LTC1778/LTC1778-1
Wide Operating Range,
No R
SENSE
TM
Step-Down Controller
FEATURES
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DESCRIPTIO
No Sense Resistor Required
True Current Mode Control
Optimized for High Step-Down Ratios
t
ON(MIN)
≤
100ns
Extremely Fast Transient Response
Stable with Ceramic C
OUT
Dual N-Channel MOSFET Synchronous Drive
Power Good Output Voltage Monitor (LTC1778)
Adjustable On-Time (LTC1778-1)
Wide V
IN
Range: 4V to 36V
±1%
0.8V Voltage Reference
Adjustable Current Limit
Adjustable Switching Frequency
Programmable Soft-Start
Output Overvoltage Protection
Optional Short-Circuit Shutdown Timer
Micropower Shutdown: I
Q
< 30µA
Available in a 16-Pin Narrow SSOP Package
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.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 5481178, 6100678, 6580258, 5847554, 6304066
APPLICATIO S
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Notebook and Palmtop Computers
Distributed Power Systems
TYPICAL APPLICATIO
R
ON
1.4MΩ
C
SS
0.1µF
RUN/SS
C
C
500pF
I
TH
R
C
20k
SGND
LTC1778
INTV
CC
BG
I
ON
V
IN
TG
SW
BOOST
M1
Si4884
C
B
0.22µF
D
B
CMDSH-3
M2
Si4874
C
VCC
4.7µF
L1
1.8µH
EFFICIENCY (%)
C
IN
10µF
50V
×3
V
IN
5V TO 28V
90
+
D1
B340A
R2
30.1k
C
OUT
180µF
4V
×2
V
OUT
2.5V
10A
+
PGOOD
PGND
V
FB
R1
14k
1778 F01a
Figure 1. High Efficiency Step-Down Converter
1778fb
U
Efficiency vs Load Current
100
V
OUT
= 2.5V
V
IN
= 5V
V
IN
= 25V
80
70
60
0.01
1
0.1
LOAD CURRENT (A)
10
1778 F01b
U
U
1
LTC1778/LTC1778-1
ABSOLUTE
AXI U
RATI GS
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
PACKAGE/ORDER I FOR ATIO
TOP VIEW
RUN/SS 1
PGOOD 2
V
RNG
3
FCB 4
I
TH
5
SGND 6
I
ON
7
V
FB
8
16 BOOST
15 TG
14 SW
13 PGND
12 BG
11 INTV
CC
10 V
IN
9
EXTV
CC
ORDER PART
NUMBER
LTC1778EGN
LTC1778IGN
GN PART MARKING
1778
1778I
GN PACKAGE
16-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 130°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 Input Current
Error Amplifier Transconductance
Forced Continuous Threshold
Forced Continuous Pin Current
On-Time
Minimum On-Time
Main Control Loop
The
●
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
V
FB
∆V
FB(LINEREG)
∆V
FB(LOADREG)
I
FB
g
m(EA)
V
FCB
I
FCB
t
ON
t
ON(MIN)
I
TH
= 1.2V (Note 3) LTC1778E
I
TH
= 1.2V (Note 3) LTC1778I
V
IN
= 4V to 30V, I
TH
= 1.2V (Note 3)
I
TH
= 0.5V to 1.9V (Note 3)
V
FB
= 0.8V
I
TH
= 1.2V (Note 3)
V
FCB
= 0.8V
I
ON
= 30µA, V
ON
= 0V (LTC1778-1)
I
ON
= 15µA, V
ON
= 0V (LTC1778-1)
I
ON
= 180µA
2
U
U
W
W W
U
W
(Note 1)
TG, BG, INTV
CC
, EXTV
CC
Peak Currents .................... 2A
TG, BG, INTV
CC
, EXTV
CC
RMS Currents .............. 50mA
Operating Ambient Temperature Range (Note 4)
LTC1778E ........................................... – 40°C to 85°C
LTC1778I .......................................... – 40°C to 125°C
Junction Temperature (Note 2) ............................ 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
TOP VIEW
RUN/SS 1
V
ON
2
V
RNG
3
FCB 4
I
TH
5
SGND 6
I
ON
7
V
FB
8
16 BOOST
15 TG
14 SW
13 PGND
12 BG
11 INTV
CC
10 V
IN
9
EXTV
CC
ORDER PART
NUMBER
LTC1778EGN-1
GN PART MARKING
17781
GN PACKAGE
16-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 130°C/ W
900
15
●
●
●
●
●
2000
30
0.808
0.812
– 0.3
±50
2
0.84
–2
268
536
100
µA
µA
V
V
%/V
%
nA
mS
V
µA
ns
ns
ns
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0.792
0.792
0.800
0.800
0.002
– 0.05
–5
1.4
0.76
198
396
1.7
0.8
–1
233
466
50
LTC1778/LTC1778-1
ELECTRICAL CHARACTERISTICS
SYMBOL
t
OFF(MIN)
V
SENSE(MAX)
PARAMETER
Minimum Off-Time
Maximum Current Sense Threshold
V
PGND
– V
SW
Minimum Current Sense Threshold
V
PGND
– V
SW
Output Overvoltage Fault Threshold
Output Undervoltage Fault Threshold
RUN Pin Start Threshold
RUN Pin Latchoff Enable Threshold
RUN Pin Latchoff Threshold
Soft-Start Charge Current
Soft-Start Discharge Current
Undervoltage Lockout
Undervoltage Lockout Release
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
PGOOD Upper Threshold
PGOOD Lower Threshold
PGOOD Hysteresis
PGOOD Low Voltage
The
●
denotes specifications which apply over the full operating
temperature range, otherwise specifications are T
A
= 25°C. V
IN
= 15V unless otherwise noted.
CONDITIONS
I
ON
= 30µA
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
●
●
●
●
MIN
113
79
158
TYP
250
133
93
186
– 67
– 47
– 93
7.5
600
1.5
4
3.5
MAX
400
153
107
214
UNITS
ns
mV
mV
mV
mV
mV
mV
V
SENSE(MIN)
∆V
FB(OV)
V
FB(UV)
V
RUN/SS(ON)
V
RUN/SS(LE)
V
RUN/SS(LT)
I
RUN/SS(C)
I
RUN/SS(D)
V
IN(UVLO)
V
IN(UVLOR)
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)
∆V
FBH
∆V
FBL
∆V
FB(HYS)
V
PGL
9.5
680
2
4.5
4.2
–3
3
3.9
4
3
3
4
2
%
mV
V
V
V
µA
µA
V
V
Ω
Ω
Ω
Ω
ns
ns
ns
ns
0.8
RUN/SS Pin Rising
RUN/SS Pin Falling
V
RUN/SS
= 0V
V
RUN/SS
= 4.5V, V
FB
= 0V
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
●
●
●
●
– 0.5
0.8
– 1.2
1.8
3.4
3.5
2
2
3
1
20
20
20
20
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
5.5
– 5.5
7.5
– 7.5
1
0.15
9.5
– 9.5
2
0.4
300
5.3
±2
V
%
V
mV
mV
%
%
%
V
PGOOD Output (LTC1778 Only)
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. The LTC1778I is guaranteed over the full – 40°C to 125°C
operating temperature range.
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LTC1778/LTC1778-1
TYPICAL PERFOR A CE CHARACTERISTICS
Transient Response
Transient Response
(Discontinuous Mode)
RUN/SS
2V/DIV
V
OUT
1V/DIV
I
L
5A/DIV
I
L
5A/DIV
I
L
5A/DIV
V
OUT
50mV/DIV
20µs/DIV
LOAD STEP 0A TO 10A
V
IN
= 15V
V
OUT
= 2.5V
FCB = 0V
FIGURE 9 CIRCUIT
Efficiency vs Load Current
100
DISCONTINUOUS
MODE
EFFICIENCY (%)
90
80
CONTINUOUS
MODE
70
V
IN
= 10V
V
OUT
= 2.5V
EXTV
CC
= 5V
FIGURE 9 CIRCUIT
0.1
0.01
1
LOAD CURRENT (A)
10
1778 G03
I
LOAD
= 1A
90
I
LOAD
= 10A
85
FREQUENCY (kHz)
EFFICIENCY (%)
60
50
0.001
Frequency vs Load Current
300
CONTINUOUS MODE
250
FREQUENCY (kHz)
∆V
OUT
(%)
150
100
DISCONTINUOUS
MODE
I
TH
VOLTAGE (V)
200
50
0
0
2
4
6
LOAD CURRENT (A)
8
10
1778 G26
4
U W
Start-Up
V
OUT
50mV/DIV
1778 G01
20µs/DIV
LOAD STEP 1A TO 10A
V
IN
= 15V
V
OUT
= 2.5V
FCB = INTV
CC
FIGURE 9 CIRCUIT
1778 G02
50ms/DIV
V
IN
= 15V
V
OUT
= 2.5V
R
LOAD
= 0.25Ω
1778 G19
Efficiency vs Input Voltage
100
FCB = 5V
FIGURE 9 CIRCUIT
300
Frequency vs Input Voltage
FCB = 0V
FIGURE 9 CIRCUIT
I
OUT
= 10A
95
280
260
240
I
OUT
= 0A
220
80
0
5
10
15
20
INPUT VOLTAGE (V)
25
30
1778 G04
200
5
10
15
INPUT VOLTAGE (V)
20
25
1778 G05
Load Regulation
0
FIGURE 9 CIRCUIT
2.5
I
TH
Voltage vs Load Current
FIGURE 9 CIRCUIT
–0.1
2.0
1.5
CONTINUOUS
MODE
1.0
DISCONTINUOUS
MODE
–0.2
–0.3
0.5
–0.4
0
2
6
4
LOAD CURRENT (A)
8
10
1778 G06
0
0
10
5
LOAD CURRENT (A)
15
1778 G07
1778fb
LTC1778/LTC1778-1
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
ON-TIME (ns)
100
0
–100
10
–200
0
0.5
1.0
1.5
2.0
I
TH
VOLTAGE (V)
On-Time vs Temperature
MAXIMUM CURRENT SENSE THRESHOLD (mV)
300
250
200
150
100
50
0
–50 –25
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
1778 G09
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)
U W
0.7V
0.5V
2.5
1778 G08
On-Time vs I
ON
Current
V
VON
= 0V
On-Time vs V
ON
Voltage
1000
I
ION
= 30µA
800
600
400
100
200
3.0
1
10
I
ON
CURRENT (µA)
100
1778 G20
0
0
2
1
V
ON
VOLTAGE (V)
3
1778 G21
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
1778 G22
1778 G10
Maximum Current Sense
Threshold vs Temperature
150
V
RNG
= 1V
0.82
Feedback Reference Voltage
vs Temperature
0.81
130
0.80
120
0.79
110
3.5
1778 G23
100
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
0.78
–50 –25
75
0
25
50
TEMPERATURE (°C)
100
125
1778 G11
1778 G12
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