LTC3808
No R
SENSE
TM
, Low EMI,
Synchronous DC/DC Controller
with Output Tracking
DESCRIPTIO
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.
, 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.
FEATURES
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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
= 9µA
Tiny Thermally Enhanced Leadless (4mm
×
3mm)
DFN or 16-lead SSOP Package
APPLICATIO S
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One or Two Cell Lithium-Ion Powered Devices
Notebook and Palmtop Computers, PDAs
Portable Instruments
Distributed DC Power Systems
TYPICAL APPLICATIO
1M
LTC3808
PLLLPF
Efficiency and Power Loss vs Load Current
100
10k
EFFICIENCY
V
IN
= 3.3V
V
IN
= 5V
80
TYPICAL POWER
LOSS (V
IN
= 4.2V)
V
IN
= 4.2V
100
High Efficiency, 550kHz Step-Down Converter
V
IN
2.75V TO 9.8V
V
IN
SENSE
+
TG
2.2µH
SW
BG
IPRG
GND
3808 TA01
90
SYNC/MODE
59k
15k
187k
220pF
PGOOD
V
FB
I
TH
RUN
EFFICIENCY (%)
10µF
+
47µF
V
OUT
2.5V
2A
70
60
FIGURE 11 CIRCUIT
V
OUT
= 2.5V
1
10
100
1k
LOAD CURRENT (mA)
50
U
U
U
1k
POWER LOSS (mW)
10
1
0.1
10k
3808 TA01b
3808f
1
LTC3808
ABSOLUTE
MAXIMUM
RATINGS
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
PACKAGE/ORDER INFORMATION
TOP VIEW
PLLLPF
SYNC/MODE
TRACK/SS
PGOOD
V
FB
I
TH
RUN
1
2
3
4
5
6
7
15
14 SW
13 SENSE
–
12 V
IN
11 SENSE
+
10 TG
9 BG
8 IPRG
ORDER PART
NUMBER
LTC3808EDE
DE PART
MARKING
3808
DE PACKAGE
14-LEAD (4mm
×
3mm) PLASTIC DFN
T
JMAX
= 125°C,
θ
JA
= 37°C/W
EXPOSED PAD (PIN 15) IS GND
(MUST BE SOLDERED TO PCB)
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
●
indicates specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 4.2V unless otherwise noted.
PARAMETER
Main Control Loops
Input DC Supply Current
Normal Operation
Sleep Mode
Shutdown
UVLO
Undervoltage Lockout Threshold
Shutdown Threshold of RUN Pin
Start-Up Current Source
Regulated Feedback Voltage
Output Voltage Line Regulation
Output Voltage Load Regulation
V
FB
Input Current
Overvoltage Protect Threshold
TRACK/SS = 0V
(Note 5)
2.75V < V
IN
< 9.8V (Note 5)
I
TH
= 0.9V (Note 5)
I
TH
= 1.7V
(Note 5)
Measured at V
FB
0.66
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
(Note 4)
RUN = 0V
V
IN
= UVLO Threshold – 200mV
V
IN
Falling
V
IN
Rising
●
●
2
U
U
W
W W
U
W
(Note 1)
TG, BG Peak Output Current (<10µs) ........................ 1A
Operating Temperature Range (Note 2)... – 40°C to 85°C
Storage Ambient Temperature Range ... – 65°C to 125°C
Junction Temperature (Note 3) ............................ 125°C
Lead Temperature (Soldering, 10 sec)
GN16 Package .................................................. 300°C
TOP VIEW
GND
PLLLPF
SYNC/MODE
TRACK/SS
PGOOD
V
FB
I
TH
RUN
1
2
3
4
5
6
7
8
16 SW
15 SENSE
–
14 V
IN
13 SENSE
+
12 TG
11 BG
10 IPRG
9
GND
ORDER PART
NUMBER
LTC3808EGN
GN PACKAGE
16-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 130°C/W
MIN
TYP
MAX
UNITS
350
105
9
9
1.95
2.15
0.8
0.65
0.591
2.25
2.45
1.1
1
0.6
0.01
0.1
–0.1
9
0.68
500
150
20
20
2.55
2.75
1.4
1.35
0.609
0.04
0.5
–0.5
50
0.7
µA
µA
µA
µA
V
V
V
µA
V
%/V
%
%
nA
V
3808f
LTC3808
The
●
indicates specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 4.2V unless otherwise noted.
PARAMETER
Overvoltage Protect Hysteresis
Auxiliary Feedback Threshold
Top Gate (TG) Drive Rise Time
Top Gate (TG) Drive Fall Time
Bottom Gate (BG) Drive Rise Time
Bottom Gate (BG) Drive Fall Time
Maximum Current Sense Voltage (∆V
SENSE(MAX)
)
(SENSE
+
– SW)
Soft-Start Time (Internal)
Oscillator and Phase-Locked Loop
Oscillator Frequency
Unsynchronized (SYNC/MODE Not Clocked)
PLLLPF = Floating
PLLLPF = 0V
PLLLPF = V
IN
SYNC/MODE Clocked
Minimum Synchronizable Frequency
Maximum Synchronizible Frequency
f
OSC
> f
SYNC/MODE
f
OSC
< f
SYNC/MODE
Minimum Switching Frequency
Maximum Switching Frequency
SYNC/MODE = 2.2V
I
PGOOD
Sinking 1mA
V
FB
with Respect to Set Output Voltage
V
FB
< 0.6V, Ramping Positive
V
FB
< 0.6V, Ramping Negative
V
FB
> 0.6V, Ramping Negative
V
FB
> 0.6V, Ramping Positive
–13
–16
7
10
480
260
650
550
300
750
200
1000
–3
3
460
635
2.6
50
–10.0
–13.3
10.0
13.3
–7
–10
13
16
600
340
825
250
kHz
kHz
kHz
kHz
kHz
µA
µA
kHz
kHz
µA
mV
%
%
%
%
C
L
= 3000pF
C
L
= 3000pF
CL = 3000pF
CL = 3000pF
IPRG = Floating (Note 6)
IPRG = 0V (Note 6)
IPRG = V
IN
(Note 6)
Time for V
FB
to Ramp from 0.05V to 0.55V
●
●
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
0.325
TYP
20
0.4
40
40
50
40
MAX
0.475
UNITS
mV
V
ns
ns
ns
ns
110
70
185
0.5
125
85
204
0.74
140
100
223
0.9
mV
mV
mV
ms
Phase-Locked Loop Lock Range
750
Phase Detector Output Current
Sinking
Sourcing
Spread Spectrum Frequency Range
SYNC/MODE Pull-Down Current
PGOOD Output
PGOOD Voltage Low
PGOOD Trip Level
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
0°C to 70°C. Specifications over the –40°C to 85°C 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
•
θ
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.
3808f
3
LTC3808
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Load Current
100
95
90
V
OUT
= 3.3V
EFFICIENCY (%)
FIGURE 11 CIRCUIT
V
OUT
= 2.5V
EFFICIENCY (%)
85
80
75
70
65
60
CURRENT LIMIT (%)
85
80
75
70
65
60
1
V
OUT
= 1.8V
V
OUT
= 1.2V
SYNC/MODE = V
IN
V
IN
= 5V
10
100
1k
LOAD CURRENT (mA)
10k
3808 G01
Load Step (Burst Mode
Operation)
V
OUT
200mV/DIV
AC COUPLED
V
OUT
200mV/DIV
AC COUPLED
I
L
2A/DIV
100µs/DIV
V
IN
= 3.3V
V
OUT
= 1.8V
I
LOAD
= 300mA TO 3A
SYNC/MODE = V
IN
FIGURE 11 CIRCUIT
Start-Up with Internal Soft-Start
(TRACK/SS = V
IN
)
V
IN
= 4.2V
R
LOAD
= 1Ω
FIGURE 11 CIRCUIT
4
U W
T
A
= 25°C unless otherwise noted.
Maximum Current Sense Voltage
vs I
TH
Pin Voltage
100
80
60
40
20
0
–20
Burst Mode OPERATION
(I
TH
RISING)
Burst Mode OPERATION
(I
TH
FALLING)
FORCED CONTINUOUS
MODE
PULSE SKIPPING
MODE
Efficiency vs Load Current
100
FIGURE 11 CIRCUIT
95 V
IN
= 5V, V
OUT
= 2.5V
90
BURST MODE
(SYNC/MODE =
V
IN
)
FORCED
CONTINUOUS
(SYNC/MODE = 0V)
55
50
1
PULSE SKIPPING
(SYNC/MODE = 0.6V)
10
100
1k
LOAD CURRENT (mA)
10k
3808 G02
0.5
1
1.5
I
TH
VOLTAGE (V)
2
3808 G03
Load Step (Forced Continuous
Mode)
V
OUT
200mV/DIV
AC COUPLED
Load Step (Pulse Skipping Mode)
I
L
2A/DIV
I
L
2A/DIV
3808 G04
100µs/DIV
V
IN
= 3.3V
V
OUT
= 1.8V
I
LOAD
= 300mA TO 3A
SYNC/MODE = 0V
FIGURE 11 CIRCUIT
3808 G05
100µs/DIV
V
IN
= 3.3V
V
OUT
= 1.8V
I
LOAD
= 300mA TO 3A
SYNC/MODE = V
FB
FIGURE 11 CIRCUIT
3808 G06
Start-Up with External Soft-Start
(C
SS
= 10nF)
V
OUT
1.8V
500mV/DIV
V
OUT
1.8V
500mV/DIV
200µs/DIV
3808 G07
1ms/DIV
V
IN
= 4.2V
R
LOAD
= 1Ω
FIGURE 11 CIRCUIT
3808 G08
3808f
LTC3808
TYPICAL PERFOR A CE CHARACTERISTICS
Start-Up with Coincident Tracking
(V
OUT
= 0V at 0s)
V
x
2.5V
V
OUT
1.8V
500mV/DIV
10ms/DIV
V
IN
= 4.2V
R
TA
= 590Ω
R
TB
= 1.18k
FIGURE 11 CIRCUIT
3808 G09
Regulated Feedback Voltage vs
Temperature
0.606
0.604
FEEDBACK VOLTAGE (V)
INPUT VOLTAGE (V)
0.602
0.600
0.598
0.596
0.594
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
2.40
2.35
2.30
V
IN
FALLING
2.25
2.20
RUN VOLTAGE (V)
Maximum Current Sense
Threshold vs Temperature
MAXIMUM CURRENT SENSE THRESHOLD (mV)
135
IPRG = FLOAT
TRACK/SS START-UP CURRENT (µA)
130
125
120
115
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
U W
80
3808 G12
T
A
= 25°C unless otherwise noted.
Start-Up with Ratiometric
Tracking (V
OUT
= 0V at 0s)
Start-Up with Coincident Tracking
(V
OUT
= 0.8V at 0s)
V
x
2.5V
V
OUT
1.8V
500mV/DIV
V
x
2.5V
V
OUT
1.8V
500mV/DIV
10ms/DIV
V
IN
= 4.2V
R
TA
= 590Ω
R
TB
= 1.18k
FIGURE 11 CIRCUIT
3808 G10
10ms/DIV
V
IN
= 4.2V
R
TA
= 590Ω
R
TB
= 1.69k
FIGURE 11 CIRCUIT
3808 G11
Undervoltage Lockout Threshold
vs Temperature
2.55
2.50
2.45
V
IN
RISING
1.15
1.20
Shutdown (RUN) Threshold vs
Temperature
1.10
1.05
100
2.15
–60 –40 –20 0
20 40 60
TEMPERATURE (°C)
80
100
1.00
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
3808 G13
3808 G14
TRACK/SS Start-Up Current vs
Temperature
1.04
TRACK/SS = 0V
1.02
1.00
0.98
0.96
80
100
0.94
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
3808 G16
3808 G17
3808f
5
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