LTC1708-PG
Dual Adjustable 5-Bit VID
High Efficiency, 2-Phase Current Mode
Synchronous Buck DC/DC Controller
FEATURES
s
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DESCRIPTIO
Out-of-Phase Controllers Reduce Input Capacitance
and Power Supply Induced Noise
OPTI-LOOP
®
Compensation Minimizes C
OUT
Power Good Output Monitors Both Outputs
5-Bit Mobile VID Control, V
OUT
: 0.9V to 2.0V
Dual N-Channel MOSFET Synchronous Drive
±1%
Output Voltage Accuracy
DC Programmed Fixed Frequency 150kHz to 300kHz
Wide V
IN
Range: 3.5V to 36V Operation
Very Low Dropout Operation: 99% Duty Cycle
Adjustable Soft-Start Current Ramping
Foldback Output Current Limiting
Latched Short-Circuit Shutdown with Defeat Option
Output Overvoltage Protection
Remote Output Voltage Sense
Low Shutdown Current: 20µA
5V and 3.3V Standby Regulators
Selectable Constant Frequency, Burst Mode
®
and
Continuous Operation
The LTC
®
1708 is a dual adjustable 5-bit VID program-
mable step-down switching regulator controller that drives
all N-Channel power MOSFET stages. A constant fre-
quency current mode architecture allows adjustment of
the frequency up to 300kHz. Power loss and noise due to
the ESR of the input capacitance are minimized by oper-
ating the two main controller output stages out of phase.
OPTI-LOOP compensation allows the transient response
to be optimized over a wide range of output capacitance
and ESR values. The precision 0.8V reference is compat-
ible with future microprocessor generations, and a wide
3.5V to 30V (36V maximum) input supply range that
encompasses all battery chemistries. A power good out-
put indicates when the output voltages are within 7.5% of
their programmed value.
A RUN/SS pin for each controller provides both soft-start
and an optional timed, short-circuit shutdown. Other
protection features include: internal foldback current lim-
iting and an output overvoltage crowbar. The forced
continuous control pin (FCB) can be used to inhibit Burst
Mode operation or to regulate a third, flyback output.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode and OPTI-LOOP are registered trademarks of Linear Technology Corporation.
APPLICATIO S
s
s
Notebook and Palmtop Computers, PDAs
Portable Instruments
TYPICAL APPLICATIO
M1
L1
1µH
+
4.7µF
D3
V
IN
TG1
C
B1
0.47µF
BOOST1
SW1
D1
M2
5 VID BITS
LTC1708-PG
BG1
VID0 TO VID4
SENSE1
R
SENSE1
0.003Ω
1000pF
SENSE1
V
OUT1
0.925V TO
2.00V
14.1A
ATTNIN
C
OUT1a
+
10µF
6.3V
CERAMIC
C
OUT1
270µF
2V
SP
×4
C
C1
1500pF
R
C1
22k
I
TH1
C
SS1
0.1µF
–
+
VIDV
CC
INTV
CC
TG2
BOOST2
SW2
BG2
PGND
SENSE2
+
PGOOD
SENSE2
–
D4
1µF
CERAMIC
M3a
C
B2
0.1µF
1000pF
EAIN2
I
TH2
C
SS2
0.1µF
C
C2
220pF
R
C2
15k
R4
63.4k
1%
C
OUT
180µF
4V
SP
RUN/SS1 SGND RUN/SS2
Figure 1. High Efficiency VID Controlled, 2-Output Step-Down Converter
1708f
U
C
IN
10µF
50V
CERAMIC
×4
L2
2.2µH
V
IN
4.75V TO 28V
M3b
D2
R
SENSE2
0.01Ω
V
OUT2
1.5V
4A
R3
20k
1%
U
U
+
M1: IRF7811
M2: 1RF7809
M3a, M3b: FDS6982
L1: VISHAY 5050CE
ATTNOUT CONNECTED TO EAIN1
1628 F01
1
LTC1708-PG
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
RUN/SS1
SENSE1
+
SENSE1
–
EAIN1
FREQSET
STBYMD
FCB
I
TH1
SGND
1
2
3
4
5
6
7
8
9
36 PGOOD
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 RUN/SS2
22 VIDV
CC
21 VID4
20 VID3
19 VID2
Input Supply Voltage (V
IN
).........................36V to – 0.3V
Topside Driver Voltages
(BOOST1, BOOST2) ...................................42V to – 0.3V
Switch Voltage (SW1, SW2) .........................36V to – 5V
INTV
CC,
EXTV
CC
, RUN/SS1, RUN/SS2,
(BOOST1-SW1), (BOOST2-SW2), ...............7V to – 0.3V
SENSE1
+
, SENSE2
+
, SENSE1
–
,
SENSE2
–
Voltages ....................... (1.1)INTV
CC
to – 0.3V
FREQSET, STBYMD, FCB, VIDV
CC
, VID0-4,
PGOOD Voltages ..........................................7V to – 0.3V
I
TH1,
I
TH2
, EAIN1, EAIN2, ATTNIN,
ATTNOUT Voltages ...................................2.7V to – 0.3V
Peak Output Current <10µs (TG1, TG2, BG1, BG2) ... 3A
INTV
CC
Peak 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
LTC1708EG-PG
3.3V
OUT
10
I
TH2
11
EAIN2 12
SENSE2
–
13
SENSE2
+
14
ATTNOUT 15
ATTNIN 16
VID0 17
VID1 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
EAIN1, 2
I
EAIN1, 2
V
REFLNREG
V
LOADREG
PARAMETER
Regulated Feedback Voltage
at EAIN Pin
Feedback Current
Reference Voltage Line Regulation
Output Voltage Load Regulation
Main Control Loops
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
RUN/SS1, 2
= 5V unless otherwise noted.
CONDITIONS
(Note 4); I
TH1, 2
Voltage = 1.2V
(Note 4)
V
IN
= 3.6V to 30V (Note 4)
(Note 4)
Measured in Servo Loop; I
TH1, 2
Voltage = 1.2V to 0.7V
q
Measured in Servo Loop; I
TH1, 2
Voltage = 1.2V to 2.0V
q
I
TH1, 2
= 1.2V; Sink/Source 5µA; (Note 4)
I
TH1, 2
= 1.2V; (g
m
• Z
L
, No Ext Load) (Note 4)
(Note 5)
EXTV
CC
Tied to GND; VID Inputs Open Circuit
V
RUN/SS1, 2
= 0V, V
STBYMD
> 2V
V
RUN/SS1, 2
= 0V, V
STBYMD
= Open
q
q
MIN
0.792
TYP
0.800
–5
0.002
0.1
– 0.1
1.3
3
850
125
20
MAX
0.808
– 50
0.02
0.5
– 0.5
UNITS
V
nA
%/V
%
%
mmho
MHz
µA
µA
µA
V
µA
V
V
1708f
g
m1, 2
g
mOL1, 2
I
Q
Transconductance Amplifier g
m
Transconductance Amplifier GBW
Input DC Supply Current
Normal Mode
Standby
Shutdown
Forced Continuous Threshold
Forced Continuous Current
Burst Inhibit Threshold
Undervoltage Lockout
35
0.840
– 0.1
4.8
4
V
FCB
I
FCB
V
BINHIBIT
UVLO
0.760
– 0.3
0.800
– 0.18
4.3
3.5
V
FCB
= 0.85V
Measured at FCB pin
V
IN
Ramping Down
2
U
W
U
U
W W
W
LTC1708-PG
ELECTRICAL CHARACTERISTICS
SYMBOL
V
OV
I
SENSE
V
STBYMD
MS
V
STBYMD
KA
DF
MAX
I
RUN/SS1, 2
V
RUN/SS1, 2
LT
I
SCL1, 2
I
SDLHO
V
SENSE(MAX)
PARAMETER
Output Overvoltage Threshold
Sense Pins Total Source Current
Master Shutdown Threshold
Keep-Alive Power On-Threshold
Maximum Duty Factor
Soft-Start Charge Current
RUN/SS Pin Latchoff Threshold
RUN/SS Discharge Current
Shutdown Latch Disable Current
Maximum Current Sense Threshold
TG Transition Time:
Rise Time
Fall Time
BG 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
Hysteresis
Oscillator Frequency
Lowest Frequency
Highest Frequency
FREQSET Input Current
3.3V Regulator Output Voltage
3.3V Regulator Load Regulation
3.3V Regulator Line Regulation
PGOOD Voltage Low
PGOOD Leakage Current
PGOOD Trip Level
V
FREQSET
= Open (Note 7)
V
FREQSET
= 0V
V
FREQSET
= 2.4V
V
FREQSET
= 0V
No Load
I
3.3
= 0 to 10mA
6V < V
IN
< 30V
I
PGOOD
= 2mA
V
PGOOD
= 5V
Relative to the 0.8V Regulated Feedback Voltage
EAIN1, 2 Ramping Negative from 0.8V
EAIN1, 2 Ramping Positive from 0.8V
– 10
5
– 7.5
7.5
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
RUN/SS1, 2
= 5V unless otherwise noted.
CONDITIONS
Measured at EAIN1, 2
(Each Channel); V
SENSE1
–
, 2
– = V
SENSE1
+
, 2
+ = 0V
V
STBYMD
Ramping Down
V
STBYMD
Ramping Up, RUN
SS1, 2
= 0V
In Dropout
V
RUN/SS1, 2
= 1.9V
V
RUN/SS1,
V
RUN/SS2
Rising
V
RUN/SS1,
V
RUN/SS2
Rising from 3V
Soft Short Condition E
AIN1, 2
= 0.5V;
V
RUN/SS1, 2
= 4.5V
EAIN1, 2 = 0.5V
V
EAIN1, 2
= 0.7V; V
SENSE1, 2
= 5V
V
EAIN1, 2
= 0.7V; V
SENSE1, 2
= 5V
C
LOAD
= 3300pF (Note 10)
C
LOAD
= 3300pF (Note 10)
C
LOAD
= 3300pF (Note 10)
C
LOAD
= 3300pF (Note 10)
C
LOAD
= 3300pF Each Driver (Note 10)
C
LOAD
= 3300pF Each Driver (Note 10)
Tested with a Square Wave (Notes 6, 10)
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
q
q
MIN
0.84
– 85
0.4
98
0.5
1.0
0.5
TYP
0.86
– 60
0.6
1.5
99.4
1.2
1.5
4.1
2
1.6
MAX
0.88
UNITS
V
µA
V
2
V
%
µA
V
RUN/SS1, 2
ON RUN/SS Pin ON Threshold
1.9
4.5
4
5
85
88
90
90
90
80
V
V
µA
µA
mV
mV
ns
ns
ns
ns
ns
ns
65
62
75
75
50
50
40
40
90
90
160
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
Oscillator
f
OSC
f
LOW
f
HIGH
I
FREQSET
V
3.3OUT
V
3.3IL
V
3.3VL
PGOOD Output
V
PGL
I
PGOOD
V
PG
200
5.2
1.0
240
ns
V
%
mV
V
V
INTV
CC
Linear Regulator
4.8
5.0
0.2
120
4.5
4.7
0.2
190
120
280
220
140
310
–2
3.25
3.35
0.5
0.05
0.1
250
170
350
–1
3.45
2
0.2
0.3
1
–5
10
kHz
kHz
kHz
µA
V
%
%
V
µA
%
%
1708f
3.3V Linear Regulator
3
LTC1708-PG
ELECTRICAL CHARACTERISTICS
SYMBOL
VID Parameters
VIDV
CC
I
VIDVCC
R
FBOUT1/SENSE1
R
RATIO
R
PULL-UP
V
IDT
I
VIDLEAK
V
PULL-UP
VID Operating Supply Voltage
VID Supply Current
Resistance Between ATTNIN/ATTNOUT
Resistor Ratio Accuracy
VID0 to VID4 Pull-Up Resistance
VID Voltage Threshold
VID Input Leakage Current
VID Pull-Up Voltage
(Note 9) VIDV
CC
< VIDV
CC
< 7V
VIDV
CC
= 3V
2.5
Programmed from 0.925V to 2.00V
(Note 9) V
DIODE
= 0.7V
0.4
–0.35
40
1.0
0.1
2.8
1.6
1
3.1
VIDV
CC
= 3.3V (Note 8)
2.7
0.01
10
5.5
5
5
0.25
V
µA
kΩ
%
kΩ
V
µA
V
PARAMETER
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
RUN/SS1, 2
= 5V unless otherwise noted.
CONDITIONS
MIN
TYP
MAX
UNITS
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC1708EG-PG 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 formulas:
LTC1708EG-PG: T
J
= T
A
+ (P
D
• 85°C/W)
Note 4:
The LTC1708-PG is tested in a feedback loop that servos V
ITH1, 2
to a specified voltage and measures the resultant EAIN1, 2.
Note 5:
The supply current is higher due to the gate charge being delivered
at the switching frequency. See Applications Information.
Note 6:
The minimum on-time condition corresponds to the on inductor
peak-to-peak ripple current
≥
40% of I
MAX
(see minimum on-time
considerations in the Applications Information section).
Note 7:
V
FREQSET
pin internally tied to 1.19V reference through a large
resistance.
Note 8:
With all five VID inputs floating (or tied to VIDV
CC
) the VIDV
CC
current is typically < 1µA. However, the VIDV
CC
current will rise and be
approximately equal to the number of grounded VID input pins times
(VIDV
CC
– 0.6V)/40k. (See the Applications Information section.)
Note 9:
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 Applications Information section.)
Note 10:
Rise and fall times are measured at 20% to 80% levels. Delay
and nonoverlap times are measured using 50% levels.
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Output Current
and Mode (Figure 13)
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0.1
CONSTANT FREQUENCY
MODE
PWM MODE
V
IN
= 15V
V
OUT1
= OFF
V
OUT2
= 1.6V
10
1
OUTPUT CURRENT (A)
100
1708 G01
Burst Mode
OPERATION
15A
EFFICIENCY (%)
60
50
40
30
20
10
0
0.01
EFFICIENCY (%)
4
U W
Efficiency vs Output Current
(Figure 13)
100
90
80
70
V
IN
= 10V
V
IN
= 15V
V
IN
= 20V
EXTV
CC
= 0V
V
IN
= 5V
15A
100
Efficiency vs Input Voltage
(Figure 13)
V
OUT1
= OFF
V
OUT2
= 1.6V
EXTV
CC
= 0V
I
OUT
= 7A
90
80
I
OUT
= 12A
70
V
FCB
= OPEN
V
OUT1
= OFF
V
OUT2
= 1.6V
1
0.1
10
OUTPUT CURRENT (A)
100
1708
G02
60
50
5
10
15
20
INPUT VOLTAGE (V)
25
28
1708
G03
1708f
LTC1708-PG
TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs Input Voltage
and Mode (Figure 13)
1000
250
INTV
CC
AND EXTV
CC
SWITCH VOLTAGE (V)
800
EXTV
CC
VOLTAGE DROP (mV)
SUPPLY CURRENT (µA)
BOTH
CONTROLLERS ON
600
400
200
STANDBY
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
60
V
SENSE
(mV)
V
SENSE
(mV)
72
V
SENSE
(mV)
40
20
0
0
1
2
3
V
RUN/SS
(V)
1708 G10
4
U W
1708 G04
1708 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
1708 G05
4.70
– 50 – 25
50
25
75
0
TEMPERATURE (°C)
100
125
1708
G06
Maximum Current Sense Threshold
vs Duty Factor
75
80
70
60
Maximum Current Sense Threshold
vs Percent of Nominal Output
Voltage (Foldback)
50
50
40
30
20
10
25
0
0
20
40
60
DUTY FACTOR (%)
80
100
1708 G08
0
50
100
0
25
75
PERCENT ON NOMINAL OUTPUT VOLTAGE (%)
1708 G09
Maximum Current Sense Threshold
vs Sense Common Mode Voltage
80
90
80
76
70
60
50
40
30
20
10
0
64
–10
–20
–30
Current Sense Threshold
vs I
TH
Voltage
68
5
6
60
0
1
3
4
2
COMMON MODE VOLTAGE (V)
5
1708 G11
0
0.5
1
1.5
V
ITH
(V)
2
2.5
1708
G12
1708f
5
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