LTC3776
Dual 2-Phase, No R
SENSE
TM
,
Synchronous Controller for
DDR/QDR Memory Termination
FEATURES
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DESCRIPTIO
No Current Sense Resistors Required
Out-of-Phase Controllers Reduce Required
Input Capacitance
V
OUT2
Tracks 1/2 V
REF
Symmetrical Source/Sink Output Current
Capability (V
OUT2
)
Spread Spectrum Operation (When Enabled)
Wide V
IN
Range: 2.75V to 9.8V
Constant Frequency Current Mode Operation
0.6V
±1.5%
Voltage Reference (V
OUT1
)
Low Dropout Operation: 100% Duty Cycle
True PLL for Frequency Locking or Adjustment
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
®
3776 is a 2-phase dual output synchronous step-
down switching regulator controller for DDR/QDR memory
termination applications. The second controller regulates
its output voltage to 1/2 V
REF
while providing symmetrical
source and sink output current capability.
The No R
SENSE
constant frequency current mode architec-
ture 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.
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 LTC3776
switching frequency can be externally synchronized from
250kHz to 850kHz, or can be enabled for spread spectrum
operation. Forced continuous operation reduces noise and
RF interference. Soft-start for V
OUT1
is provided internally
and can be extended using an external capacitor.
The LTC3776 is available in the tiny thermally enhanced
(4mm
×
4mm) QFN package or 24-lead SSOP narrow
package.
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode
is a registered trademark of Linear Technology Corporation.
No R
SENSE
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, 6144194, 6580258,
6304066, 6611131, 6498466, patent pending on Spread Spectrum.
APPLICATIO S
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DDR, DDR II and QDR Memory
SSTL, HSTL Termination Supplies
Servers, RAID Systems
Distributed DC Power Systems
TYPICAL APPLICATIO
High Efficiency, 2-Phase, DDR Memory (V
DDQ
and V
TT
) Supplies
V
IN
3.3V
V
IN
SENSE1
+
SENSE2
+
TG1
1.5µH
SW1
LTC3776
BG1
PGND
(V
DDQ
)V
OUT1
2.5V
4A
187k
V
REF
V
FB1
470pF
59k
15k
I
TH1
SGND
V
FB2
I
TH2
2200pF
6.2k
3776 TA01a
10µF
×2
TG2
SW2
BG2
PGND
V
OUT2
(V
TT
)
1.25V
±4A
47µF
1.5µH
EFFICIENCY (%)
47µF
U
Efficiency vs Load Current
100
90
80
70
60
50
40
30
20
10
0
10
FIGURE 14 CIRCUIT
CHANNEL 2 (V
IN
= 3.3V)
CHANNEL 1 (V
IN
= 5V)
CHANNEL 1 (V
IN
= 3.3V)
CHANNEL 2 (V
IN
= 5V)
100
1000
LOAD CURRENT (mA)
10000
3776 TA01b
U
U
3776f
1
LTC3776
ABSOLUTE
AXI U
RATI GS
Input Supply Voltage (V
IN
) ........................ – 0.3V to 10V
PLLLPF, RUN/SS, SYNC/SSEN,
V
REF
, SENSE1
+
, SENSE2
+
, V
FB2
IPRG1, IPRG2 Voltages ................. – 0.3V to (V
IN
+ 0.3V)
V
FB1
, I
TH1
, I
TH2
Voltages ........................... – 0.3V to 2.4V
SW1, SW2 Voltages .............. –2V to V
IN
+ 1V (10V Max)
PGOOD ..................................................... – 0.3V to 10V
PACKAGE/ORDER I FOR ATIO
TOP VIEW
SENSE1
+
IPRG1
PGND
SW1
V
FB1
BG1
ORDER PART
NUMBER
SW1
24 23 22 21 20 19
I
TH1
1
IPRG2 2
PLLLPF 3
SGND 4
V
IN
5
V
REF
6
7
8
9 10 11 12
25
18 SYNC/SSEN
17 TG1
16 PGND
15 TG2
14 RUN/SS
13 BG2
LTC3776EUF
UF PART MARKING
3776
PGOOD
SW2
SENSE2
+
PGND
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 Operation
Shutdown
UVLO
Undervoltage Lockout Threshold
Shutdown Threshold at RUN/SS
Start-Up Current Source
Regulated Feedback Voltage (V
FB1
)
Regulated Feedback Voltage (V
FB2
)
Output Voltage Line Regulation (V
FB1
)
Output Voltage Line Regulation (V
FB2
)
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
(Note 4)
I
TH1
= I
TH2
= 1.3V
RUN/SS = 0V
V
IN
< UVLO Threshold –200mV
V
IN
Falling
V
IN
Rising
RUN/SS = 0V
0°C to 85°C (Note 5)
–40°C to 85°C
V
REF
= 2.5V
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)
(LTC3776EGN) ..................................................... 300°C
TOP VIEW
1
2
3
4
5
6
7
8
9
24 SENSE1
+
23 PGND
22 BG1
21 SYNC/SSEN
20 TG1
19 PGND
18 TG2
17 RUN/SS
16 BG2
15 PGND
14 SENSE2
+
13 SW2
ORDER PART
NUMBER
LTC3776EGN
IPRG1
V
FB1
I
TH1
IPRG2
PLLLPF
SGND
V
IN
V
REF
V
FB2
10
I
TH2
11
PGOOD 12
GN PACKAGE
24-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 130°C/ W
MIN
TYP
MAX
UNITS
575
9
3
1.95
2.15
0.45
0.4
0.591
0.588
1.232
2.25
2.45
0.65
0.7
0.6
0.6
1.250
0.05
0.02
850
20
10
2.55
2.75
0.85
1
0.609
0.612
1.268
0.2
0.1
µA
µA
µA
V
V
V
µA
V
V
V
mV/V
mV/V
3776f
LTC3776
ELECTRICAL CHARACTERISTICS
PARAMETER
Output Voltage Load Regulation
V
FB1
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 (Channel 1)
(SENSE1
+
– SW1)(∆V
SENSE(MAX)
) (SOURCE)
Maximum Current Sense Voltage (Channel 2)
(SENSE2
+
– SW2)(∆V
SENSE(MAX)
) (SOURCE)
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)
Measured at V
FB
with Respect to
Regulated Feedback Voltage
C
L
= 3000pF
C
L
= 3000pF
C
L
= 3000pF
C
L
= 3000pF
IPRG1 = Floating (Note 6)
IPRG1 = 0V
IPRG1 = V
IN
IPRG2 = Floating (Note 6)
IPRG2 = 0V
IPRG2 = V
IN
●
●
●
●
●
●
●
●
●
MIN
TYP
0.12
–0.12
10
MAX
0.5
–0.5
50
16
UNITS
%
%
nA
%
%
ns
ns
ns
ns
10
13.3
3
40
40
50
40
110
70
185
127
85
215
–130
–90
–208
0.667
125
85
204
147
100
245
–112
–75
–188
0.833
140
100
223
167
105
275
–94
–60
–168
1
mV
mV
mV
mV
mV
mV
mV
mV
mV
ms
Minimum Current Sense Voltage (Channel 2 Only) IPRG2 = Floating (Note 6)
(SENSE2
+
– SW2)(∆V
SENSE(MAX)
) (SINK)
IPRG2 = 0V
IPRG2 = V
IN
Soft-Start Time
Oscillator and Phase-Locked Loop
Oscillator Frequency
Spread Spectrum Disabled (SYNC/SSEN = GND)
PLLLPF = Floating
PLLLPF = 0V
PLLLPF = V
IN
SYNC/SSEN = V
IN
Minimum Switching Frequency
Maximum Switching Frequency
SYNC/SSEN Clocked
Minimum Synchronizable Frequency
Maximum Synchronizable Frequency
f
OSC
> f
SYNC/FCB
f
OSC
< f
SYNC/FCB
I
PGOOD
Sinking 1mA
V
FB
with Respect to Set Output Voltage
V
FB
< Regulated Feedback Voltage, Ramping Positive
V
FB
< Regulated Feedback Voltage, Ramping Negative
V
FB
> Regulated Feedback Voltage, Ramping Negative
V
FB
> Regulated Feedback Voltage, Ramping Positive
Time for V
FB1
to Ramp from 0.05V to 0.55V
●
●
●
460
260
650
550
300
750
450
580
610
340
825
kHz
kHz
kHz
kHz
kHz
Spread Spectrum Frequency Range
Phase-Locked Loop Lock Range
●
●
850
200
1150
–4
4
125
250
kHz
kHz
µA
µA
mV
Phase Detector Output Current
Sinking
Sourcing
PGOOD Output
PGOOD Voltage Low
PGOOD Trip Level
–13
–16
7
10
–10.0
–13.3
10.0
13.3
–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 LTC3776E 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 LTC3776 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.
3776f
3
LTC3776
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Load Current
100
90
80
70
EFFICIENCY (%)
60
50
40
30
20
10
0
10
FIGURE 14 CIRCUIT
CHANNEL 2 (V
IN
= 3.3V)
CHANNEL 1 (V
IN
= 5V)
CHANNEL 1 (V
IN
= 3.3V)
CHANNEL 2 (V
IN
= 5V)
100
1000
LOAD CURRENT (mA)
10000
3776 G01
Load Step (Load Connected
Between V
OUT1
and V
OUT2
)
V
OUT1
100mV/DIV
AC-COUPLED
V
OUT2
100mV/DIV
AC-COUPLED
LOAD CURRENT
1A/DIV
20µs/DIV
V
IN
= 3.3V
FIGURE 14 CIRCUIT
V
OUT1
SOURCING
V
OUT2
SINKING
3776 G03
Tracking Start-Up with External
Soft-Start (C
SS
= 0.15µF)
5
NORMALIZED FREQUENCY SHIFT (%)
2
1
0
–1
–2
–3
–4
–5
2
3
4
8
6
5
7
INPUT VOLTAGE (V)
9
10
CURRENT LIMIT (%)
V
OUT1
=
2.5V
500mV/DIV
V
OUT2
=
1.25V
4ms/DIV
V
IN
= 3.3V
FIGURE 14 CIRCUIT
4
U W
T
A
= 25°C unless otherwise noted.
Step from Sinking to Sourcing
Load Current (CH2)
V
OUT2
100mV/DIV
AC-COUPLED
LOAD CURRENT
500mA/DIV
20µs/DIV
V
IN
= 3.3V
FIGURE 14 CIRCUIT
3776 G02
Tracking Start-Up with Internal
Soft-Start (C
SS
= 0µF)
V
OUT1
=
2.5V
500mV/DIV
V
OUT2
=
1.25V
500µs/DIV
V
IN
= 3.3V
FIGURE 14 CIRCUIT
3776 G04
Oscillator Frequency
vs Input Voltage
100
80
60
40
20
0
–20
4
3
Maximum Current Sense Voltage
vs I
TH1
Pin Voltage (CH1)
3776 G05
0.5
1
1.5
I
TH
VOLTAGE (V)
2
3776 G07
3736 G06
3776f
LTC3776
TYPICAL PERFOR A CE CHARACTERISTICS
Maximum Current Sense Voltage
vs I
TH2
Pin Voltage (CH2)
100
80
60
1.2625
1.2600
FEEDBACK VOLTAGE (V
FB2
) (V)
1.2575
1.2550
1.2525
1.2500
1.2475
1.2450
1.2425
1.2400
40
20
0
–20
–40
–60
–80
–100
0
1.5
1.0
I
TH2
VOLTAGE (V)
2.0
3776 G08
FEEDBACK VOLTAGE (V
FB1
) (V)
CURRENT LIMIT (%)
Shutdown (RUN) Threshold
vs Temperature
1.0
0.9
RUN/SS PULL-UP CURRENT (µA)
1.0
0.9
0.8
0.7
0.6
0.5
0.4
–60 –40 –20 0
20 40 60
TEMPERATURE (°C)
MAXIMUM CURRENT SENSE THRESHOLD (mV)
0.8
RUN/SS VOLTAGE (V)
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
Maximum Current Sense
Threshold (CH2) vs Temperature
MAXIMUM CURRENT SENSE THRESHOLD (mV)
150
I
PRG2
= FLOAT
NROMALIZED FREQUENCY (%)
INPUT (V
IN
) VOLTAGE (V)
145
140
135
130
125
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
U W
3736 G11
T
A
= 25°C unless otherwise noted.
Regulated Feedback Voltage
(CH1) vs Temperature
0.612
0.608
0.604
0.600
0.596
0.592
0.588
–60 –40 –20 0
20 40 60
TEMPERATURE (°C)
Regulated Feedback Voltage
(CH2) vs Temperature
V
REF
= 2.500V
1.2375
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80
100
80
100
3776 G09
3776 G10
RUN/SS Pull-Up Current
vs Temperature
135
Maximum Current Sense Threshold
(CH1) vs Temperature
I
PRG1
= FLOAT
130
125
120
80
100
115
–60 –40 –20 0
20 40 60
TEMPERATURE (°C)
80
100
3736 G12
3736 G13
Oscillator Frequency
vs Temperature
10
8
6
4
2
0
–2
–4
–6
–8
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
3776 G14
–10
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
2.10
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
3736 G15
3736 G16
3776f
5
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