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1
LTC3778
3778f
FEATURES
DESCRIPTIO
U
APPLICATIO S
U
TYPICAL APPLICATIO
U
Wide Operating Range,
No R
SENSE
TM
Step-Down Controller
s
Wide V
IN
Range: 4V to 36V
s
Sense Resistor Optional
s
True Current Mode Control
s
2% to 90% Duty Cycle at 200kHz
s
t
ON(MIN)
100ns
s
Stable with Ceramic C
OUT
s
Dual N-Channel MOSFET Synchronous Drive
s
Power Good Output Voltage Monitor
s
1% 0.6V 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 1mm 20-Lead TSSOP Package
s
Notebook and Palmtop Computers, PDAs
s
Battery Chargers
s
Distributed Power Systems
s
DDR Memory Power Supply
s
Automobile DC Power Supply
The LTC
3778 is a synchronous step-down switching
regulator controller for computer memory, automobile
and other DC/DC power supplies. The controller uses a
valley current control architecture to deliver very low duty
cycles without requiring a sense resistor. Operating fre-
quency is selected by an external resistor and is compen-
sated for variations in V
IN
and V
OUT
.
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 when the main output is lightly
loaded. SENSE
+
and
SENSE
pins provide true Kelvin
sensing across the optional sense resistor or the
sychronous MOSFET.
Fault protection is provided by internal foldback current
limiting, an output overvoltage comparator, optional short-
circuit shutdown timer and input undervoltage lockout.
Soft-start capability for supply sequencing is accom-
plished using an external timing capacitor. The regulator
current limit level is user programmable. Wide supply
range allows operation from 4V to 36V at the input and
from 0.6V 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
LOAD CURRENT (A)
0.01
EFFICIENCY (%)
80
90
3778 F01b
70
60
0.1
1
10
100
V
IN
= 5V
V
IN
= 25V
+
D
B
CMDSH-3
D1
B340A
L1
1.8
H
C
VCC
4.7
F
C
IN
10
F
35V
3
V
IN
5V TO 28V
V
OUT
2.5V
10A
+
C
OUT
180
F
4V
2
M2
Si4874
3778 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
SENSE
SENSE
+
V
FB
PGOOD
C
B
0.22
F
R
C
20k
LTC3778
C
C
500pF
R2
40.2k
R1
12.7k
DRV
CC
2
LTC3778
3778f
(Note 1)
Input Supply Voltage (V
IN
, I
ON
)..................36V to 0.3V
Boosted Topside Driver Supply Voltage
(BOOST) ................................................... 42V to 0.3V
SENSE
+
,
SW Voltage .................................... 36V to 5V
DRV
CC
, EXTV
CC
, (BOOST SW),
FCB,
RUN/SS, PGOOD Voltages .................... 7V to 0.3V
V
ON,
V
RNG
Voltages ................... INTV
CC
+ 0.3V to 0.3V
I
TH
, V
FB
Voltages...................................... 2.7V to 0.3V
TG, BG, INTV
CC
Peak Currents .................................. 2A
TG, BG, INTV
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
ORDER PART
NUMBER
LTC3778EF
Consult LTC Marketing for parts specified with wider operating temperature ranges.
T
JMAX
= 125
C,
JA
= 110
C/ W
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.
ABSOLUTE AXI U RATI GS
W
W
W
U
PACKAGE/ORDER I FOR ATIO
U
U
W
ELECTRICAL CHARACTERISTICS
1
2
3
4
5
6
7
8
9
10
TOP VIEW
20
19
18
17
16
15
14
13
12
11
RUN/SS
V
ON
PGOOD
V
RNG
I
TH
FCB
SGND
I
ON
VFB
EXTV
CC
BOOST
TG
SW
SENSE
+
SENSE
PGND
BG
DRV
CC
INTV
CC
V
IN
F PACKAGE
20-LEAD PLASTIC TSSOP
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
0.594
0.600
0.606
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)
0.05
0.3
%
I
FB
Feedback Pin Input Current
5
100
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
0.57
0.6
0.63
V
I
FCB
Forced Continuous Pin Current
V
FCB
= 0.6V
1
2
A
t
ON
On-Time
I
ON
= 60
A, V
ON
= 1.5V
200
250
300
ns
I
ON
= 60
A, V
ON
= 0V
110
ns
t
ON(MIN)
Minimum On-Time
I
ON
= 180
A, V
ON
= 0V
50
100
ns
t
OFF(MIN)
Minimum Off-Time
I
ON
= 60
A, V
ON
= 1.5V
250
400
ns
V
SENSE(MAX)
Maximum Current Sense Threshold
V
RNG
= 1V, V
FB
= 0.56V
q
113
133
153
mV
V
SENSE
V
SENSE
+
V
RNG
= 0V, V
FB
= 0.56V
q
79
93
107
mV
V
RNG
= INTV
CC
, V
FB
= 0.56V
q
158
186
214
mV
V
SENSE(MIN)
Minimum Current Sense Threshold
V
RNG
= 1V, V
FB
= 0.64V
67
mV
V
SENSE
+
V
SENSE
V
RNG
= 0V, V
FB
= 0.64V
47
mV
V
RNG
= INTV
CC
, V
FB
= 0.64V
93
mV
V
FB(OV)
Output Overvoltage Fault Threshold
7.5
10
12.5
%
V
FB(UV)
Output Undervoltage Fault Threshold
340
400
460
mV
3
LTC3778
3778f
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:
LTC3778E: T
J
= T
A
+ (P
D
110
C/W)
Note 3: The LTC3778 is tested in a feedback loop that adjusts V
FB
to achieve
a specified error amplifier output voltage (I
TH
).
Note 4: The LTC3778E 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
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
0.5
1.2
3
A
I
RUN/SS(D)
Soft-Start Discharge Current
0.8
1.8
3
A
V
IN(UVLO)
Undervoltage Lockout Threshold
V
IN
Falling
q
3.4
3.9
V
V
IN(UVLOR)
Undervoltage Lockout Threshold
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
V
FBH
PGOOD Upper Threshold
V
FB
Rising
7.5
10
12.5
%
V
FBL
PGOOD Lower Threshold
V
FB
Falling
7.5
10
12.5
%
V
FB(HYS)
PGOOD Hysteresis
V
FB
Returning
1
2.5
%
V
PGL
PGOOD Low Voltage
I
PGOOD
= 5mA
0.15
0.4
V
4
LTC3778
3778f
Efficiency vs Load Current
Efficiency vs Input Voltage
Frequency vs Input Voltage
Load Regulation
I
TH
Voltage vs Load Current
Current Sense Threshold
vs I
TH
Voltage
Transient Response
(Discontinuous Mode)
Transient Response
V
OUT
50mV/DIV
I
L
5A/DIV
20
s/DIV
3778 G01
LOAD STEP 0A TO 10A
V
IN
= 15V
V
OUT
= 2.5V
FCB = 0V
FIGURE 1 CIRCUIT
20
s/DIV
3778 G02
LOAD STEP 1A TO 10A
V
IN
= 15V
V
OUT
= 2.5V
FCB = INTV
CC
FIGURE 1 CIRCUIT
Start-Up
50ms/DIV
3778 G19
V
IN
= 15V
V
OUT
= 2.5V
R
LOAD
= 0.125
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
V
OUT
50mV/DIV
I
L
5A/DIV
RUN/SS
2V/DIV
I
L
5A/DIV
V
OUT
1V/DIV
LOAD CURRENT (A)
0.001
EFFICIENCY (%)
70
80
10
3778 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 1 CIRCUIT
INPUT VOLTAGE (V)
0
80
EFFICIENCY (%)
85
90
95
100
5
10
15
20
3778 G04
25
30
I
LOAD
= 1A
I
LOAD
= 10A
FCB = 5V
FIGURE 1 CIRCUIT
INPUT VOLTAGE (V)
5
FREQUENCY (kHz)
240
260
25
3778 G05
220
200
10
15
20
300
280
I
OUT
= 10A
FCB = 0V
FIGURE 1 CIRCUIT
I
OUT
= 0A
LOAD CURRENT (A)
0
V
OUT
(%)
0.2
0.1
8
3778 G06
0.3
0.4
2
4
6
10
0
FIGURE 1 CIRCUIT
LOAD CURRENT (A)
0
I
TH
VOLTAGE (V)
1.0
1.5
3778 G07
0.5
0
5
10
15
2.5
2.0
CONTINUOUS
MODE
DISCONTINUOUS
MODE
FIGURE 1 CIRCUIT
V
RNG
= 1V
I
TH
VOLTAGE (V)
0
200
CURRENT SENSE THRESHOLD (mV)
100
0
100
200
300
0.5
1.0
1.5
2.0
3778 G08
2.5
3.0
V
RNG
=
1V
0.7V
0.5V
1.4V
2V
5
LTC3778
3778f
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
Error Amplifier g
m
vs Temperature
Current Limit Foldback
On-Time vs I
ON
Current
On-Time vs V
ON
Voltage
On-Time vs Temperature
Maximum Current Sense
Threshold vs RUN/SS Voltage
I
ON
CURRENT (
A)
1
10
ON-TIME (ns)
100
1k
10k
10
100
3778 G20
V
VON
= 0V
V
ON
VOLTAGE (V)
0
ON-TIME (ns) 400
600
3778 G21
200
0
1
2
3
1000
I
ION
= 30
A
800
TEMPERATURE (
C)
50
ON-TIME (ns)
200
250
300
25
75
3778 G22
150
100
25
0
50
100
125
50
0
I
ION
= 30
A
V
ON
= 0V
V
FB
(V)
0
0
MAXIMUM CURRENT SENSE THRESHOLD (mV)
25
50
75
100
125
150
V
RNG
= 1V
0.15
0.30
0.45
0.60
3778 G09
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
3778 G10
1.75
2.0
250
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
3778 G23
TEMPERATURE (
C)
50
25
100
MAXIMUM CURRENT SENSE THRESHOLD (mV)
120
150
0
50
75
3778 G11
110
140
130
25
100
125
V
RNG
= 1V
TEMPERATURE (
C)
50
0.58
FEEDBACK REFERENCE VOLTAGE (V)
0.59
0.60
0.61
0.62
25
0
25
50
3778 G12
75
100
125
TEMPERATURE (
C)
50
25
1.0
g
m
(mS)
1.4
2.0
0
50
75
3778 G13
1.2
1.8
1.6
25
100
125
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