LTC3778
Wide Operating Range,
No R
SENSE
TM
Step-Down Controller
FEATURES
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
DESCRIPTIO
Wide V
IN
Range: 4V to 36V
Sense Resistor Optional
True Current Mode Control
2% to 90% Duty Cycle at 200kHz
t
ON(MIN)
≤
100ns
Stable with Ceramic C
OUT
Dual N-Channel MOSFET Synchronous Drive
Power Good Output Voltage Monitor
±1%
0.6V 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 1mm 20-Lead TSSOP Package
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.
APPLICATIO S
s
s
s
s
s
Notebook and Palmtop Computers, PDAs
Battery Chargers
Distributed Power Systems
DDR Memory Power Supply
Automobile DC Power Supply
TYPICAL APPLICATIO
C
SS
0.1µF
RUN/SS
C
C
500pF
I
TH
R
C
20k
SGND
LTC3778
INTV
CC
DRV
CC
BG
SENSE
+
SENSE
–
PGOOD
PGND
V
FB
I
ON
V
IN
TG
SW
BOOST
R
ON
1.4M
V
IN
5V TO 28V
M1
Si4884
C
B
0.22µF
D
B
CMDSH-3
M2
Si4874
L1
1.8µH
EFFICIENCY (%)
+
D1
B340A
R2
40.2k
C
IN
10µF
35V
V
OUT
×3
2.5V
C
OUT
10A
180µF
4V
×2
100
V
IN
= 5V
90
V
IN
= 25V
80
+
C
VCC
4.7µF
70
60
0.01
R1
12.7k
3778 F01a
Figure 1. High Efficiency Step-Down Converter
3778f
U
Efficiency vs Load Current
1
0.1
LOAD CURRENT (A)
10
3778 F01b
U
U
1
LTC3778
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
RUN/SS 1
V
ON
2
PGOOD 3
V
RNG
4
I
TH
5
FCB 6
SGND 7
I
ON
8
VFB 9
EXTV
CC
10
20 BOOST
19 TG
18 SW
17 SENSE
+
16 SENSE
–
15 PGND
14 BG
13 DRV
CC
12 INTV
CC
11 V
IN
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
F PACKAGE
20-LEAD PLASTIC TSSOP
T
JMAX
= 125°C,
θ
JA
= 110°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 Pin Input Current
Error Amplifier Transconductance
Forced Continuous Threshold
Forced Continuous Pin Current
On-Time
Minimum On-Time
Minimum Off-Time
Maximum Current Sense Threshold
V
SENSE–
– V
SENSE+
Minimum Current Sense Threshold
V
SENSE+
– V
SENSE–
Output Overvoltage Fault Threshold
Output Undervoltage Fault Threshold
Main Control Loop
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.
CONDITIONS
MIN
TYP
MAX
UNITS
900
15
I
TH
= 1.2V (Note 3)
V
IN
= 4V to 30V, I
TH
= 1.2V (Note 3)
I
TH
= 0.5V to 1.9V (Note 3)
I
TH
= 1.2V (Note 3)
V
FCB
= 0.6V
I
ON
= 60µA, V
ON
= 1.5V
I
ON
= 60µA, V
ON
= 0V
I
ON
= 180µA, V
ON
= 0V
I
ON
= 60µA, V
ON
= 1.5V
V
RNG
= 1V, V
FB
= 0.56V
V
RNG
= 0V, V
FB
= 0.56V
V
RNG
= INTV
CC
, V
FB
= 0.56V
V
RNG
= 1V, V
FB
= 0.64V
V
RNG
= 0V, V
FB
= 0.64V
V
RNG
= INTV
CC
, V
FB
= 0.64V
7.5
340
q
q
q
q
q
q
2000
30
0.606
– 0.3
±100
2
0.63
–2
300
100
400
153
107
214
V
FB
∆V
FB(LINEREG)
∆V
FB(LOADREG)
I
FB
g
m(EA)
V
FCB
I
FCB
t
ON
t
ON(MIN)
t
OFF(MIN)
V
SENSE(MAX)
0.594
0.600
0.002
– 0.05
–5
1.4
0.57
200
1.7
0.6
–1
250
110
50
250
113
79
158
133
93
186
67
47
93
10
400
V
SENSE(MIN)
∆V
FB(OV)
∆V
FB(UV)
12.5
460
2
U
µA
µA
V
%/V
%
nA
mS
V
µA
ns
ns
ns
ns
mV
mV
mV
mV
mV
mV
%
mV
3778f
W
U
U
W W
W
LTC3778
ELECTRICAL CHARACTERISTICS
SYMBOL
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)
PGOOD Output
∆V
FBH
∆V
FBL
∆V
FB(HYS)
V
PGL
PGOOD Upper Threshold
PGOOD Lower Threshold
PGOOD Hysteresis
PGOOD Low Voltage
PARAMETER
RUN Pin Start Threshold
RUN Pin Latchoff Enable Threshold
RUN Pin Latchoff Threshold
Soft-Start Charge Current
Soft-Start Discharge Current
Undervoltage Lockout Threshold
Undervoltage Lockout Threshold
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
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.
CONDITIONS
q
MIN
0.8
TYP
1.5
4
3.5
MAX
2
4.5
4.2
–3
3
3.9
4
3
3
4
2
UNITS
V
V
V
µA
µA
V
V
Ω
Ω
Ω
Ω
ns
ns
ns
ns
RUN/SS Pin Rising
RUN/SS Pin Falling
– 0.5
0.8
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
q
q
q
q
– 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
7.5
– 7.5
10
– 10
1
0.15
12.5
– 12.5
2.5
0.4
300
5.3
±2
V
%
V
mV
mV
%
%
%
V
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.
3778f
3
LTC3778
TYPICAL PERFOR A CE CHARACTERISTICS
Transient Response
V
OUT
50mV/DIV
V
OUT
50mV/DIV
I
L
5A/DIV
20µs/DIV
LOAD STEP 0A TO 10A
V
IN
= 15V
V
OUT
= 2.5V
FCB = 0V
FIGURE 1 CIRCUIT
Efficiency vs Load Current
100
DISCONTINUOUS
MODE
EFFICIENCY (%)
100
90
EFFICIENCY (%)
80
CONTINUOUS
MODE
70
V
IN
= 10V
V
OUT
= 2.5V
EXTV
CC
= 5V
FIGURE 1 CIRCUIT
0.1
0.01
1
LOAD CURRENT (A)
10
3778 G03
I
LOAD
= 1A
90
I
LOAD
= 10A
85
FREQUENCY (kHz)
60
50
0.001
Load Regulation
0
FIGURE 1 CIRCUIT
–0.1
I
TH
VOLTAGE (V)
∆V
OUT
(%)
2.0
CURRENT SENSE THRESHOLD (mV)
–0.2
–0.3
–0.4
0
2
6
4
LOAD CURRENT (A)
4
U W
8
10
3778 G06
Transient Response
(Discontinuous Mode)
RUN/SS
2V/DIV
V
OUT
1V/DIV
I
L
5A/DIV
I
L
5A/DIV
Start-Up
3778 G01
20µs/DIV
LOAD STEP 1A TO 10A
V
IN
= 15V
V
OUT
= 2.5V
FCB = INTV
CC
FIGURE 1 CIRCUIT
3778 G02
50ms/DIV
V
IN
= 15V
V
OUT
= 2.5V
R
LOAD
= 0.125Ω
3778 G19
Efficiency vs Input Voltage
FCB = 5V
FIGURE 1 CIRCUIT
Frequency vs Input Voltage
300
FCB = 0V
FIGURE 1 CIRCUIT
I
OUT
= 10A
95
280
260
240
I
OUT
= 0A
220
80
0
5
10
15
20
INPUT VOLTAGE (V)
25
30
3778 G04
200
5
10
15
INPUT VOLTAGE (V)
20
25
3778 G05
I
TH
Voltage vs Load Current
2.5
FIGURE 1 CIRCUIT
V
RNG
= 1V
300
Current Sense Threshold
vs I
TH
Voltage
V
RNG
=
2V
1.4V
1V
100
0.7V
0.5V
200
1.5
CONTINUOUS
MODE
1.0
DISCONTINUOUS
MODE
0
0.5
–100
0
–200
0
10
5
LOAD CURRENT (A)
15
3778 G07
0
0.5
1.0
1.5
2.0
I
TH
VOLTAGE (V)
2.5
3.0
3778 G08
3778f
LTC3778
TYPICAL PERFOR A CE CHARACTERISTICS
On-Time vs I
ON
Current
10k
V
VON
= 0V
ON-TIME (ns)
ON-TIME (ns)
600
ON-TIME (ns)
1k
100
10
1
10
I
ON
CURRENT (µA)
100
3778 G20
Current Limit Foldback
MAXIMUM CURRENT SENSE THRESHOLD (mV)
MAXIMUM CURRENT SENSE THRESHOLD (mV)
MAXIMUM CURRENT SENSE THRESHOLD (mV)
150
125
100
75
50
25
0
V
RNG
= 1V
0
0.15
0.30
V
FB
(V)
0.45
Maximum Current Sense
Threshold vs Temperature
MAXIMUM CURRENT SENSE THRESHOLD (mV)
150
V
RNG
= 1V
FEEDBACK REFERENCE VOLTAGE (V)
140
0.60
120
g
m
(mS)
130
110
100
–50 –25
50
25
0
75
TEMPERATURE (°C)
U W
3778 G09
On-Time vs V
ON
Voltage
1000
I
ION
= 30µA
On-Time vs Temperature
300
250
200
150
100
I
ION
= 30µA
V
ON
= 0V
800
400
200
50
0
–50 –25
0
0
2
1
V
ON
VOLTAGE (V)
3
3778 G21
50
25
75
0
TEMPERATURE (°C)
100
125
3778 G22
Maximum Current Sense
Threshold vs V
RNG
Voltage
300
250
200
150
100
50
0
150
125
100
75
50
25
0
Maximum Current Sense
Threshold vs RUN/SS Voltage
V
RNG
= 1V
0.60
0.5
0.75
1.0
1.25
1.5
V
RNG
VOLTAGE (V)
1.75
2.0
1.5
2
2.5
3
RUN/SS VOLTAGE (V)
3.5
3778 G23
3778 G10
Feedback Reference Voltage
vs Temperature
0.62
Error Amplifier g
m
vs Temperature
2.0
1.8
0.61
1.6
1.4
0.59
1.2
100
125
0.58
–50 –25
75
0
25
50
TEMPERATURE (°C)
100
125
1.0
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
3778 G11
3778 G12
3778 G13
3778f
5
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