LTC1876
High Efficiency, 2-Phase,
Dual Synchronous Step-Down Switching
Controller and Step-Up Regulator
DESCRIPTIO
The LTC
®
1876 is a high performance triple output switching
regulator. It incorporates a dual step-down switching con-
troller that drives all N-channel synchronous power MOSFET
stages. A step-up regulator with an internal 1A, 36V switch
provides the third output.
The step-down controllers minimize power loss and noise
by operating the output stage of each controller out of
phase. OPTI-LOOP compensation allows the transient
response to be optimized over a wide range of output
capacitance and ESR values. A RUN/SS pin for each
controller provides both soft-start and an optional timed,
short-circuit shutdown that can be configured to latch off
one or both controllers. Current foldback provides
additional short-circuit protection. In an overvoltage
condition, the bottom MOSFET is latched on until V
OUT
returns to normal. The FCB pin can be used to inhibit Burst
Mode operation or to enable regulation of a secondary
output voltage.
The step-up regulator operates at 1.2MHz, allowing the
use of tiny low cost capacitors and inductors. In addition,
its internal 1A switch allows high current outputs to be
generated. Its current mode control scheme provides
excellent line and load regulation.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode and OPTI-LOOP are trademarks of Linear Technology Corporation.
FEATURES
Step-Down Controller
s
Out-of-Phase Controllers Reduce Required Input
Capacitance and Power Supply Induced Noise
s
Power Good Output Voltage Indicator
s
OPTI-LOOP
TM
Compensation Minimizes C
OUT
s
DC Programmed Fixed Frequency 150kHz to 300kHz
s
Wide V Range: 3.5V to 36V Operation
IN
s
Very Low Dropout Operation: 99% Duty Cycle
s
Adjustable Soft-Start Current Ramping
s
Latched Short-Circuit Shutdown with Defeat Option
s
Remote Output Voltage Sense and OV Protection
s
5V and 3.3V Standby Regulators
s
Selectable Const. Freq. or Burst Mode
TM
Operation
Step-Up Regulator
s
High Operating Switching Frequency of 1.2MHz
s
Low Internal V
CESAT
Switch: 400mV @ 1A, V
IN
= 3V
s
Wide V Range: 2.6V to 16V Operation
IN
s
High Output Voltage: Up to 34V
APPLICATIO S
s
s
s
3.3V Input Step-Down Converter
Notebook and Palmtop Computers, PDAs
Battery-Operated Digital Devices
TYPICAL APPLICATIO
V
IN
5.2V
TO 28V 33µF
35V
ALUM
+
10µF
35V
CER
M3
1µF
CER
0.1µF
+
INTV
CC
AUXV
IN
TG2
BOOST2
SW2
LTC1876
V
IN
TG1
BOOST1
SW1
BG1
AUXSW
PGND
PGOOD
AUXV
FB
AUXSD
SENSE1
+
SENSE1
–
V
OSENSE1
I
TH1
220pF
15k
4.7µF
10V
M1
10µF
20V
0.1µF
6.3µH
M4
BG2
0.01Ω
V
OUT2
3.3V
5A
1000pF
SENSE2
+
SENSE2
–
V
OSENSE2
I
TH2
220pF
15k
1000pF
+
56µF
4V
SP
63.4k
1%
20k
1%
RUN/SS2 SGND RUN/SS1
0.1µF
0.1µF
10.2k
1%
Figure 1. High Efficiency Triple 5V/3.3V/12V Power Supply
U
U
U
10µH
V
OUT3
12V
200mA
+
6.8µH
M2
86.6k,
1%
0.01Ω
V
OUT1
5V
4A
105k
1%
20k
1%
M1, M2, M3, M4: FDS6680A
+
47µF
6.3V
SP
1876 TA01
1876fa
1
LTC1876
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
RUN/SS1
SENSE1
+
SENSE1
–
V
OSENSE1
FREQSET
STBYMD
FCB
I
TH1
SGND
1
2
3
4
5
6
7
8
9
36 PGOOD
35 TG1
34 SW1
33 BOOST1
32 V
IN
31 BG1
30 EXTV
CC
29 INTV
CC
28 PGND
27 BG2
26 BOOST2
25 SW2
24 TG2
23 RUN/SS2
22 AUXSD
21 AUXV
IN
20 AUXPGND
19 AUXPGND
G PACKAGE
36-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 95°C/W
Input Supply Voltage (V
IN
)......................... 36V to –0.3V
Topside Driver Voltages
(BOOST1, BOOST2) ................................... 42V to –0.3V
Switch Voltage (SW1, SW2) ......................... 36V to –5V
INTV
CC,
EXTV
CC
, RUN/SS1, RUN/SS2, PGOOD,
(BOOST1-SW1), (BOOST2-SW2), ...............7V to – 0.3V
SENSE1
+
, SENSE2
+
, SENSE1
–
, SENSE2
–
Voltages ................................... (1.1)INTV
CC
to – 0.3V
FREQSET, STBYMD, FCB, PGOOD
Voltages ..................................................7V 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
AUXV
IN ..................................................................
16V to –0.3V
AUXSD ..................................................................... 10V
AUXSW ..................................................... 36V to –0.3V
AUXV
FB
Voltage ....................................... 2.5V to –0.3V
Current into AUXV
FB .......................................................
±1mA
Operating Temperature Range (Note 2) ...–40°C to 85°C
Junction Temperature (Note 3) ............................. 125°C
Storage Temperature Range ..................–65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LTC1876EG
3.3V
OUT
10
I
ITH2
11
V
OSENSE2
12
SENSE2
–
13
SENSE2
+
14
AUXSGND 15
AUXV
FB
16
AUXSW 17
AUXSW 18
Consult factory for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
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
Main Control Loops
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, AUXV
IN
= 3V unless otherwise noted.
CONDITIONS
I
TH1, 2
Voltage = 1.2V (Note 4)
(Note 4)
V
IN
= 3.6V to 30V (Note 4)
(Note 4)
Measured in Servo Loop;
∆I
TH
Voltage = 1.2V to 0.7V
Measured in Servo Loop;
∆I
TH
Voltage = 1.2V to 2V
I
TH1, 2
= 1.2V; Sink/Source 5µA; (Note 4)
I
TH1, 2
= 1.2V; (Note 4)
(Note 5)
V
IN
= 15V; EXTV
CC
Tied to V
OUT1
; V
OUT1
= 5V
V
RUN/SS1, 2
= 0V, V
STBYMD
> 2V
V
RUN/SS1, 2
= 0V, V
STBYMD
= Open
q
q
q
q
MIN
0.792
TYP
0.800
–5
0.002
0.1
–0.1
1.3
3
350
125
20
MAX
0.808
–50
0.02
0.5
–0.5
UNITS
V
nA
%/V
%
%
mmho
MHz
µA
µA
µA
V
µA
V
g
m1, 2
g
mOL1, 2
I
Q
Transconductance Amplifier g
m
Transconductance Amplifier GBW
Input DC Supply Current
Normal Mode
Standby
Shutdown
Forced Continuous Threshold
Forced Continuous Current
Burst Inhibit (Constant Frequency)
Threshold
35
0.84
–0.1
4.8
V
FCB
I
FCB
V
BINHIBIT
0.76
–0.3
0.800
–0.18
4.3
V
FCB
= 0.85V
Measured at FCB pin
1876fa
2
U
W
U
U
W W
W
LTC1876
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, AUXV
IN
= 3V unless otherwise noted.
SYMBOL
UVLO
V
OVL
I
SENSE
V
STBYMD
MS
V
STBYMD
KA
DF
MAX
I
RUN/SS1, 2
V
RUN/SS1, 2
LT
I
SCL1, 2
I
SDLHO
V
SENSE(MAX)
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
Oscillator
f
OSC
f
LOW
f
HIGH
I
FREQSET
Oscillator frequency
Lowest Frequency
Highest Frequency
FREQSET Input Current
V
FREQSET
= Open (Note 8)
V
FREQSET
= 0V
V
FREQSET
= 2.4V
V
FREQSET
= 2.4V
190
120
280
220
140
310
–2
250
160
360
–1
kHz
kHz
kHz
µA
PARAMETER
Undervoltage Lockout
Overvoltage Feedback Threshold
Sense Pins Total Source Current
Master Shutdown Threshold
Keep-Alive Power On-Threshold
Maximum Duty Factor
Soft-Start Charge Current
RUN/SS Pin Latchoff Arming Threshold
RUN/SS Discharge Current
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
CONDITIONS
V
IN
Ramping Down
Measured at V
OSENSE1, 2
(Each Channel); V
SENSE1–, 2 –
= V
SENSE1+, 2+
= 0V
V
STBYMD
Ramping Down
V
STBYMD
Ramping Up, RUN
SS1, 2
= 0V
In Dropout
V
RUN/SS1, 2
= 1.9V
V
RUN/SS1,
V
RUN/SS2
Rising
V
RUN/SS1,
V
RUN/SS2
Rising from 3V
Soft Short Condition V
OSENSE1, 2
= 0.5V;
V
RUN/SS1, 2
= 4.5V
V
OSENSE1, 2
=0.5V
V
OSENSE1, 2
= 0.7V
,
V
SENSE1–, 2–
= 5V
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF Each Driver
C
LOAD
= 3300pF Each Driver
Tested with a Square Wave (Note 7)
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
q
q
q
ELECTRICAL CHARACTERISTICS
MIN
0.84
–85
0.4
98
0.5
1.0
0.5
TYP
3.5
0.86
–60
0.6
1.5
99.4
1.2
1.5
4.1
2
1.6
MAX
4
0.88
UNITS
V
V
µA
V
2
V
%
µA
V
RUN/SS1, 2
ON RUN/SS Pin ON Threshold
1.9
4.5
4
5
88
90
90
90
80
V
V
µA
µA
mV
ns
ns
ns
ns
ns
ns
ns
62
75
50
50
40
40
90
90
180
INTV
CC
Linear Regulator
4.8
5.0
0.2
80
4.5
4.7
0.2
5.2
1.0
160
V
%
mV
V
V
1876fa
3
LTC1876
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, AUXV
IN
= 3V unless otherwise noted.
SYMBOL
V
3.3OUT
V
3.3IL
V
3.3VL
PGOOD Output
V
PGL
I
PGOOD
V
PG
PGOOD Voltage Low
PGOOD Leakage Current
PGOOD Trip Level, Either Controller
I
PGOOD
= 2mA
V
PGOOD
= 5V
V
OSENSE
with Respect to Set Output Voltage
V
OSENSE
Ramping Negative
V
OSENSE
Ramping Positive
q
q
q
ELECTRICAL CHARACTERISTICS
PARAMETER
3.3V Regulator Output Voltage
3.3V Regulator Load Regulation
3.3V Regulator Line Regulation
3.3V Linear Regulator
CONDITIONS
No Load
I
3.3
= 0mA to 10mA
6V < V
IN
< 30V
q
MIN
3.25
TYP
3.35
0.5
0.05
0.1
MAX
3.45
2
0.2
0.3
±1
UNITS
V
%
%
V
µA
%
%
V
V
nA
mA
µA
%/V
MHz
%
A
mV
µA
V
V
µA
µA
–6
6
–7.5
7.5
2.4
–9.5
9.5
2.6
1.28
360
Aux Output
AUXV
INMIN
AUXV
FB
AUXI
FB
AUXI
Q
AUX Minimum Operating Voltage
AUX Regulated Feedback Voltage
AUX Feedback Pin Bias Current
AUX Input DC Supply Current
Normal Mode
Shutdown
AUX Line Regulation
AUX Oscillator Frequency
AUX Oscillator Maximum Duty Cycle
AUX Switch Current Limit
AUX Switch Saturation Voltage
AUX Switch Leakage Current
AUX Shutdown Input Voltage
AUX Shutdown Upper Trip Point
AUX Shutdown Lower Trip Point
AUXSD Pin Bias Current
V
AUXSD
= 3V
V
AUXSD
= 0V
(Note 9)
I
SW
= 900mA (Note 10)
V
SW
= 5V
2.4
0.5
16
0.01
32
0.1
V
AUXSD
= 2.4V, Not Switching
V
AUXSD
= 0V
2.6V
≤
AUXV
IN
≤
16V
q
q
1.23
1.26
120
4
0.01
0.01
1
0.05
1.6
2
550
1
AUXV
LINEREG
AUXf
OSC
AUXDC
MAX
AUXI
LIMIT
AUXV
CESAT
AUXI
LEAKAGE
AUXV
AUXSD
0.8
84
1
1.2
86
1.4
330
0.01
I
AUXSD
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC1876E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the – 40°C to 85°C operating
temperature 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 formulas:
LTC1876EG: T
J
= T
A
+ (P
D
• 95°C/W)
Note 4:
The LTC1876 is tested in a feedback loop that servos V
ITH1, 2
to a
specified voltage and measures the resultant V
OSENSE1, 2.
Note 5:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.
Note 6:
Rise and fall times are measured using 10% and 90% levels.
Delay times are measured using 50% levels.
Note 7:
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).
Note 8:
V
FREQSET
pin internally tied to 1.19V reference through a large
resistance.
Note 9:
Current limit guaranteed by design and/or correlation to static test.
Note 10:
100% tested at wafer level.
1876fa
4
LTC1876
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Output Current and
Mode (Figure 1)
100
90
80
Burst Mode
OPERATION
100
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
1876 G01
V
IN
Supply Current vs Input
Voltage and Mode (Figure 1)
1000
INTV
CC
AND EXTV
CC
SWITCH VOLTAGE (V)
EXTV
CC
VOLTAGE DROP (mV)
800
SUPPLY CURRENT (µA)
600
BOTH
CONTROLLERS ON
400
200
STANDBY
SHUTDOWN
0
0
5
20
15
10
25
INPUT VOLTAGE (V)
30
35
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
1876 G07
V
SENSE
(mV)
V
SENSE
(mV)
U W
1876 G04
Efficiency vs Output Current
(Figure 1)
V
OUT
= 5V
100
V
IN
= 7V
90
V
IN
= 10V
V
IN
= 15V
V
IN
= 20V
70
Efficiency vs Input Voltage
(Figure 1)
V
OUT
= 5V
I
OUT
= 3A
90
80
70
60
60
50
0.001
0.1
0.01
1
OUTPUT CURRENT (A)
50
10
1876 G02
5
25
15
INPUT VOLTAGE (V)
35
1876 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
1876 G05
4.70
– 50 – 25
50
25
75
0
TEMPERATURE (°C)
100
125
1876 G06
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
20
40
60
DUTY FACTOR (%)
80
100
1876 G08
0
50
0
100
25
75
PERCENT ON NOMINAL OUTPUT VOLTAGE (%)
1876 G09
1876fa
5
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