LTC1628-SYNC
High Efficiency, 2-Phase
Synchronous Step-Down Switching Regulator
FEATURES
s
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DESCRIPTIO
Out-of-Phase Controllers Reduce Required Input
Capacitance and Power Supply Induced Noise
OPTI-LOOP
TM
Compensation Minimizes C
OUT
Dual N-Channel MOSFET Synchronous Drive
±1%
Output Voltage Accuracy
Power Good Output Voltage Monitor
Phase-Lockable Fixed Frequency 150kHz to 300kHz
Wide V
IN
Range: 3.5V to 36V Operation
Very Low Dropout Operation: 99% Duty Cycle
Adjustable Soft-Start Current Ramping
Foldback Output Current Limiting
Latched Short-Circuit Shutdown with Defeat Option
Output Overvoltage Protection
Remote Output Voltage Sense
Low Shutdown I
Q
: 20µA
5V and 3.3V Standby Regulators
Small 28-Lead SSOP Package
Selectable Constant Frequency or Burst Mode
TM
Operation
The LTC
®
1628-SYNC is a high performance dual step-
down switching regulator controller that drives all
N-channel synchronous power MOSFET stages. A con-
stant frequency current mode architecture allows phase-
lockable frequency of up to 300kHz. Power loss and noise
due to the ESR of the input capacitors are minimized by
operating the two controller output stages out of phase.
OPTI-LOOP compensation allows the transient response
to be optimized over a wide range of output capacitance and
ESR values. The precision 0.8V reference and power good
output indicator are compatible with future microproces-
sor generations, and a wide 3.5V to 30V (36V maximum)
input supply range encompasses all battery chemistries.
A RUN/SS pin for each controller provides both soft-start
and optional timed, short-circuit shutdown. Current
foldback limits MOSFET dissipation during short-circuit
conditions when overcurrent latchoff is disabled. Output
overvoltage protection circuitry latches on the bottom
MOSFET until V
OUT
returns to normal. The FCB mode pin
can select among Burst Mode, constant frequency mode
and continuous inductor current mode or regulate a
secondary winding. The LTC1628-SYNC includes a power
good output pin that indicates when both outputs are
within 7.5% of their designed set point.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode and OPTI-LOOP are trademarks of Linear Technology Corporation.
APPLICATIO S
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Notebook and Palmtop Computers, PDAs
Telecom Systems
Portable Instruments
Battery-Operated Digital Devices
DC Power Distribution Systems
TYPICAL APPLICATIO
L1
6.3µH
+
4.7µF
M1
C
B1
, 0.1µF
D3
V
IN
PGOOD INTV
CC
TG1
BOOST1
SW1
BG1
f
IN
LTC1628-SYNC
TG2
BOOST2
SW2
BG2
PGND
+
D4
1µF
CERAMIC
M3
C
B2
, 0.1µF
D1
M2
PLLIN
SENSE1
SENSE2
+
1000pF
SENSE2
–
V
OSENSE2
I
TH2
C
SS2
0.1µF
C
C2
220pF
R
C2
15k
R4
63.4k
1%
C
OUT
56µF
6V
SP
R
SENSE2
0.01Ω
V
OUT2
3.3V
5A
R
SENSE1
0.01Ω
V
OUT1
5V
5A
1000pF
SENSE1
–
V
OSENSE1
R2
105k
1%
+
C
OUT1
47µF
6V
SP
R1
20k
1%
C
C1
220pF
R
C1
15k
I
TH1
C
SS1
0.1µF
RUN/SS1 SGND RUN/SS2
M1, M2, M3, M4: FDS6680A
Figure 1. High Efficiency Dual 5V/3.3V Step-Down Converter
U
V
IN
5.2V TO 28V
C
IN
22µF
50V
CERAMIC
L2
6.3µH
M4
D2
R3
20k
1%
U
U
+
1628 F01
1
LTC1628-SYNC
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
RUN/SS1
SENSE1
+
SENSE1
–
V
OSENSE1
PLLFLTR
PLLIN
FCB
I
TH1
SGND
1
2
3
4
5
6
7
8
9
28 PGOOD
27 TG1
26 SW1
25 BOOST1
24 V
IN
23 BG1
22 EXTV
CC
21 INTV
CC
20 PGND
19 BG2
18 BOOST2
17 SW2
16 TG2
15 RUN/SS2
Input Supply Voltage (V
IN
).........................36V to – 0.3V
Top Side Driver Voltages
(BOOST1, BOOST2) ...................................42V to – 0.3V
Switch Voltage (SW1, SW2) .........................36V to – 5V
INTV
CC,
EXTV
CC
, RUN/SS1, RUN/SS2, (BOOST1-SW1),
(BOOST2-SW2), PGOOD .............................7V to – 0.3V
SENSE1
+
, SENSE2
+
, SENSE1
–
,
SENSE2
–
Voltages ........................ (1.1)INTV
CC
to – 0.3V
PLLIN, PLLFLTR, FCB, Voltage ............ INTV
CC
to – 0.3V
I
TH1,
I
TH2
, V
OSENSE1
, V
OSENSE2
Voltages ...2.7V to – 0.3V
Peak Output Current <10µs (TG1, TG2, BG1, BG2) ... 3A
INTV
CC
Peak Output Current ................................ 50mA
Operating Temperature Range
LTC1628CG-SYNC ................................... 0°C to 85°C
LTC1628IG-SYNC ............................... – 40°C to 85°C
Junction Temperature (Note 2) ............................. 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LTC1628CG-SYNC
LTC1628IG-SYNC
3.3V
OUT
10
I
TH2
11
V
OSENSE2
12
SENSE2
–
13
SENSE2
+
14
G PACKAGE
28-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 95°C/W
*PGOOD ON THE LTC1628-SYNC
Consult factory for Military grade parts.
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 15V, V
RUN/SS1, 2
= 5V unless otherwise noted.
SYMBOL
V
OSENSE1, 2
I
VOSENSE1, 2
V
REFLNREG
V
LOADREG
PARAMETER
Regulated Feedback Voltage
Feedback Current
Reference Voltage Line Regulation
Output Voltage Load Regulation
CONDITIONS
(Note 3); I
TH1, 2
Voltage = 1.2V
(Note 3)
V
IN
= 3.6V to 30V (Note 3)
(Note 3)
Measured in Servo Loop;
∆I
TH
Voltage = 1.2V to 0.7V
Measured in Servo Loop;
∆I
TH
Voltage = 1.2V to 2.0V
I
TH1, 2
= 1.2V; Sink/Source 5uA; (Note 3)
I
TH1, 2
= 1.2V; (Note 3)
(Note 4)
V
IN
= 15V; EXTV
CC
Tied to V
OUT1
; V
OUT1
= 5V
V
RUN/SS1, 2
= 0V, V
STBYMD
= Open;
q
q
q
q
ELECTRICAL CHARACTERISTICS
MIN
0.792
TYP
0.800
–5
0.002
0.1
– 0.1
1.3
3
350
20
MAX
0.808
– 50
0.02
0.5
– 0.5
UNITS
V
nA
%/V
%
%
mmho
MHz
µA
µA
V
µA
V
V
V
µA
Main Control Loops
g
m1, 2
g
mGBW1, 2
I
Q
Transconductance Amplifier g
m
Transconductance Amplifier GBW
Input DC Supply Current
Normal Mode
Shutdown
Forced Continuous Threshold
Forced Continuous Pin Current
Burst Inhibit (Constant Frequency)
Threshold
Undervoltage Lockout
Feedback Overvoltage Lockout
Sense Pins Total Source Current
35
0.84
– 0.1
4.8
4
0.88
V
FCB
I
FCB
V
BINHIBIT
UVLO
V
OVL
I
SENSE
0.76
– 0.30
0.800
– 0.18
4.3
V
FCB
= 0.85V
Measured at FCB pin
V
IN
Ramping Down
Measured at V
OSENSE1, 2
(Each Channel); V
SENSE1
–
, 2
– = V
SENSE1
+
, 2
+ = 0V
q
q
3.5
0.84
– 85
0.86
– 60
2
U
W
U
U
W W
W
LTC1628-SYNC
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 15V, V
RUN/SS1, 2
= 5V unless otherwise noted.
SYMBOL
DF
MAX
I
RUN/SS1, 2
PARAMETER
Maximum Duty Factor
Soft-Start Charge Current
CONDITIONS
In Dropout
V
RUN/SS1, 2
= 1.9V
V
RUN/SS1,
V
RUN/SS2
Rising
MIN
98
0.5
1.0
0.5
TYP
99.4
1.2
1.5
4.1
2
1.6
q
ELECTRICAL CHARACTERISTICS
MAX
UNITS
%
µA
V
RUN/SS1, 2
ON RUN/SS Pin ON Threshold
1.9
4.5
4
5
85
88
90
90
90
80
V
V
µA
µA
mV
mV
ns
ns
ns
ns
ns
ns
ns
V
RUN/SS1, 2
LT RUN/SS Pin Latchoff Arming Threshold V
RUN/SS1,
V
RUN/SS2
Rising from 3V
I
SCL1, 2
RUN/SS Discharge Current
Soft Short Condition V
OSENSE1, 2
= 0.5V;
V
RUN/SS1, 2
= 4.5V
I
SDLHO
V
SENSE(MAX)
Shutdown Latch Disable Current
Maximum Current Sense Threshold
TG Transition Time:
Rise Time
Fall Time
BG Transition Time:
Rise Time
Fall Time
Top Gate Off to Bottom Gate On Delay
Synchronous Switch-On Delay Time
Bottom Gate Off to Top Gate On Delay
Top Switch-On Delay Time
Minimum On-Time
Internal V
CC
Voltage
INTV
CC
Load Regulation
EXTV
CC
Voltage Drop
EXTV
CC
Switchover Voltage
EXTV
CC
Hysteresis
Nominal Frequency
Lowest Frequency
Highest Frequency
PLLIN Input Resistance
Phase Detector Output Current
Sinking Capability
Sourcing Capability
3.3V Regulator Output Voltage
3.3V Regulator Load Regulation
3.3V Regulator Line Regulation
PGOOD Voltage Low
PGOOD Leakage Current
PGOOD Trip Level, Either Controller
f
PLLIN
< f
OSC
f
PLLIN
> f
OSC
No Load
I
3.3
= 0 to 10mA
6V < V
IN
< 30V
I
PGOOD
= 2mA
V
PGOOD
= 5V
V
OSENSE
with Respect to Set Output Voltage
V
OSENSE
Ramping Negative
V
OSENSE
Ramping Positive
q
V
OSENSE1, 2
= 0.5V
V
OSENSE1, 2
= 0.7V,V
SENSE1–, 2
–
= 5V
V
OSENSE1, 2
= 0.7V,V
SENSE1–, 2
–
= 5V
(Note 5)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
(Note 5)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF Each Driver
C
LOAD
= 3300pF Each Driver
Tested with a Square Wave (Note 6)
6V < V
IN
< 30V, V
EXTVCC
= 4V
I
CC
= 0 to 20mA, V
EXTVCC
= 4V
I
CC
= 20mA, V
EXTVCC
= 5V
I
CC
= 20mA, EXTV
CC
Ramping Positive
q
65
62
75
75
50
50
40
40
90
90
180
TG1, 2 t
r
TG1, 2 t
f
BG1, 2 t
r
BG1, 2 t
f
TG/BG t
1D
BG/TG t
2D
t
ON(MIN)
V
INTVCC
V
LDO
INT
V
LDO
EXT
V
EXTVCC
V
LDOHYS
f
NOM
f
LOW
f
HIGH
R
PLLIN
I
PLLFLTR
INTV
CC
Linear Regulator
4.8
5.0
0.2
80
4.5
4.7
0.2
V
PLLFLTR
= 1.2V
V
PLLFLTR
= 0V
V
PLLFLTR
≥
2.4V
190
120
280
220
140
310
50
–15
15
3.25
3.35
0.5
0.05
0.1
3.45
2
0.2
0.3
±1
–6
6
–7.5
7.5
– 9.5
9.5
250
160
360
5.2
1.0
160
V
%
mV
V
V
kHz
kHz
kHz
kΩ
µA
µA
V
%
%
V
µA
%
%
Oscillator and Phase-Locked Loop
3.3V Linear Regulator
V
3.3OUT
V
3.3IL
V
3.3VL
PGOOD Output
V
PGL
I
PGOOD
V
PG
3
LTC1628-SYNC
ELECTRICAL CHARACTERISTICS
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formulas:
LTC1628-SYNC: T
J
= T
A
+ (P
D
• 95
°C/W)
Note 3:
The LTC1628-SYNC is tested in a feedback loop that servos V
ITH1,
2
to a specified voltage and measures the resultant
V
OSENSE1, 2.
Note 4:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.
Note 5:
Rise and fall times are measured using 10% and 90% levels. Delay
times are measured using 50% levels.
Note 6:
The minimum on-time condition is specified for an inductor
peak-to-peak ripple current
≥
40% of I
MAX
(see minimum on-time
considerations in the Applications Information section).
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Output Current
and Mode (Figure 14)
100
90
80
Burst Mode
OPERATION
100
V
IN
= 7V
90
90
EFFICIENCY (%)
EFFICIENCY (%)
60
50
40
30
20
10
0
0.001
80
EFFICIENCY (%)
70
FORCED
CONTINUOUS
MODE
CONSTANT
FREQUENCY
(BURST DISABLE)
V
IN
= 15V
V
OUT
= 5V
0.1
0.01
1
OUTPUT CURRENT (A)
10
1628 G01
Supply Current vs Input Voltage
and Mode (Figure 14)
1000
INTV
CC
AND EXTV
CC
SWITCH VOLTAGE (V)
800
EXTV
CC
VOLTAGE DROP (mV)
SUPPLY CURRENT (µA)
600
BOTH
CONTROLLERS ON
400
200
STANDBY
SHUTDOWN
0
0
5
20
15
10
25
INPUT VOLTAGE (V)
30
35
4
U W
1628 G04
Efficiency vs Output Current
(Figure 14)
100
Efficiency vs Input Voltage
(Figure 14)
V
OUT
= 5V
I
OUT
= 3A
V
IN
= 10V
V
IN
= 15V
V
IN
= 20V
80
70
70
60
V
OUT
= 5V
0.1
0.01
1
OUTPUT CURRENT (A)
10
1628 G02
60
50
0.001
50
5
25
15
INPUT VOLTAGE (V)
35
1628 G03
EXTV
CC
Voltage Drop
250
5.05
5.00
4.95
4.90
4.85
4.80
4.75
INTV
CC
and EXTV
CC
Switch
Voltage vs Temperature
INTV
CC
VOLTAGE
200
150
100
50
EXTV
CC
SWITCHOVER THRESHOLD
0
0
10
30
20
CURRENT (mA)
40
50
1628 G05
4.70
– 50 – 25
50
25
75
0
TEMPERATURE (°C)
100
125
1628 G06
LTC1628-SYNC
TYPICAL PERFOR A CE CHARACTERISTICS
Internal 5V LDO Line Regulation
5.1
5.0
I
LOAD
= 1mA
INTV
CC
VOLTAGE (V)
4.9
4.8
4.7
4.6
4.5
4.4
0
5
20
15
25
10
INPUT VOLTAGE (V)
30
35
V
SENSE
(mV)
V
SENSE
(mV)
0
20
40
60
DUTY FACTOR (%)
80
100
1628 G08
Maximum Current Sense Threshold
vs V
RUN/SS
(Soft-Start)
80
V
SENSE(CM)
= 1.6V
80
60
V
SENSE
(mV)
V
SENSE
(mV)
72
V
SENSE
(mV)
40
20
64
0
0
1
2
3
V
RUN/SS
(V)
1628 G10
4
Load Regulation
0.0
NORMALIZED V
OUT
(%)
–0.1
–0.2
V
ITH
(V)
1.5
I
SENSE
(µA)
–0.3
–0.4
0
1
3
2
LOAD CURRENT (A)
U W
1628 G07
Maximum Current Sense Threshold
vs Duty Factor
75
80
70
60
Maximum Current Sense Threshold
vs Percent of Nominal Output
Voltage (Foldback)
50
50
40
30
20
10
25
0
0
50
100
0
25
75
PERCENT ON NOMINAL OUTPUT VOLTAGE (%)
1628 G09
Maximum Current Sense Threshold
vs Sense Common Mode Voltage
90
80
76
70
60
50
40
30
20
10
0
–10
–20
60
–30
Current Sense Threshold
vs I
TH
Voltage
68
5
6
0
1
3
4
2
COMMON MODE VOLTAGE (V)
5
1628 G11
0
0.5
1
1.5
V
ITH
(V)
2
2.5
1628 G12
V
ITH
vs V
RUN/SS
2.5
FCB = 0V
V
IN
= 15V
FIGURE 1
SENSE Pins Total Source Current
100
V
OSENSE
= 0.7V
2.0
50
0
1.0
–50
0.5
0
4
5
1628 G13
0
1
2
3
V
RUN/SS
(V)
4
5
6
1628 G14
–100
0
2
4
6
1628 G15
V
SENSE
COMMON MODE VOLTAGE (V)
5
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