LTC3736-1
Dual 2-Phase, No R
SENSE
TM
,
Synchronous Controller with
Spread Spectrum
FEATURES
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DESCRIPTIO
Spread Spectrum Operation
Tracking Function
No Current Sense Resistors Required
Out-of-Phase Controllers Reduce Required
Input Capacitance
Wide V
IN
Range: 2.75V to 9.8V
Current Mode Operation
0.6V
±1.5%
Voltage Reference
Low Dropout Operation: 100% Duty Cycle
Pulse Skipping Operation at Light Loads
Internal Soft-Start Circuitry
Power Good Output Voltage Monitor
Output Overvoltage Protection
Micropower Shutdown: I
Q
= 9µA
Tiny Low Profile (4mm
×
4mm) QFN and Narrow
SSOP Packages
The LTC
®
3736-1 is a 2-phase dual synchronous step-
down switching regulator controller with tracking 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 sense resistors and improves efficiency.
Power loss and noise due to the ESR of the input capaci-
tance are minimized by operating the two controllers out
of phase.
A unique spread spectrum architecture randomly varies
the LTC3736-1’s switching frequency from 450kHz to
580kHz, significantly reducing the peak radiated and con-
ducted noise on both the input and output supplies,
making it easier to comply with electromagnetic interfer-
ence (EMI) standards.
Pulse skipping operation provides high efficiency at light
loads. 100% duty cycle capability provides low dropout op-
eration, extending operating time in battery-powered sys-
tems. The high switching frequencies allow for the use of
small surface mount inductors and capacitors.
The LTC3736-1 is available in the low profile (0.75mm)
24-pin thermally enhanced (4mm
×
4mm) QFN package
and 24-lead narrow SSOP packages.
APPLICATIO S
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One or Two Lithium-Ion Powered Devices
Notebook and Palmtop Computers, PDAs
Portable Instruments
Distributed DC Power Systems
, LTC and LT are registered trademarks of Linear Technology Corporation.
No R
SENSE
is a trademark of Linear Technology Corporation.
Protected by U.S. Patents including 5481178, 5929620, 6144194, 6580258, 6304066,
6611131, 6498466.
TYPICAL APPLICATIO
Low Noise, 2-Phase, Dual Synchronous DC/DC Step-Down Converter
V
IN
2.75V TO 9.8V
V
IN
SENSE1
+
SENSE2
+
TG1
2.2µH
TG2
2.2µH
10µF
×2
–10
AMPLITUDE (dBm)
SW1
SW2
LTC3736-1
BG1
BG2
PGND
V
FB2
I
TH2
SGND
15k
220pF
15k
PGND
V
FB1
V
OUT1
2.5V
187k
118k
V
OUT2
1.8V
+
47µF
220pF
59k
I
TH1
+
47µF
59k
37361 TA01a
U
Output Voltage Frequency
Spectrum
FIGURE 13 CIRCUIT
SPREAD SPECTRUM
–20 V
OUT
= 2.5V
DISABLED
R
BW
= 30Hz
–30
SPREAD SPECTRUM
–40
ENABLED
–50
–60
–70
–80
–90
–100
–110
410k
450k
490k
530k
FREQUENCY (Hz)
570k
610k
37361 TA01b
U
U
37361f
1
LTC3736-1
ABSOLUTE
AXI U
RATI GS
Input Supply Voltage (V
IN
) ........................ – 0.3V to 10V
FREQ, RUN/SS, SSDIS,
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
PACKAGE/ORDER I FOR ATIO
TOP VIEW
SENSE1
+
IPRG1
PGND
ORDER PART
NUMBER
BG1
SW1
SW1
V
FB1
24 23 22 21 20 19
I
TH1
1
IPRG2 2
FREQ 3
SGND 4
V
IN
5
TRACK 6
7
8
9 10 11 12
25
18 SSDIS
17 TG1
16 PGND
15 TG2
14 RUN/SS
13 BG2
LTC3736EUF-1
UF PART MARKING
3736-1
PGOOD
SENSE2
+
PGND
SW2
V
FB2
I
TH2
UF PACKAGE
24-LEAD (4mm
×
4mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 37°C/W
EXPOSED PAD (PIN 25) IS PGND
MUST BE SOLDERED TO PCB
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER
Main Control Loops
Input DC Supply Current
Normal Mode
Shutdown
UVLO
Undervoltage Lockout Threshold
Shutdown Threshold at RUN/SS
Start-Up Current Source
Regulated Feedback Voltage
Output Voltage Line Regulation
(Note 4)
The
●
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.
CONDITIONS
MIN
TYP
MAX
UNITS
RUN/SS = 0V
V
IN
< UVLO Threshold
V
IN
Falling
V
IN
Rising
RUN/SS = 0V
0°C to 85°C (Note 5)
–40°C to 85°C
2.75V < V
IN
< 9.8V (Note 5)
●
●
●
2
U
U
W
W W
U
W
(Note 1)
TG1, TG2, BG1, BG2 Peak Output Current (<10µs) ..... 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-1) .................................................. 300°C
TOP VIEW
1
2
3
4
5
6
7
8
9
24 SENSE1
+
23 PGND
22 BG1
21 SSDIS
20 TG1
19 PGND
18 TG2
17 RUN/SS
16 BG2
15 PGND
14 SENSE2
+
13 SW2
ORDER PART
NUMBER
LTC3736EGN-1
IPRG1
V
FB1
I
TH1
IPRG2
FREQ
SGND
V
IN
TRACK
V
FB2
10
I
TH2
11
PGOOD 12
GN PACKAGE
24-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 130°C/ W
500
9
3
1.95
2.15
0.45
0.5
0.591
0.588
2.25
2.45
0.65
0.7
0.6
0.6
0.05
850
20
10
2.55
2.75
0.85
1
0.609
0.612
0.2
µA
µA
µA
V
V
V
µA
V
V
mV/V
37361f
LTC3736-1
ELECTRICAL CHARACTERISTICS
PARAMETER
Output Voltage Load Regulation
V
FB1,2
Input Current
TRACK Input Current
Overvoltage Protect Threshold
Overvoltage Protect Hysteresis
Top Gate (TG) Drive 1, 2 Rise Time
Top Gate (TG) Drive 1, 2 Fall Time
Bottom Gate (BG) Drive 1, 2 Rise Time
Bottom Gate (BG) Drive 1, 2 Fall Time
Maximum Current Sense Voltage
(SENSE
+
– SW)(∆V
SENSE(MAX)
)
Soft-Start Time
Spread Spectrum Oscillator
Oscillator Frequency
The
●
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.
CONDITIONS
I
TH
= 0.9V (Note 5)
I
TH
= 1.7V
(Note 5)
TRACK = 0.6V
Measured at V
FB
C
L
= 3000pF
C
L
= 3000pF
C
L
= 3000pF
C
L
= 3000pF
IPRG = Floating (Note 6)
IPRG = 0V
IPRG = V
IN
Time for V
FB1
to Ramp from 0.05V to 0.55V
Spread Spectrum Disabled (SSDIS = V
IN
)
V
FREQ
= Floating
V
FREQ
= 0V
V
FREQ
= V
IN
SSDIS = GND
Minimum Switching Frequency
Maximum Switching Frequency
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
●
●
●
MIN
TYP
0.12
–0.12
10
10
MAX
0.5
–0.5
50
50
0.7
UNITS
%
%
nA
nA
V
mV
ns
ns
ns
ns
0.66
0.68
20
40
40
50
40
110
70
185
0.667
125
85
204
0.833
140
100
223
1
mV
mV
mV
ms
●
●
●
480
260
650
550
300
750
450
580
125
–10.0
–13.3
10.0
13.3
600
340
825
kHz
kHz
kHz
kHz
kHz
mV
Spread Spectrum Frequency Range
PGOOD Output
PGOOD Voltage Low
PGOOD Trip Level
–7
–10
13
16
%
%
%
%
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC3736E-1 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 LTC3736-1 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 2.
37361f
3
LTC3736-1
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency and Power Lost vs
Load Current
10
FIGURE 13 CIRCUIT
V
IN
= 5V
100
95
90
85
80
0.1
75
70
65
60
V
OUT
= 2.5V 55
V
OUT
= 1.8V
50
100
1000
10000
10
LOAD CURRENT (mA)
37361 G01
1
POWER LOST (W)
0.01
0.001
1
Input Voltage Noise
(Spread Spectrum Disabled)
–10
FIGURE 13 CIRCUIT
–20 V
IN
= 5V
= 2.5V
V
–30 R
OUT
= 30Hz
BW
–40 SSDIS = V
IN
–50
–60
–70
–80
–90
–100
–110
410k
450k
490k
530k
FREQUENCY (Hz)
570k
610k
37361 G04
AMPLITUDE (dBm)
AMPLITUDE (dBm)
Tracking Start-Up with External
Soft-Start (C
SS
= 0.15µF)
V
OUT1
2.5V
V
OUT2
1.8V
500mV/
DIV
NORMALIZED FREQUENCY SHIFT (%)
40ms/DIV
V
IN
= 5V
R
LOAD1
= R
LOAD2
= 1Ω
SSDIS = V
IN
FIGURE 15 CIRCUIT
37361 G07
4
U W
T
A
= 25°C unless otherwise noted.
Load Step
(Spread Spectrum Disabled)
V
OUT
AC-COUPLED
100mV/DIV
Load Step
(Spread Spectrum Enabled)
V
OUT
AC-COUPLED
100mV/DIV
EFFICIENCY (%)
I
L
2A/DIV
I
L
2A/DIV
V
IN
= 3.3V
100µs/DIV
V
OUT
= 1.8V
I
LOAD
= 300mA TO 3A
SSDIS = GND
FIGURE 15 CIRCUIT
37361 G02
100µs/DIV
V
IN
= 3.3V
V
OUT
= 1.8V
I
LOAD
= 300mA TO 3A
SSDIS = V
IN
FIGURE 15 CIRCUIT
37361 G03
Input Voltage Noise
(Spread Spectrum Enabled)
–10
FIGURE 13 CIRCUIT
–20 V
IN
= 5V
= 2.5V
V
–30 R
OUT
= 30Hz
BW
–40 SSDIS = GND
–50
–60
–70
–80
–90
–100
–110
410k
450k
490k
530k
FREQUENCY (Hz)
570k
610k
37361 G05
Tracking Start-Up with Internal
Soft-Start (C
SS
= 0µF)
V
OUT1
2.5V
V
OUT2
1.8V
500mV/
DIV
V
IN
= 5V
200µs/DIV
R
LOAD1
= R
LOAD2
= 1Ω
SSDIS = GND
FIGURE 15 CIRCUIT
37361 G06
Output Voltage Ripple
5
Oscillator Frequency
vs Input Voltage
SSDIS = V
IN
CONSTANT
550kHz
OPERATION
50mV/DIV
AC-COUPLED
SSDIS = GND
SPREAD
SPECTRUM
4
3
2
1
0
–1
–2
–3
–4
–5
2
3
4
8
6
5
7
INPUT VOLTAGE (V)
9
10
SSDIS = V
IN
1µs/DIV
FIGURE 15 CIRCUIT
ENVELOPE OF 100 SAMPLES
37361 G20
37361 G08
37361f
LTC3736-1
TYPICAL PERFOR A CE CHARACTERISTICS
Maximum Current Sense Voltage
vs I
TH
Pin Voltage
100
80
1
CURRENT LIMIT (%)
60
40
20
0
–20
0.001
0.5
1
1.5
I
TH
VOLTAGE (V)
2
37361 G09
POWER LOST (W)
80
0.1
75
70
0.01
65
V
IN
= 5V
60
V
IN
= 3.3V
V
IN
= 4.2V 55
V
IN
= 7.2V
50
10
100
1000
10000
LOAD CURRENT (mA)
37361 G10
EFFICIENCY (%)
Regulated Feedback Voltage
vs Temperature
0.609
0.607
1.0
0.9
RUN/SS PULL-UP CURRENT (µA)
FEEDBACK VOLTAGE (V)
0.605
RUN/SS VOLTAGE (V)
0.603
0.601
0.599
0.597
0.595
0.593
0.591
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
Maximum Current Sense Threshold
vs Temperature
MAXIMUM CURRENT SENSE THRESHOLD (mV)
135
I
PRG
= FLOAT
NORMALIZED FREQUENCY (%)
130
INPUT (V
IN
) VOLTAGE (V)
125
120
115
–60 –40 –20 0
20 40 60
TEMPERATURE (°C)
U W
37361 G11
T
A
= 25°C unless otherwise noted.
Efficiency and Power Lost vs
Load Current
10
FIGURE 13 CIRCUIT
V
OUT
= 2.5V
100
95
90
85
100
Efficiency vs Load Current
FIGURE 13 CIRCUIT
95 V
OUT
= 2.5V
V
IN
= 5V
90
85
EFFICIENCY (%)
80
75
70
65
60
55
50
1
SPREAD SPECTRUM
CONSTANT 550kHz
10
100
1000
LOAD CURRENT (mA)
10000
37361 G19
1
Shutdown (RUN) Threshold
vs Temperature
1.0
0.9
0.8
0.7
0.6
0.5
RUN/SS Pull-Up Current
vs Temperature
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80
100
0.4
–60 –40 –20 0
20 40 60
TEMPERATURE (°C)
80
100
37361 G12
37361 G13
Oscillator Frequency
vs Temperature
10
8
6
4
2
0
–2
–4
–6
–8
SSDIS = V
IN
Undervoltage Lockout Threshold
vs Temperature
2.50
2.45
2.40
2.35
2.30
V
IN
FALLING
2.25
2.20
2.15
80
100
V
IN
RISING
80
100
–10
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
2.10
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
37361 G14
37361 G15
37361 G16
37361f
5
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