise noted.
CONDITIONS
(Note 5)
EXTV
CC
Tied to V
OUT
, V
OUT
= 5V
V
RUN/SS
= 0V
V
RUN/SS
= 1.9V
V
RUN/SS
Rising
V
RUN/SS
Rising from 3V
Soft Short Condition V
EAIN
= 0.5V, V
RUN/SS
= 4.5V
V
EAIN
= 0.5V
Each Channel: V
SENSE1
–
, 2
– = V
SENSE1
+
, 2
+ = 0V
In Dropout
(Note 6)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
(Note 6)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF Each Driver (Note 6)
C
LOAD
= 3300pF Each Driver (Note 6)
Tested with a Square Wave (Note 7)
6V < V
IN
< 30V, V
EXTVCC
= 4V
I
CC
= 0 to 20mA, V
EXTVCC
= 4V
I
CC
= 20mA, V
EXTVCC
= 5V
I
CC
= 20mA, EXTV
CC
Ramping Positive
I
CC
= 20mA, EXTV
CC
Ramping Negative
2.7
10
– 0.25
40
0.8
0.25
q
MIN
TYP
470
20
MAX
UNITS
µA
µA
µA
V
V
µA
µA
µA
%
40
1.9
4.5
4
5
I
RUN/SS
V
RUN/SS
V
RUN/SSLO
I
SCL
I
SDLHO
I
SENSE
DF
MAX
TG1, 2 t
r
TG1, 2 t
f
BG1, 2 t
r
BG1, 2 t
f
TG/BG t
1D
BG/TG t
2D
t
ON(MIN)
V
INTVCC
V
LDO
INT
V
LDO
EXT
V
EXTVCC
V
LDOHYS
V
BIAS
R
ATTEN
ATTEN
ERR
R
PULLUP
VID
THLOW
VID
THHIGH
VID
LEAK
f
NOM
f
LOW
f
HIGH
R
PLLIN
I
PLLFLTR
– 0.5
1.0
0.5
– 85
98
–1.2
1.5
4.1
2
1.6
– 60
99.5
30
40
30
20
90
90
180
90
90
90
90
ns
ns
ns
ns
ns
ns
ns
Internal V
CC
Regulator
4.8
5.0
0.2
80
4.5
4.7
0.2
5.5
5.2
1.0
160
V
%
mV
V
V
V
kΩ
%
kΩ
V
V
µA
kHz
kHz
kHz
kΩ
µA
µA
Deg
170985f
VID Parameters
VID25mV to VID3 Logic Threshold Low V
BIAS
= 3.3V
VID25mV to VID3 Logic Threshold High V
BIAS
= 3.3V
VID25mV to VID3 Leakage
Nominal Frequency
Lowest Frequency
Highest Frequency
PLLIN Input Resistance
Phase Detector Output Current
Sinking Capability
Sourcing Capability
Controller 2-Controller 1 Phase
V
BIAS
< VID25mV–VID3 < 7V
V
PLLFLTR
= 1.2V
V
PLLFLTR
= 0V
V
PLLFLTR
≥
2.4V
Oscillator and Phase-Locked Loop
R
RELPHS
3
LTC1709-85
ELECTRICAL CHARACTERISTICS
SYMBOL
V
PGL
I
PGOOD
V
PG
PARAMETER
PGOOD Voltage Low
PGOOD Leakage Current
PGOOD Trip Level, Either Controller
PGOOD Output
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 15V, V
BIAS
= 3.3V, V
RUN/SS
= 5V unless otherwise noted.
CONDITIONS
I
PGOOD
= 2mA
V
PGOOD
= 5V
V
EAIN
with Respect to Set Output Voltage
V
EAIN
Ramping Negative
V
EAIN
Ramping Positive
–6
6
0.995
0V < V
CM
< 5V
Measured at V
OS
+ Input
46
– 7.5
7.5
1
55
80
MIN
TYP
0.1
MAX
0.3
±1
– 9.5
9.5
1.005
UNITS
V
µA
%
%
V/V
dB
kΩ
Differential Amplifier/Op Amp Gain Block
A
DA
CMRR
DA
R
IN
Gain
Common Mode Rejection Ratio
Input Resistance
Note 1:
Absolute Maximum Ratings are those values beyond which the
life of a device may be impaired.
Note 2:
The LTC1709EG-85 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
according to the following formula:
LTC1709EG-85: T
J
= T
A
+ (P
D
• 85°C/W)
Note 4:
The LTC1709-85 is tested in a feedback loop that servos V
ITH
to a
specified voltage and measures the resultant V
EAIN
.
Note 5:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.
Note 6:
Rise and fall times are measured using 10% and 90% levels. Delay
times are measured using 50% levels.
Note 7:
The minimum on-time condition corresponds to the on inductor
peak-to-peak ripple current
≥
40% I
MAX
(see Minimum On-Time
Considerations in the Applications Information section).
Note 8:
Each built-in pull-up resistor attached to the VID inputs also has a
series diode to allow input voltages higher than the VIDV
CC
supply without
damage or clamping (see the Applications Information section).
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Load Current
(3 Operating Modes)
100
90
80
Burst Mode
OPERATION
80
V
IN
= 5V
90
V
IN
= 8V
V
IN
= 12V
V
IN
= 20V
40
V
OUT
= 1.825V
V
EXTVCC
= 0V
FREQ = 200kHz
V
FCB
= 0V
0.1
1
10
OUTPUT CURRENT (A)
100
170985 G02
EFFICIENCY (%)
EFFICIENCY (%)
60
50
40
30
20
10
0
0.01
0.1
60
EFFICIENCY (%)
70
FORCED
CONTINUOUS
MODE
CONSTANT
FREQUENCY
(BURST DISABLE)
V
IN
= 5V
V
OUT
= 1.6V
FREQ = 200kHz
10
1
LOAD CURRENT (A)
100
170985 G01
4
U W
Efficiency vs Output Current
100
100
Efficiency vs Input Voltage
I
OUT
= 20A
V
OUT
= 1.825V
80
70
20
60
0
50
5
15
10
INPUT VOLTAGE (V)
20
170985 G03
170985f
LTC1709-85
TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs Input Voltage
and Mode
1000
ON
EXTV
CC
VOLTAGE DROP (mV)
250
INTV
CC
AND EXTV
CC
SWITCH VOLTAGE (V)
800
SUPPLY CURRENT (µA)
600
400
200
SHUTDOWN
0
0
5
20
15
10
25
INPUT VOLTAGE (V)
30
35
Internal 5V LDO Line Reg
5.1
5.0
I
LOAD
= 1mA
INTV
CC
VOLTAGE (V)
4.9
4.8
4.7
4.6
4.5
4.4
0
5
20
15
25
10
INPUT VOLTAGE (V)
30
35
V
SENSE
(mV)
V
SENSE
(mV)
Maximum Current Sense Threshold
vs V
RUN/SS
(Soft-Start)
80
V
SENSE(CM)
= 1.6V
80
60
V
SENSE
(mV)
V
SENSE
(mV)
72
V
SENSE
(mV)
40
20
64
0
0
1
2
3
V
RUN/SS
(V)
170985 G10
4
U W
170985 G04
170985 G07
EXTV
CC
Voltage Drop
5.05
5.00
4.95
4.90
4.85
4.80
4.75
INTV
CC
and EXTV
CC
Switch
Voltage vs Temperature
INTV
CC
VOLTAGE
200
150
100
50
EXTV
CC
SWITCHOVER THRESHOLD
0
0
10
30
20
CURRENT (mA)
40
50
170985 G05
4.70
– 50 – 25
50
25
75
0
TEMPERATURE (°C)
100
125
170985 G06
Maximum Current Sense Threshold
vs Duty Factor
75
Maximum Current Sense Threshold
vs Percent of Nominal Output
Voltage (Foldback)
80
70
60
50
50
40
30
20
10
25
0
0
20
40
60
DUTY FACTOR (%)
80
100
170985 G08
0
50
100
0
25
75
PERCENT OF NOMINAL OUTPUT VOLTAGE (%)
170985 G09
Maximum Current Sense Threshold
vs Sense Common Mode Voltage
90
80
76
70
60
50
40
30
20
10
0
–10
–20
–30
Current Sense Threshold
vs I
TH
Voltage
68
5
6
60
0
1
1.5
0.5
COMMON MODE VOLTAGE (V)
2
170985 G11
0
0.5
1
1.5
V
ITH
(V)
2
2.5
170985 G12
170985f
5
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