1
LTC3808
3808f
No R
SENSE
TM
, Low EMI,
Synchronous DC/DC Controller
with Output Tracking
Programmable Output Voltage Tracking
Sense Resistor Optional
Spread Spectrum Modulation for Low Noise
Constant Frequency Current Mode Operation for
Excellent Line and Load Transient Response
Wide V
IN
Range: 2.75V to 9.8V
Wide V
OUT
Range: 0.6V to V
IN
0.6V 1.5% Reference
Low Dropout Operation: 100% Duty Cycle
True PLL for Frequency Locking or Adjustment
(Frequency Range: 250kHz to 750kHz)
Selectable Burst Mode
/Pulse Skipping/Forced
Continuous Operation
Auxiliary Winding Regulation
Internal Soft-Start Circuitry
Power Good Output Voltage Monitor
Output Overvoltage Protection
Micropower Shutdown: I
Q
= 9A
Tiny Thermally Enhanced Leadless (4mm 3mm)
DFN or 16-lead SSOP Package
One or Two Cell Lithium-Ion Powered Devices
Notebook and Palmtop Computers, PDAs
Portable Instruments
Distributed DC Power Systems
High Efficiency, 550kHz Step-Down Converter
DESCRIPTIO
U
FEATURES
APPLICATIO S
U
TYPICAL APPLICATIO
U
+
LTC3808
2.2H
10F
V
IN
2.75V TO 9.8V
V
OUT
2.5V
2A
47F
3808 TA01
15k
59k
1M
187k
220pF
GND
PLLLPF
SYNC/MODE
PGOOD
V
FB
I
TH
RUN
SW
V
IN
SENSE
+
TG
BG
IPRG
The LTC
3808 is a synchronous step-down switching
regulator controller that drives external complementary
power MOSFETs using few external components. The
constant frequency current mode architecture with MOSFET
V
DS
sensing eliminates the need for a current sense
resistor and improves efficiency.
Burst Mode operation provides high efficiency operation
at light loads. 100% duty cycle provides low dropout
operation, extending operating time in battery-powered
systems.
The switching frequency can be programmed up to 750kHz,
allowing the use of small surface mount inductors and
capacitors. For noise sensitive applications, the LTC3808
can be externally synchronized from 250kHz to 750kHz.
Burst Mode is inhibited during synchronization or when
the SYNC/MODE pin is pulled low to reduce noise and RF
interference. To further reduce EMI, the LTC3808 incorpo-
rates a novel spread spectrum frequency modulation
technique.
The LTC3808 is available in the tiny footprint thermally
enhanced DFN package or 16-lead SSOP package.
Efficiency and Power Loss vs Load Current
LOAD CURRENT (mA)
EFFICIENCY (%)
100
90
80
70
60
50
POWER LOSS (mW)
10k
1k
100
10
1
0.1
1
100
1k
10k
3808 TA01b
10
V
IN
= 3.3V
V
IN
= 5V
V
IN
= 4.2V
FIGURE 11 CIRCUIT
V
OUT
= 2.5V
TYPICAL POWER
LOSS (V
IN
= 4.2V)
EFFICIENCY
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode
is a registered trademark of Linear Technology Corporation.
No RSENSE is a trademark of
Linear Technology Corporation.
All other trademarks are the property of their respective
owners.
Protected by U.S. Patents including 5481178,
5929620,
6580258, 6304066,
5847554, 6611131, 6498466. Other Patents pending.
2
LTC3808
3808f
ORDER PART
NUMBER
Input Supply Voltage (V
IN
)........................ 0.3V to 10V
PLLLPF, RUN, SYNC/MODE, TRACK/SS,
SENSE
+
, IPRG Voltages ............ 0.3V to (V
IN
+ 0.3V)
V
FB
, I
TH
Voltages...................................... 0.3V to 2.4V
SW, SENSE
Voltages ......... 2V to V
IN
+ 1V (10V Max)
PGOOD ..................................................... 0.3V to 10V
LTC3808EDE
T
JMAX
= 125C,
JA
= 130C/W
ABSOLUTE
M
AXI
M
U
M
RATINGS
W
W
W
U
PACKAGE/ORDER I
N
FOR
M
ATIO
N
W
U
U
Consult LTC Marketing for parts specified with wider operating temperature ranges.
(Note 1)
TG, BG Peak Output Current (<10s)........................ 1A
Operating Temperature Range (Note 2) ... 40C to 85C
Storage Ambient Temperature Range ... 65C to 125C
Junction Temperature (Note 3) ............................ 125C
Lead Temperature (Soldering, 10 sec)
GN16 Package .................................................. 300C
1
2
3
4
5
6
7
14
13
12
11
10
9
8
SW
SENSE
V
IN
SENSE
+
TG
BG
IPRG
PLLLPF
SYNC/MODE
TRACK/SS
PGOOD
V
FB
I
TH
RUN
TOP VIEW
15
DE PACKAGE
14-LEAD (4mm 3mm) PLASTIC DFN
ORDER PART
NUMBER
LTC3808EGN
GN PACKAGE
16-LEAD PLASTIC SSOP
1
2
3
4
5
6
7
8
TOP VIEW
16
15
14
13
12
11
10
9
GND
PLLLPF
SYNC/MODE
TRACK/SS
PGOOD
V
FB
I
TH
RUN
SW
SENSE
V
IN
SENSE
+
TG
BG
IPRG
GND
T
JMAX
= 125C,
JA
= 37C/W
EXPOSED PAD (PIN 15) IS GND
(MUST BE SOLDERED TO PCB)
DE PART
MARKING
3808
The
indicates specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25C. V
IN
= 4.2V unless otherwise noted.
ELECTRICAL CHARACTERISTICS
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Main Control Loops
Input DC Supply Current
(Note 4)
Normal Operation
350
500
A
Sleep Mode
105
150
A
Shutdown
RUN = 0V
9
20
A
UVLO
V
IN
= UVLO Threshold 200mV
9
20
A
Undervoltage Lockout Threshold
V
IN
Falling
1.95
2.25
2.55
V
V
IN
Rising
2.15
2.45
2.75
V
Shutdown Threshold of RUN Pin
0.8
1.1
1.4
V
Start-Up Current Source
TRACK/SS = 0V
0.65
1
1.35
A
Regulated Feedback Voltage
(Note 5)
0.591
0.6
0.609
V
Output Voltage Line Regulation
2.75V < V
IN
< 9.8V (Note 5)
0.01
0.04
%/V
Output Voltage Load Regulation
I
TH
= 0.9V (Note 5)
0.1
0.5
%
I
TH
= 1.7V
0.1
0.5
%
V
FB
Input Current
(Note 5)
9
50
nA
Overvoltage Protect Threshold
Measured at V
FB
0.66
0.68
0.7
V
3
LTC3808
3808f
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
The
indicates specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25C. V
IN
= 4.2V 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: The LTC3808E is guaranteed to meet specified performance from
0C to 70C. Specifications over the 40C to 85C operating range are
assured by design characterization, and correlation with statistical process
controls.
Note 3: T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formula:
T
J
= T
A
+ (P
D
JA
C/W)
Note 4: Dynamic supply current is higher due to gate charge being
delivered at the switching frequency.
Note 5: The LTC3808 is tested in a feedback loop that servos I
TH
to a
specified voltage and measures the resultant V
FB
voltage.
Note 6: Peak current sense voltage is reduced dependent on duty cycle to
a percentage of value as shown in Figure 1.
Overvoltage Protect Hysteresis
20
mV
Auxiliary Feedback Threshold
0.325
0.4
0.475
V
Top Gate (TG) Drive Rise Time
C
L
= 3000pF
40
ns
Top Gate (TG) Drive Fall Time
C
L
= 3000pF
40
ns
Bottom Gate (BG) Drive Rise Time
CL = 3000pF
50
ns
Bottom Gate (BG) Drive Fall Time
CL = 3000pF
40
ns
Maximum Current Sense Voltage (V
SENSE(MAX)
)
IPRG = Floating (Note 6)
110
125
140
mV
(SENSE
+
SW)
IPRG = 0V (Note 6)
70
85
100
mV
IPRG = V
IN
(Note 6)
185
204
223
mV
Soft-Start Time (Internal)
Time for V
FB
to Ramp from 0.05V to 0.55V
0.5
0.74
0.9
ms
Oscillator and Phase-Locked Loop
Oscillator Frequency
Unsynchronized (SYNC/MODE Not Clocked)
PLLLPF = Floating
480
550
600
kHz
PLLLPF = 0V
260
300
340
kHz
PLLLPF = V
IN
650
750
825
kHz
Phase-Locked Loop Lock Range
SYNC/MODE Clocked
Minimum Synchronizable Frequency
200
250
kHz
Maximum Synchronizible Frequency
750
1000
kHz
Phase Detector Output Current
Sinking
f
OSC
> f
SYNC/MODE
3
A
Sourcing
f
OSC
< f
SYNC/MODE
3
A
Spread Spectrum Frequency Range
Minimum Switching Frequency
460
kHz
Maximum Switching Frequency
635
kHz
SYNC/MODE Pull-Down Current
SYNC/MODE = 2.2V
2.6
A
PGOOD Output
PGOOD Voltage Low
I
PGOOD
Sinking 1mA
50
mV
PGOOD Trip Level
V
FB
with Respect to Set Output Voltage
V
FB
< 0.6V, Ramping Positive
13
10.0
7
%
V
FB
< 0.6V, Ramping Negative
16
13.3
10
%
V
FB
> 0.6V, Ramping Negative
7
10.0
13
%
V
FB
> 0.6V, Ramping Positive
10
13.3
16
%
4
LTC3808
3808f
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
Efficiency vs Load Current
Efficiency vs Load Current
Maximum Current Sense Voltage
vs I
TH
Pin Voltage
Load Step (Burst Mode
Operation)
Load Step (Forced Continuous
Mode)
Load Step (Pulse Skipping Mode)
Start-Up with Internal Soft-Start
(TRACK/SS = V
IN
)
Start-Up with External Soft-Start
(C
SS
= 10nF)
T
A
= 25C unless otherwise noted.
LOAD CURRENT (mA)
EFFICIENCY (%)
100
90
80
70
95
85
75
65
60
1
100
1k
10k
3808 G01
10
V
OUT
= 3.3V
V
OUT
= 2.5V
V
OUT
= 1.8V
V
OUT
= 1.2V
FIGURE 11 CIRCUIT
SYNC/MODE = V
IN
V
IN
= 5V
LOAD CURRENT (mA)
EFFICIENCY (%)
100
90
80
70
95
85
75
65
50
55
60
3808 G02
1
100
1k
10k
10
FIGURE 11 CIRCUIT
V
IN
= 5V, V
OUT
= 2.5V
FORCED
CONTINUOUS
(SYNC/MODE = 0V)
PULSE SKIPPING
(SYNC/MODE = 0.6V)
BURST MODE
(SYNC/MODE =
V
IN
)
V
OUT
200mV/DIV
AC COUPLED
I
L
2A/DIV
100s/DIV
V
IN
= 3.3V
V
OUT
= 1.8V
I
LOAD
= 300mA TO 3A
SYNC/MODE = V
IN
FIGURE 11 CIRCUIT
3808 G04
V
OUT
200mV/DIV
AC COUPLED
I
L
2A/DIV
100s/DIV
V
IN
= 3.3V
V
OUT
= 1.8V
I
LOAD
= 300mA TO 3A
SYNC/MODE = 0V
FIGURE 11 CIRCUIT
3808 G05
V
OUT
200mV/DIV
AC COUPLED
I
L
2A/DIV
100s/DIV
V
IN
= 3.3V
V
OUT
= 1.8V
I
LOAD
= 300mA TO 3A
SYNC/MODE = V
FB
FIGURE 11 CIRCUIT
3808 G06
V
OUT
1.8V
500mV/DIV
200s/DIV
V
IN
= 4.2V
R
LOAD
= 1
FIGURE 11 CIRCUIT
3808 G07
V
OUT
1.8V
500mV/DIV
1ms/DIV
V
IN
= 4.2V
R
LOAD
= 1
FIGURE 11 CIRCUIT
3808 G08
I
TH
VOLTAGE (V)
0.5
20
CURRENT LIMIT (%)
0
20
40
60
100
1
1.5
3808 G03
2
80
Burst Mode OPERATION
(I
TH
RISING)
Burst Mode OPERATION
(I
TH
FALLING)
FORCED CONTINUOUS
MODE
PULSE SKIPPING
MODE
5
LTC3808
3808f
Start-Up with Coincident Tracking
(V
OUT
= 0V at 0s)
Start-Up with Coincident Tracking
(V
OUT
= 0.8V at 0s)
Start-Up with Ratiometric
Tracking (V
OUT
= 0V at 0s)
Regulated Feedback Voltage vs
Temperature
Undervoltage Lockout Threshold
vs Temperature
Shutdown (RUN) Threshold vs
Temperature
Maximum Current Sense
Threshold vs Temperature
TRACK/SS Start-Up Current vs
Temperature
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
T
A
= 25C unless otherwise noted.
V
OUT
1.8V
V
x
2.5V
500mV/DIV
10ms/DIV
V
IN
= 4.2V
R
TA
= 590
R
TB
= 1.18k
FIGURE 11 CIRCUIT
3808 G09
V
OUT
1.8V
V
x
2.5V
500mV/DIV
10ms/DIV
V
IN
= 4.2V
R
TA
= 590
R
TB
= 1.18k
FIGURE 11 CIRCUIT
3808 G10
V
OUT
1.8V
V
x
2.5V
500mV/DIV
10ms/DIV
V
IN
= 4.2V
R
TA
= 590
R
TB
= 1.69k
FIGURE 11 CIRCUIT
3808 G11
TEMPERATURE (C)
60
0.594
FEEDBACK VOLTAGE (V)
0.596
0.598
0.600
0.602
20
20
60
100
3808 G12
0.604
0.606
40
0
40
80
TEMPERATURE (C)
60
2.15
2.20
2.25
INPUT VOLTAGE (V)
2.30
2.35
2.40
2.45
20
20
60
100
3808 G13
2.50
2.55
40
0
40
80
V
IN
RISING
V
IN
FALLING
TEMPERATURE (C)
60
1.00
RUN VOLTAGE (V)
1.05
20
20
60
100
3808 G14
1.15
1.10
1.20
40
0
40
80
TEMPERATURE (C)
60
115
MAXIMUM CURRENT SENSE THRESHOLD (mV)
120
125
130
20
20
60
100
3808 G16
135
40
0
40
80
IPRG = FLOAT
TEMPERATURE (C)
60
0.94
TRACK/SS START-UP CURRENT (
A)
20
20
60
100
3808 G17
0.96
0.98
1.04
1.02
1.00
40
0
40
80
TRACK/SS = 0V
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