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LTC3736
3736f
Dual 2-Phase, No R
SENSE
TM
,
Synchronous Controller
with Output Tracking
High Efficiency, 2-Phase, Dual Synchronous DC/DC Step-Down Converter
s
No Current Sense Resistors Required
s
Out-of-Phase Controllers Reduce Required
Input Capacitance
s
Tracking Function
s
Wide V
IN
Range: 2.75V to 9.8V
s
Constant Frequency Current Mode Operation
s
0.6V
1.5% Voltage Reference
s
Low Dropout Operation: 100% Duty Cycle
s
True PLL for Frequency Locking or Adjustment
s
Selectable Burst Mode
/Forced Continuous Operation
s
Auxiliary Winding Regulation
s
Internal Soft-Start Circuitry
s
Power Good Output Voltage Monitor
s
Output Overvoltage Protection
s
Micropower Shutdown: I
Q
= 9
A
s
Tiny Low Profile (4mm
4mm) QFN and Narrow
SSOP Packages
, LTC and LT are registered trademarks of Linear Technology Corporation.
The LTC
3736 is a 2-phase dual synchronous step-down
switching regulator controller with tracking that drives
external complementary power MOSFETs using few exter-
nal components. The constant frequency current mode
architecture with MOSFET V
DS
sensing eliminates the
need for sense resistors and improves efficiency. Power
loss and noise due to the ESR of the input capacitance are
minimized by operating the two controllers out of phase.
Burst Mode operation provides high efficiency at light loads.
100% duty cycle capability 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 ca-
pacitors. For noise sensitive applications, the LTC3736
switching frequency can be externally synchronized from
250kHz to 850kHz. Burst Mode operation is inhibited dur-
ing synchronization or when the SYNC/FCB pin is pulled low
in order to reduce noise and RF interference. Automatic soft-
start is internally controlled.
The LTC3736 is available in the tiny thermally enhanced
(4mm
4mm) QFN package or 24-lead SSOP narrow
package.
Burst Mode is a registered trademark of Linear Technology Corporation.
No R
SENSE
is a trademark of Linear Technology Corporation.
s
One or Two Lithium-Ion Powered Devices
s
Notebook and Palmtop Computers, PDAs
s
Portable Instruments
s
Distributed DC Power Systems
+
SENSE1
+
V
IN
LTC3736
SGND
SENSE2
+
TG1
TG2
SW1
SW2
BG1
BG2
PGND
PGND
V
FB1
V
FB2
220pF
V
OUT1
2.5V
V
OUT2
1.8V
47
F
+
47
F
15k
220pF
15k
59k
59k
187k
118k
2.2
H
2.2
H
I
TH1
3736 TA01a
I
TH2
10
F
2
V
IN
2.75V TO 9.8V
LOAD CURRENT (mA)
65
EFFICIENCY (%)
95
100
60
55
90
75
85
80
70
1
100
1000
10000
3736 TA01b
50
10
V
IN
= 3.3V
FIGURE 16 CIRCUIT
V
OUT
= 2.5V
V
IN
= 5V
V
IN
= 4.2V
Efficiency vs Load Current
FEATURES
DESCRIPTIO
U
APPLICATIO S
U
TYPICAL APPLICATIO
U
2
LTC3736
3736f
Input Supply Voltage (V
IN
) ........................ 0.3V to 10V
PLLLPF, RUN/SS, SYNC/FCB,
TRACK, SENSE1
+
, SENSE2
+
,
IPRG1, IPRG2 Voltages ................. 0.3V to (V
IN
+ 0.3V)
V
FB1
, V
FB2
, I
TH1
, I
TH2
Voltages .................. 0.3V to 2.4V
SW1, SW2 Voltages ............ 2V to V
IN
+ 1V or 10V Max
PGOOD ..................................................... 0.3V to 10V
ABSOLUTE AXI U RATI GS
W
W
W
U
(Note 1)
TG1, TG2, BG1, BG2 Peak Output Current (<10
s) ..... 1A
Operating Temperature Range (Note 2) ... 40
C to 85
C
Storage Temperature Range .................. 65
C to 125
C
Junction Temperature (Note 3) ............................ 125
C
Lead Temperature (Soldering, 10 sec)
(LTC3736EGN) ..................................................... 300
C
PACKAGE/ORDER I FOR ATIO
U
U
W
24 23 22 21 20 19
7
8
9
TOP VIEW
25
UF PACKAGE
24-LEAD (4mm
4mm) PLASTIC QFN
10 11 12
6
5
4
3
2
1
13
14
15
16
17
18
I
TH1
IPRG2
PLLLPF
SGND
V
IN
TRACK
SYNC/FCB
TG1
PGND
TG2
RUN/SS
BG2
V
FB1
IPRG1
SW1
SENSE1
+
PGND
BG1
V
FB2
I
TH2
PGOOD
SW2
SENSE2
+
PGND
T
JMAX
= 125
C,
JA
= 37
C/W
EXPOSED PAD (PIN 25) IS PGND
MUST BE SOLDERED TO PCB
1
2
3
4
5
6
7
8
9
10
11
12
TOP VIEW
GN PACKAGE
24-LEAD PLASTIC SSOP
24
23
22
21
20
19
18
17
16
15
14
13
SW1
IPRG1
V
FB1
I
TH1
IPRG2
PLLLPF
SGND
V
IN
TRACK
V
FB2
I
TH2
PGOOD
SENSE1
+
PGND
BG1
SYNC/FCB
TG1
PGND
TG2
RUN/SS
BG2
PGND
SENSE2
+
SW2
ORDER PART
NUMBER
UF PART MARKING
3736
LTC3736EUF
T
JMAX
= 125
C,
JA
= 130
C/ W
ORDER PART
NUMBER
LTC3736EGN
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications that apply over the full operating temperature
range, otherwise specifications are at T
A
= 25
C. V
IN
= 4.2V unless otherwise specified.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Main Control Loops
Input DC Supply Current
(Note 4)
Sleep Mode
300
425
A
Shutdown
RUN/SS = 0V
9
20
A
UVLO
V
IN
< UVLO Threshold
3
10
A
Undervoltage Lockout Threshold
V
IN
Falling
q
1.95
2.25
2.55
V
V
IN
Rising
q
2.15
2.45
2.75
V
Shutdown Threshold at RUN/SS
0.45
0.65
0.85
V
Start-Up Current Source
RUN/SS = 0V
0.5
0.7
1
A
Regulated Feedback Voltage
0
C to 85
C (Note 5)
0.591
0.6
0.609
V
40
C to 85
C
q
0.588
0.6
0.612
V
Output Voltage Line Regulation
2.7V < V
IN
< 9.8V (Note 5)
0.05
0.2
mV/V
3
LTC3736
3736f
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications that apply over the full operating temperature
range, otherwise specifications are at T
A
= 25
C. V
IN
= 4.2V unless otherwise specified.
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The LTC3736E 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
JA
C/W)
Note 4: Dynamic supply current is higher due to gate charge being
delivered at the switching frequency.
Note 5: The LTC3736 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.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Output Voltage Load Regulation
I
TH
= 0.9V (Note 5)
0.12
0.5
%
I
TH
= 1.7V
0.12
0.5
%
V
FB1,2
Input Current
(Note 5)
10
50
nA
TRACK Input Current
TRACK = 0.6V
10
50
nA
Overvoltage Protect Threshold
Measured at V
FB
0.66
0.68
0.7
V
Overvoltage Protect Hysteresis
20
mV
Auxiliary Feedback Threshold
SYNC/FCB Ramping Negative
0.525
0.6
0.675
V
Top Gate (TG) Drive 1, 2 Rise Time
C
L
= 3000pF
40
ns
Top Gate (TG) Drive 1, 2 Fall Time
C
L
= 3000pF
40
ns
Bottom Gate (BG) Drive 1, 2 Rise Time
C
L
= 3000pF
50
ns
Bottom Gate (BG) Drive 1, 2 Fall Time
C
L
= 3000pF
40
ns
Maximum Current Sense Voltage
IPRG = Floating
115
125
135
mV
(SENSE
+
SW)(
V
SENSE(MAX)
)
q
110
125
140
mV
IPRG = 0V
75
85
95
mV
q
70
85
100
mV
IPRG = V
IN
193
204
215
mV
q
185
204
223
mV
Soft-Start Time
Time for V
FB1
to Ramp from 0.05V to 0.55V
0.667
0.833
1
ms
Oscillator and Phase-Locked Loop
Oscillator Frequency
Unsynchronized (SYNC/FCB Not Clocked)
V
PLLLPF
= Floating
q
480
550
600
kHz
V
PLLLPF
= 0V
q
260
300
340
kHz
V
PLLLPF
= V
IN
q
650
750
825
kHz
Phase-Locked Loop Lock Range
SYNC/FCB Clocked
Minimum Synchronizable Frequency
q
200
250
kHz
Maximum Synchronizable Frequency
q
850
1150
kHz
Phase Detector Output Current
Sinking
f
OSC
> f
SYNC/FCB
4
A
Sourcing
f
OSC
< f
SYNC/FCB
4
A
PGOOD Output
PGOOD Voltage Low
I
PGOOD
Sinking 1mA
125
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
LTC3736
3736f
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
Efficiency vs Load Current
LOAD CURRENT (mA)
65
EFFICIENCY (%)
95
100
60
55
90
75
85
80
70
1
100
1000
10000
3736 G01
50
10
V
IN
= 5V
SYNC/FCB = V
IN
V
OUT
= 3.3V
V
OUT
= 2.5V
V
OUT
= 1.8V
V
OUT
= 1.2V
FIGURE 15 CIRCUIT
V
OUT
AC-COUPLED
100mV/DIV
V
IN
= 3.3V
V
OUT
= 1.8V
I
LOAD
= 300mA TO 3A
SYNC/FCB = V
IN
FIGURE 17 CIRCUIT
100
s/DIV
3736 G02
I
L
2A/DIV
V
OUT
AC-COUPLED
100mV/DIV
V
IN
= 3.3V
V
OUT
= 1.8V
I
LOAD
= 300mA TO 3A
SYNC/FCB = 0V
FIGURE 17 CIRCUIT
100
s/DIV
3736 G03
I
L
2A/DIV
Load Step (Burst Mode Operation)
Load Step
(Forced Continuous Mode)
Load Step (Pulse Skipping Mode)
V
OUT
AC-COUPLED
100mV/DIV
V
IN
= 3.3V
V
OUT
= 1.8V
I
LOAD
= 300mA TO 3A
SYNC/FCB = 550kHz EXTERNAL CLOCK
FIGURE 17 CIRCUIT
100
s/DIV
3736 G04
I
L
2A/DIV
Burst Mode
OPERATION
SYNC/FCB = V
IN
FORCED
CONTINUOUS
MODE
SYNC/FCB = 0V
V
IN
= 3.3V
V
OUT
= 1.8V
I
LOAD
= 200mA
FIGURE 17 CIRCUIT
4
s/DIV
3736 G05
PULSE
SKIPPING MODE
SYNC/FCB = 550kHz
I
L
1A/DIV
V
IN
= 5V
R
LOAD1
= R
LOAD2
= 1
FIGURE 15 CIRCUIT
200
s/DIV
3736 G06
500mV/
DIV
V
OUT1
2.5V
V
OUT2
1.8V
Inductor Current at Light Load
Tracking Start-Up with Internal
Soft-Start (C
SS
= 0
F)
V
IN
= 5V
R
LOAD1
= R
LOAD2
= 1
FIGURE 15 CIRCUIT
40ms/DIV
3736 G07
500mV/
DIV
V
OUT1
2.5V
V
OUT2
1.8V
INPUT VOLTAGE (V)
2
5
NORMALIZED FREQUENCY SHIFT (%)
4
2
1
0
5
2
4
6
7
3736 G08
3
3
4
1
3
5
8
9
10
Oscillator Frequency
vs Input Voltage
Tracking Start-Up with External
Soft-Start (C
SS
= 0.15
F)
T
A
= 25
C unless otherwise noted.
5
LTC3736
3736f
Maximum Current Sense Voltage
vs I
TH
Pin Voltage
I
TH
VOLTAGE (V)
0.5
20
CURRENT LIMIT (%)
0
20
40
60
100
1
1.5
3736 G09
2
80
Burst Mode OPERATION
(I
TH
RISING)
Burst Mode OPERATION
(I
TH
FALLING)
FORCED CONTINUOUS
MODE
PULSE SKIPPING
MODE
LOAD CURRENT (mA)
65
EFFICIENCY (%)
95
100
60
55
90
75
85
80
70
1
100
1000
10000
3736 G10
50
10
PULSE SKIPPING
MODE
(SYNC/FCB = 550kHz)
FIGURE 15 CIRCUIT
V
IN
= 5V
V
OUT
= 2.5V
Burst Mode
OPERATION
(SYNC/FCB = V
IN
)
FORCED
CONTINUOUS
(SYNC/FCB = 0V)
TEMPERATURE (
C)
60
FEEDBACK VOLTAGE (V)
0.601
0.603
0.605
100
3736 G11
0.599
0.597
0.591
20
20
60
40
0
40
80
0.595
0.593
0.609
0.607
Efficiency vs Load Current
Regulated Feedback Voltage
vs Temperature
Shutdown (RUN) Threshold
vs Temperature
RUN/SS Pull-Up Current
vs Temperature
Maximum Current Sense Threshold
vs Temperature
TEMPERATURE (
C)
60
0
RUN/SS VOLTAGE (V)
0.1
0.3
0.4
0.5
1.0
0.7
20
20
40
3736 G12
0.2
0.8
0.9
0.6
40
0
60
80
100
TEMPERATURE (
C)
60
0.4
RUN/SS PULL-UP CURRENT (
A)
0.5
0.6
0.7
0.8
20
20
60
100
3736 G13
0.9
1.0
40
0
40
80
TEMPERATURE (
C)
60
115
MAXIMUM CURRENT SENSE THRESHOLD (mV)
120
125
130
135
40 20
0
20
3736 G14
40
60
80
100
I
PRG
= FLOAT
Oscillator Frequency
vs Temperature
TEMPERATURE (
C)
60
10
NROMALIZED FREQUENCY (%)
8
4
2
0
10
4
20
20
40
3736 G15
6
6
8
2
40
0
60
80
100
TEMPERATURE (
C)
60
INPUT (V
IN
) VOLTAGE (V) 2.30
2.40
100
3736 G16
2.20
2.10
20
20
60
40
0
40
80
2.50
2.25
2.35
2.15
2.45
V
IN
RISING
V
IN
FALLING
Undervoltage Lockout Threshold
vs Temperature
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
T
A
= 25
C unless otherwise noted.
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