FEATURES
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LTC1735-1
High Efficiency
Synchronous Step-Down
Switching Regulator
DESCRIPTIO
The LTC
®
1735-1 is a synchronous step-down switching
regulator controller optimized for CPU power. OPTI-LOOP
compensation allows the transient response to be opti-
mized over a wide range of output capacitance and ESR
values.
The operating frequency (synchronizable up to 500kHz) is
set by an external capacitor allowing maximum flexibility
in optimizing efficiency. The output voltage is monitored
by a power good window comparator that indicates when
the output is within 7.5% of its programmed value, con-
forming to Intel Mobile CPU Specifications.
Protection features include internal foldback current lim-
iting, output overvoltage crowbar and optional short-
circuit shutdown. Soft-start is provided by an external
capacitor that can be used to properly sequence supplies.
The operating current level is user-programmable via an
external current sense resistor. Wide input supply range
allows operation from 3.5V to 30V (36V maximum).
Pin defeatable Burst Mode
TM
operation provides high
efficiency at low load currents while 99% duty cycle
provides low dropout operation.
s
s
s
s
Dual N-Channel MOSFET Synchronous Drive
Programmable/Synchronizable Fixed Frequency
V
OUT
Range: 0.8V to 7V
Wide V
IN
Range: 3.5V to 36V Operation
Very Low Dropout Operation: 99% Duty Cycle
OPTI-LOOP
TM
Compensation Minimizes C
OUT
±
1% Output Voltage Accuracy
Power Good Output Voltage Monitor
Internal Current Foldback
Output Overvoltage Crowbar Protection
Latched Short-Circuit Shutdown Timer
with Defeat Option
Optional Programmable Soft-Start
Remote Output Voltage Sense
Logic Controlled Micropower Shutdown: I
Q
< 25µA
Available in 16-Lead Narrow SSOP and SO Packages
APPLICATIO S
s
s
s
Notebook and Palmtop Computers, PDAs
Power Supply for Mobile Pentium
®
III Processor with
SpeedStep
TM
Technology
Cellular Telephones and Wireless Modems
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode and OPTI-LOOP are trademarks of Linear Technology Corporation.
Pentium is a registered trademark of Intel Corporation. SpeedStep is a trademark of Intel
Corporation.
TYPICAL APPLICATIO
PGOOD
C
OSC
47pF
C
SS
0.1µF
C
C2
330pF
R
C1
33k
C
C1
47pF
1
2
3
4
5
47pF
1000pF 6
7
8
V
IN
4.5V TO 24V
C
OSC
RUN/SS
I
TH
TG
BOOST
SW
V
IN
INTV
CC
BG
PGND
EXTV
CC
10Ω
10Ω
16
15
14
13
12
11
10
9
5V
(OPTIONAL)
D1
CMDSH-3
C
B
0.22µF
Q1
FDS6680A
L1
1.2µH
D2
MBRS340T3
Q2, Q3
FDS6680A
×2
C
IN
22µF
50V
CERAMIC
×2
R
SENSE
0.004Ω
LTC1735-1
–
PGOOD
SENSE
SENSE
+
V
OSENSE
SGND
+
4.7µF
Figure 1. CPU Core DC/DC Converter with Dynamic Voltage Selection from SpeedStep Enabled Processors
U
U
U
C
IN
: MARCON THCR70E1H226ZT
C
OUT
: PANASONIC EEFUE06181R
L1: PANASONIC ETQP6RZ1R20HFA
R
SENSE
: IRC CRF2010-01-R004J
V
OUT
1.35V TO 1.60V
12A
C
OUT
180µF
4V
PANASONIC SP
×4
V
SEL
= 1: V
OUT
= 1.60V
V
SEL
= 0: V
OUT
= 1.35V
GND
1735-1 F01
47pF
R1
10k
0.5%
+
47pF
R2
14.3k
0.5%
R3
33.2k
1%
Q4
2N7002
1
LTC1735-1
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
C
OSC
1
RUN/SS 2
I
TH
3
PGOOD 4
SENSE
–
5
SENSE
+
6
V
OSENSE
7
SGND 8
16 TG
15 BOOST
14 SW
13 V
IN
12 INTV
CC
11 BG
10 PGND
9
EXTV
CC
Input Supply Voltage (V
IN
)........................ 36V to – 0.3V
Topside Driver Supply Voltage (BOOST)... 42V to – 0.3V
Switch Voltage (SW) ................................... 36V to – 5V
INTV
CC
, EXTV
CC
(BOOST, SW) Voltages ..... 7V to – 0.3V
SENSE
+
, SENSE
–
,
PGOOD Voltages ................ 1.1(INTV
CC
+ 0.3V) to – 0.3V
I
TH
, V
OSENSE
, C
OSC
Voltages .....................2.7V to – 0.3V
RUN/SS Voltage ....................(INTV
CC
+ 0.3V) to – 0.3V
Peak Driver Output Current <10µs (TG, BG) .............. 3A
INTV
CC
Output Current ......................................... 50mA
Operating Ambient Temperature Range
LTC1735C-1 ............................................ 0°C to 85°C
LTC1735I-1 ........................................ – 40°C to 85°C
Junction Temperature (Note 2) ............................. 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LTC1735CGN-1
LTC1735CS-1
LTC1735IGN-1
LTC1735IS-1
GN PACKAGE
S PACKAGE
16-LEAD PLASTIC SSOP 16-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 130°C/W (GN)
T
JMAX
= 125°C,
θ
JA
= 110°C/W (S)
Consult factory for Military grade parts.
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 15V, V
RUN/SS
= 5V unless otherwise noted.
SYMBOL
I
VOSENSE
V
OSENSE
∆V
LINEREG
∆V
LOADREG
PARAMETER
Feedback Current
Feedback Voltage
Reference Voltage Line Regulation
Output Voltage Load Regulation
CONDITIONS
(Note 3)
(Note 3)
V
IN
= 3.6V to 30V (Note 3)
(Note 3)
Measured in Servo Loop; V
ITH
= 0.7V
Measured in Servo Loop; V
ITH
= 2V
In Dropout
q
q
q
q
ELECTRICAL CHARACTERISTICS
Main Control Loop
MIN
TYP
–4
MAX
– 25
0.808
0.02
0.3
– 0.3
UNITS
nA
V
%/V
%
%
%
mmho
0.792
0.8
0.001
0.1
– 0.1
DF Max
g
m
V
OVL
I
Q
Maximum Duty Factor
Transconductance Amplifier g
m
Feedback Overvoltage Lockout
Input DC Supply Current
Normal Mode
Shutdown
Run Pin Start Threshold
Run Pin Begin Latchoff Threshold
Soft-Start Charge Current
RUN/SS Discharge Current
Undervoltage Lockout
Maximum Current Sense Threshold
SENSE Pins Total Source Current
Minimum On-Time
98
0.84
99.4
1.3
0.86
450
15
0.88
(Note 5)
3.6V < V
IN
< 30V
V
RUN/SS
= 0V
V
RUN/SS
, Ramping Positive
V
RUN/SS
, Ramping Positive
V
RUN/SS
= 0V
Soft Short Condition, V
OSENSE
= 0.5V,
V
RUN/SS
= 4.5V
Measured at V
IN
Pin (Ramping Negative)
V
OSENSE
= 0.7V
V
SENSE–
= V
SENSE+
= 0V
Tested with a Square Wave (Note 4)
q
q
25
1.9
4.5
4
3.9
85
80
200
V
RUN/SS
I
RUN/SS
I
SCL
UVLO
∆V
SENSE(MAX)
I
SENSE
t
ON(MIN)
1.0
– 0.7
0.5
1.5
4.1
– 1.2
2
3.5
60
75
60
160
2
U
V
µA
µA
V
V
µA
µA
V
mV
µA
ns
W
U
U
W W
W
LTC1735-1
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 15V, V
RUN/SS
= 5V unless otherwise noted.
SYMBOL
TG t
r
TG t
f
BG t
r
BG t
f
TG/BG T1D
TG/BG T2D
PARAMETER
TG Transition Time:
Rise Time
Fall Time
BG Transition Time:
Rise Time
Fall Time
Top Gate Off to Synchronous
Gate-On Delay Time
Synchronous Gate Off to Top
Gate-On Delay Time
Internal V
CC
Voltage
INTV
CC
Load Regulation
EXTV
CC
Drop Voltage
EXTV
CC
Switchover Voltage
EXTV
CC
Hysteresis
Oscillator Frequency
Maximum Sync Frequency Ratio
PGOOD Threshold for Sync
PGOOD Threshold for Force Cont.
PGOOD Voltage Low
PGOOD Pull-Up Current
PGOOD Trip Level
I
PGOOD
= 2mA
V
PGOOD
= 0.85V
V
OSENSE
With Respect to Set Output Voltage
V
OSENSE
Ramping Negative
V
OSENSE
Ramping Positive
– 6.0
6.0
Ramping Negative
0.9
0.76
(Note 6), C
OSC
= 43pF
265
CONDITIONS
(Note 7)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
(Note 7)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF Each Driver
C
LOAD
= 3300pF Each Driver
MIN
TYP
50
50
50
40
100
70
MAX
90
90
90
80
UNITS
ns
ns
ns
ns
ns
ns
ELECTRICAL CHARACTERISTICS
Internal V
CC
Regulator
V
INTVCC
V
LDO(INT)
V
LDO(EXT)
V
EXTVCC
V
EXTVCC(HYS)
Oscillator
f
OSC
f
H
/f
OSC
PGOOD Pin
V
PG(SYNC)
V
PG(FC)
V
PGL
I
PGOOD
V
PG
1.2
0.8
110
– 0.17
–7.5
7.5
–9.5
9.5
0.84
200
V
V
mV
µA
%
%
300
1.3
335
kHz
6V < V
IN
< 30V, V
EXTVCC
= 4V
I
CC
= 0mA to 20mA, V
EXTVCC
= 4V
I
CC
= 20mA, V
EXTVCC
= 5V
I
CC
= 20mA, EXTV
CC
Ramping Positive
q
5.0
5.2
0.2
130
5.4
1
200
V
%
mV
V
V
4.5
4.7
0.2
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
Note 2:
T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formulas:
LTC1735CS-1, LTC1735IS-1: T
J
= T
A
+ (P
D
• 110
°C/W)
LTC1735CGN-1, LTC1735IGN-1: T
J
= T
A
+ (P
D
• 130°C/W)
Note 3:
The LTC1735-1 is tested in a feedback loop that servos V
OSENSE
to the balance point for the error amplifier (V
ITH
= 1.2V).
Note 4:
The minimum on-time condition corresponds to an inductor
peak-to-peak ripple current > 40% of I
MAX
(see Minimum On-Time
Considerations in the Applications Information section).
Note 5:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.
Note 6:
Oscillator frequency is tested by measuring the C
OSC
charge
current (I
OSC
) and applying the formula:
–1
8.477(10
8
)
1
1
f
OSC
(kHz)
=
+
C
OSC
(pF)
+
11
I
CHG
I
DIS
Note 7:
Rise and fall times are measured using 10% to 90% levels.
Delay times are measured using 50% levels.
3
LTC1735-1
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Load Current
(3 Operating Modes)
100
90
80
EFFICIENCY (%)
EXTV
CC
OPEN
BURST
70
CONT
60
50
40
30
20
0.001
V
IN
= 10V
V
OUT
= 3.3V
R
S
= 0.01Ω
f
O
= 300kHz
0.1
0.01
1
LOAD CURRENT (A)
10
1735-1 G01
EFFICIENCY (%)
EFFICIENCY (%)
SYNC
Efficiency vs Input Voltage
100
EXTV
CC
OPEN
V
OUT
= 1.6V
95 FIGURE 1
90
I
OUT
= 5A
85
I
OUT
= 0.5A
80
75
70
–0.4
0
NORMALIZED V
OUT
(%)
EFFICIENCY (%)
V
IN
– V
OUT
(mV)
0
5
10
15
20
INPUT VOLTAGE (V)
Input and Shutdown Currents
vs Input Voltage
500
EXTV
CC
OPEN
400
80
100
4
3
2
1
0
0
5
20
15
25
10
INPUT VOLTAGE (V)
30
35
300
60
EXTV
CC
– INTV
CC
(mV)
INPUT CURRENT (µA)
INTV
CC
VOLTAGE (V)
200
SHUTDOWN
100
EXTV
CC
= 5V
0
0
5
20
15
10
25
INPUT VOLTAGE (V)
30
35
4
U W
25
1735-1 G04
1735-1 G07
Efficiency vs Load Current
100
90
V
IN
= 5V
80
70
60
50
40
10mA
V
IN
= 24V
V
IN
= 15V
90
EXTV
CC
= 5V
V
OUT
= 1.6V
100
95
Efficiency vs Input Voltage
EXTV
CC
= 5V
V
OUT
= 1.6V
FIGURE 1
I
OUT
= 5A
85
I
OUT
= 0.5A
80
75
70
100mA
1A
LOAD CURRENT (A)
10A
1735-1 G02
0
5
10
15
20
INPUT VOLTAGE (V)
25
30
1735-1 G03
Load Regulation
500
FCB = 0V
V
IN
= 15V
FIGURE 1
V
IN
– V
OUT
Dropout Voltage
vs Load Current
R
SENSE
= 0.005Ω
V
OUT
= 5V – 5% DROP
400
–0.1
300
–0.2
200
–0.3
100
0
0
2
6
4
LOAD CURRENT (A)
8
10
1735-1 G05
30
0
2
4
6
LOAD CURRENT (A)
8
10
1735-1 G06
INTV
CC
Line Regulation
6
5
1mA LOAD
500
EXTV
CC
Switch Drop
vs INTV
CC
Load Current
400
SHUTDOWN CURRENT (µA)
300
40
200
20
100
0
0
0
10
30
40
20
INTV
CC
LOAD CURRENT (mA)
50
1735-1 G09
1735-1 G08
LTC1735-1
TYPICAL PERFOR A CE CHARACTERISTICS
Maximum Current Sense Threshold
vs Normalized Output Voltage
(Foldback)
MAXIMUM CURRENT SENSE THRESHOLD (mV)
MAXIMUM CURRENT SENSE THRESHOLD (mV)
V
SENSE(CM)
= 1.6V
70
60
50
40
30
20
10
0
0
50
25
75
NORMALIZED OUTPUT VOLTAGE (%)
100
MAXIMUM CURRENT SENSE THRESHOLD (mV)
80
Maximum Current Sense Voltage
vs I
TH
Voltage
MAXIMUM CURRENT SENSE THRESHOLD (mV)
90
MAXIMUM CURRENT SENSE VOLTAGE (V)
80
70
60
50
40
30
20
10
0
–10
–20
–30
0
0.5
1
1.5
V
ITH
(V)
2
2.5
1735-1 G13
70
V
ITH
(V)
SENSE Pins Total Source Current
100
50
2.0
I
TH
VOLTAGE (V)
V
IN
= 10V
V
OUT
= 3.3V
R
SENSE
= 0.01Ω
f
O
= 300kHz
CONTINUOUS
MODE
SYNCHRONIZED f = f
O
AVERAGE OUTPUT CURRENT I
OUT
/I
MAX
(%)
I
SENSE
(µA)
0
–50
–100
0
2
4
V
SENSE
COMMON MODE VOLTAGE (V)
1735-1 G16
U W
1735-1 G10
Maximum Current Sense Threshold
vs V
RUN/SS
80
80
Maximum Current Sense Threshold
vs Sense Common Mode Voltage
60
76
72
40
68
20
64
0
0
1
2
3
V
RUN/SS
(V)
1735-1 G11
4
5
6
60
0
1
3
4
2
COMMON MODE VOLTAGE (V)
5
1735-1 G12
Maximum Current Sense Threshold
vs Temperature
80
2.5
V
SENSE(CM)
= 1.6V
2.0
V
ITH
vs V
RUN/SS
V
OSENSE
= 0.7V
75
1.5
1.0
65
0.5
60
–40 –15
85
10
35
60
TEMPERATURE (°C)
110
135
0
0
1
2
3
V
RUN/SS
(V)
4
5
6
1735-1 G15
1735-1 G18
I
TH
Voltage vs Load Current
2.5
100
Output Current vs Duty Cycle
I
OUT
/I
MAX
(SYNCHRONIZED)
80
I
OUT
/I
MAX
(FREE RUN)
1.5
60
1.0
Burst Mode
OPERATION
40
0.5
20
f
SYNC
= f
O
0
0
20
40
60
DUTY CYCLE (%)
80
100
1735-1 G14
0
6
0
1
2
3
4
LOAD CURRENT (A)
5
6
1735-1 G17
5
FPGA/CPLD
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