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1
LTC1778/LTC1778-1
1778fa
Wide Operating Range,
No R
SENSE
TM
Step-Down Controller
s
No Sense Resistor Required
s
True Current Mode Control
s
Optimized for High Step-Down Ratios
s
t
ON(MIN)
100ns
s
Extremely Fast Transient Response
s
Stable with Ceramic C
OUT
s
Dual N-Channel MOSFET Synchronous Drive
s
Power Good Output Voltage Monitor (LTC1778)
s
Adjustable On-Time (LTC1778-1)
s
Wide V
IN
Range: 4V to 36V
s
1% 0.8V Voltage Reference
s
Adjustable Current Limit
s
Adjustable Switching Frequency
s
Programmable Soft-Start
s
Output Overvoltage Protection
s
Optional Short-Circuit Shutdown Timer
s
Micropower Shutdown: I
Q
< 30
A
s
Available in a 16-Pin Narrow SSOP Package
s
Notebook and Palmtop Computers
s
Distributed Power Systems
The LTC
1778 is a synchronous step-down switching
regulator controller optimized for CPU power. The con-
troller uses a valley current control architecture to deliver
very low duty cycles with excellent transient response
without requiring a sense resistor. Operating frequency is
selected by an external resistor and is compensated for
variations in V
IN
.
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 opera-
tion 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 user program-
mable. Wide supply range allows operation from 4V to 36V
at the input and from 0.8V up to (0.9)V
IN
at the output.
, LTC and LT are registered trademarks of Linear Technology Corporation.
No R
SENSE
is a trademark of Linear Technology Corporation.
Figure 1. High Efficiency Step-Down Converter
Efficiency vs Load Current
+
D
B
CMDSH-3
D1
B340A
L1
1.8
H
C
VCC
4.7
F
C
IN
10
F
50V
3
V
IN
5V TO 28V
V
OUT
2.5V
10A
+
C
OUT
180
F
4V
2
M2
Si4874
1778 F01a
M1
Si4884
R
ON
1.4M
C
SS
0.1
F
I
ON
V
IN
TG
SW
BOOST
RUN/SS
I
TH
SGND
INTV
CC
BG
PGND
V
FB
PGOOD
C
B
0.22
F
R
C
20k
LTC1778
C
C
500pF
R2
30.1k
R1
14k
LOAD CURRENT (A)
0.01
EFFICIENCY (%)
80
90
1778 F01b
70
60
0.1
1
10
100
V
IN
= 5V
V
OUT
= 2.5V
V
IN
= 25V
DESCRIPTIO
U
FEATURES
APPLICATIO S
U
TYPICAL APPLICATIO
U
2
LTC1778/LTC1778-1
1778fa
Input Supply Voltage (V
IN
, I
ON
)................. 36V to 0.3V
Boosted Topside Driver Supply Voltage
(BOOST) ................................................... 42V to 0.3V
SW Voltage .................................................. 36V to 5V
EXTV
CC
, (BOOST SW), RUN/SS,
PGOOD Voltages ....................................... 7V to 0.3V
FCB, V
ON
, V
RNG
Voltages .......... INTV
CC
+ 0.3V to 0.3V
I
TH
, V
FB
Voltages...................................... 2.7V to 0.3V
ORDER PART
NUMBER
LTC1778EGN
T
JMAX
= 125
C,
JA
= 130
C/ W
ABSOLUTE AXI U RATI GS
W
W
W
U
PACKAGE/ORDER I FOR ATIO
U
U
W
GN PART MARKING
1778
TG, BG, INTV
CC
, EXTV
CC
Peak Currents .................... 2A
TG, BG, INTV
CC
, EXTV
CC
RMS Currents .............. 50mA
Operating Ambient Temperature
Range (Note 4) ................................... 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
TOP VIEW
GN PACKAGE
16-LEAD PLASTIC SSOP
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
RUN/SS
PGOOD
V
RNG
FCB
I
TH
SGND
I
ON
V
FB
BOOST
TG
SW
PGND
BG
INTV
CC
V
IN
EXTV
CC
ORDER PART
NUMBER
LTC1778EGN-1
GN PART MARKING
17781
TOP VIEW
GN PACKAGE
16-LEAD PLASTIC SSOP
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
RUN/SS
V
ON
V
RNG
FCB
I
TH
SGND
I
ON
V
FB
BOOST
TG
SW
PGND
BG
INTV
CC
V
IN
EXTV
CC
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.
ELECTRICAL CHARACTERISTICS
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Main Control Loop
I
Q
Input DC Supply Current
Normal
900
2000
A
Shutdown Supply Current
15
30
A
V
FB
Feedback Reference Voltage
I
TH
= 1.2V (Note 3)
q
0.792
0.800
0.808
V
V
FB(LINEREG)
Feedback Voltage Line Regulation
V
IN
= 4V to 30V, I
TH
= 1.2V (Note 3)
0.002
%/V
V
FB(LOADREG)
Feedback Voltage Load Regulation
I
TH
= 0.5V to 1.9V (Note 3)
q
0.05
0.3
%
I
FB
Feedback Input Current
V
FB
= 0.8V
5
50
nA
g
m(EA)
Error Amplifier Transconductance
I
TH
= 1.2V (Note 3)
q
1.4
1.7
2
mS
V
FCB
Forced Continuous Threshold
q
0.76
0.8
0.84
V
I
FCB
Forced Continuous Pin Current
V
FCB
= 0.8V
1
2
A
t
ON
On-Time
I
ON
= 30
A, V
ON
= 0V (LTC1778-1)
198
233
268
ns
I
ON
= 15
A, V
ON
= 0V (LTC1778-1)
396
466
536
ns
t
ON(MIN)
Minimum On-Time
I
ON
= 180
A
50
100
ns
T
JMAX
= 125
C,
JA
= 130
C/ W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
(Note 1)
3
LTC1778/LTC1778-1
1778fa
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.
ELECTRICAL CHARACTERISTICS
Note 1: Absolute Maximum Ratings are those values beyond which the life of
a device may be impaired.
Note 2: T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
as follows:
LTC1778E: T
J
= T
A
+ (P
D
130
C/W)
Note 3: The LTC1778 is tested in a feedback loop that adjusts V
FB
to achieve
a specified error amplifier output voltage (I
TH
).
Note 4: The LTC1778E 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.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
t
OFF(MIN)
Minimum Off-Time
I
ON
= 30
A
250
400
ns
V
SENSE(MAX)
Maximum Current Sense Threshold
V
RNG
= 1V, V
FB
= 0.76V
q
113
133
153
mV
V
PGND
V
SW
V
RNG
= 0V, V
FB
= 0.76V
q
79
93
107
mV
V
RNG
= INTV
CC
, V
FB
= 0.76V
q
158
186
214
mV
V
SENSE(MIN)
Minimum Current Sense Threshold
V
RNG
= 1V, V
FB
= 0.84V
67
mV
V
PGND
V
SW
V
RNG
= 0V, V
FB
= 0.84V
47
mV
V
RNG
= INTV
CC
, V
FB
= 0.84V
93
mV
V
FB(OV)
Output Overvoltage Fault Threshold
5.5
7.5
9.5
%
V
FB(UV)
Output Undervoltage Fault Threshold
520
600
680
mV
V
RUN/SS(ON)
RUN Pin Start Threshold
q
0.8
1.5
2
V
V
RUN/SS(LE)
RUN Pin Latchoff Enable Threshold
RUN/SS Pin Rising
4
4.5
V
V
RUN/SS(LT)
RUN Pin Latchoff Threshold
RUN/SS Pin Falling
3.5
4.2
V
I
RUN/SS(C)
Soft-Start Charge Current
V
RUN/SS
= 0V
0.5
1.2
3
A
I
RUN/SS(D)
Soft-Start Discharge Current
V
RUN/SS
= 4.5V, V
FB
= 0V
0.8
1.8
3
A
V
IN(UVLO)
Undervoltage Lockout
V
IN
Falling
q
3.4
3.9
V
V
IN(UVLOR)
Undervoltage Lockout Release
V
IN
Rising
q
3.5
4
V
TG R
UP
TG Driver Pull-Up On Resistance
TG High
2
3
TG R
DOWN
TG Driver Pull-Down On Resistance
TG Low
2
3
BG R
UP
BG Driver Pull-Up On Resistance
BG High
3
4
BG R
DOWN
BG Driver Pull-Down On Resistance
BG Low
1
2
TG t
r
TG Rise Time
C
LOAD
= 3300pF
20
ns
TG t
f
TG Fall Time
C
LOAD
= 3300pF
20
ns
BG t
r
BG Rise Time
C
LOAD
= 3300pF
20
ns
BG t
f
BG Fall Time
C
LOAD
= 3300pF
20
ns
Internal V
CC
Regulator
V
INTVCC
Internal V
CC
Voltage
6V < V
IN
< 30V, V
EXTVCC
= 4V
q
4.7
5
5.3
V
V
LDO(LOADREG)
Internal V
CC
Load Regulation
I
CC
= 0mA to 20mA, V
EXTVCC
= 4V
0.1
2
%
V
EXTVCC
EXTV
CC
Switchover Voltage
I
CC
= 20mA, V
EXTVCC
Rising
q
4.5
4.7
V
V
EXTVCC
EXTV
CC
Switch Drop Voltage
I
CC
= 20mA, V
EXTVCC
= 5V
150
300
mV
V
EXTVCC(HYS)
EXTV
CC
Switchover Hysteresis
200
mV
PGOOD Output (LTC1778 Only)
V
FBH
PGOOD Upper Threshold
V
FB
Rising
5.5
7.5
9.5
%
V
FBL
PGOOD Lower Threshold
V
FB
Falling
5.5
7.5
9.5
%
V
FB(HYS)
PGOOD Hysteresis
V
FB
Returning
1
2
%
V
PGL
PGOOD Low Voltage
I
PGOOD
= 5mA
0.15
0.4
V
4
LTC1778/LTC1778-1
1778fa
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
LOAD CURRENT (A)
0.001
EFFICIENCY (%)
70
80
10
1778 G03
60
50
0.01
0.1
1
100
90
DISCONTINUOUS
MODE
CONTINUOUS
MODE
V
IN
= 10V
V
OUT
= 2.5V
EXTV
CC
= 5V
FIGURE 9 CIRCUIT
Efficiency vs Load Current
Efficiency vs Input Voltage
INPUT VOLTAGE (V)
0
80
EFFICIENCY (%)
85
90
95
100
5
10
15
20
1778 G04
25
30
I
LOAD
= 1A
I
LOAD
= 10A
FCB = 5V
FIGURE 9 CIRCUIT
Frequency vs Input Voltage
INPUT VOLTAGE (V)
5
FREQUENCY (kHz)
240
260
25
1778 G05
220
200
10
15
20
300
280
I
OUT
= 10A
FCB = 0V
FIGURE 9 CIRCUIT
I
OUT
= 0A
Load Regulation
LOAD CURRENT (A)
0
V
OUT
(%)
0.2
0.1
8
1778 G06
0.3
0.4
2
4
6
10
0
FIGURE 9 CIRCUIT
I
TH
Voltage vs Load Current
LOAD CURRENT (A)
0
I
TH
VOLTAGE (V)
1.0
1.5
1778 G07
0.5
0
5
10
15
2.5
2.0
CONTINUOUS
MODE
DISCONTINUOUS
MODE
FIGURE 9 CIRCUIT
Transient Response
(Discontinuous Mode)
Transient Response
V
OUT
50mV/DIV
I
L
5A/DIV
20
s/DIV
1778 G01
LOAD STEP 0A TO 10A
V
IN
= 15V
V
OUT
= 2.5V
FCB = 0V
FIGURE 9 CIRCUIT
V
OUT
50mV/DIV
I
L
5A/DIV
20
s/DIV
1778 G02
LOAD STEP 1A TO 10A
V
IN
= 15V
V
OUT
= 2.5V
FCB = INTV
CC
FIGURE 9 CIRCUIT
Start-Up
RUN/SS
2V/DIV
I
L
5A/DIV
50ms/DIV
1778 G19
V
IN
= 15V
V
OUT
= 2.5V
R
LOAD
= 0.25
V
OUT
1V/DIV
LOAD CURRENT (A)
0
0
FREQUENCY (kHz)
50
100
150
200
250
300
2
4
6
8
1778 G26
10
CONTINUOUS MODE
DISCONTINUOUS
MODE
Frequency vs Load Current
5
LTC1778/LTC1778-1
1778fa
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
Maximum Current Sense
Threshold vs Temperature
Maximum Current Sense
Threshold vs V
RNG
Voltage
Feedback Reference Voltage
vs Temperature
V
RNG
VOLTAGE (V)
0.5
0
MAXIMUM CURRENT SENSE THRESHOLD (mV)
50
100
150
200
300
0.75
1.0
1.25
1.5
1778 G10
1.75
2.0
250
TEMPERATURE (
C)
50
25
100
MAXIMUM CURRENT SENSE THRESHOLD (mV)
120
150
0
50
75
1778 G11
110
140
130
25
100
125
V
RNG
= 1V
TEMPERATURE (
C)
50
0.78
FEEDBACK REFERENCE VOLTAGE (V)
0.79
0.80
0.81
0.82
25
0
25
50
1778 G12
75
100
125
Current Limit Foldback
V
FB
(V)
0
0
MAXIMUM CURRENT SENSE THRESHOLD (mV)
25
50
75
100
125
150
V
RNG
= 1V
0.2
0.4
0.6
0.8
1778 G09
On-Time vs I
ON
Current
I
ON
CURRENT (
A)
1
10
ON-TIME (ns)
100
1k
10k
10
100
1778 G20
V
VON
= 0V
V
ON
VOLTAGE (V)
0
ON-TIME (ns) 400
600
1778 G21
200
0
1
2
3
1000
I
ION
= 30
A
800
TEMPERATURE (
C)
50
ON-TIME (ns)
200
250
300
25
75
1778 G22
150
100
25
0
50
100
125
50
0
I
ION
= 30
A
V
VON
= 0V
On-Time vs V
ON
Voltage
On-Time vs Temperature
RUN/SS VOLTAGE (V)
1.5
0
MAXIMUM CURRENT SENSE THRESHOLD (mV)
25
50
75
100
125
150
V
RNG
= 1V
2
2.5
3
3.5
1778 G23
Maximum Current Sense
Threshold vs RUN/SS Voltage
Current Sense Threshold
vs I
TH
Voltage
I
TH
VOLTAGE (V)
0
200
CURRENT SENSE THRESHOLD (mV)
100
0
100
200
300
0.5
1.0
1.5
2.0
1778 G08
2.5
3.0
V
RNG
=
1V
0.7V
0.5V
1.4V
2V
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