LTC1874
Dual Constant Frequency
Current Mode Step-Down
DC/DC Controller
FEATURES
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DESCRIPTIO
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High Efficiency: Up to 94%
High Output Currents Easily Achieved
Wide V
IN
Range: 2.5V to 9.8V
Constant Frequency 550kHz Operation
Burst Mode
TM
Operation at Light Load
Low Dropout: 100% Duty Cycle
0.8V Reference Allows Low Output Voltages
Current Mode Operation for Excellent
Line and Load Transient Response
Low Quiescent Current: 270µA (Each Controller)
Separate Shutdown Pin for Each Controller
Shutdown Mode Draws Only
8µA Supply Current (Each Controller)
±2.5%
Reference Accuracy
Available in 16-Lead Narrow SSOP
Each Controller Functions Independent of the Other
The LTC
®
1874 is a dual constant frequency current mode
step-down DC/DC controller with excellent AC and DC load
and line regulation. Each controller has an accurate
undervoltage lockout that shuts down the individual con-
troller when the input voltage falls below 2.0V.
The LTC1874 boasts
±2.5%
output voltage accuracy and
consumes only 270µA of quiescent current per controller.
The LTC1874 is configured with Burst Mode operation,
which enhances efficiency at low output current for appli-
cations where efficiency is a prime consideration.
To further maximize the life of a battery source, each
external P-channel MOSFET is turned on continuously in
dropout (100% duty cycle). In shutdown, each controller
draws a mere 8µA. High constant operating frequency of
550kHz allows the use of small external inductors.
The LTC1874 is available in a small footprint 16-lead
narrow SSOP.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
APPLICATIO S
s
s
s
s
1- or 2-Cell Lithium-Ion-Powered Applications
Personal Information Appliances
Portable Computers
Distributed 3.3V, 2.5V or 1.8V Power Systems
TYPICAL APPLICATION
V
IN
3.5V
TO 9.5V
C
IN
10µF
16V
×2
L1 M1
4.7µH
R1
0.04Ω
1
2
3
4
10k
220pF
13
14
15
80.6k
16
LTC1874
8
PV
IN2
V
IN1
7
SENSE1
–
PGATE2
6
PGND2
GND1
5
I
TH
/RUN2
V
FB1
12
V
FB2
I
TH
/RUN1
11
PGND1
GND2
10
PGATE1 SENSE2
–
9
PV
IN1
V
IN2
R2
0.04Ω
M2
10k
D2
220pF
L2
4.7µH
V
OUT1
3.3V
1A
C1, C2: SANYO POSCAP 6TPA47M
C
IN
: TAIYO YUDEN CERAMIC
EMK325BJ106MNT (× 2)
D1, D2: MBRM120
L1, L2: COILCRAFT D01608C-472
M1, M2: Si3443DV
R1, R2: DALE 0.25W
249k
+
C1
D1
47µF
6V
Figure 1. LTC1874 3.5V-9.5V Input to 3.3V/1A and 1.8V/1A Dual Step-Down Converter
U
+
C2
47µF
6V
V
OUT2
1.8V
1A
100k
80.6k
1874 TA01
U
U
1
LTC1874
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
V
IN1
SENSE1
–
Input Supply Voltage (V
IN
, PV
IN
) ...............– 0.3V to 10V
SENSE
–
, PGATE Voltages ............. – 0.3V to (V
IN
+ 0.3V)
V
FB
, I
TH
/RUN Voltages ..............................– 0.3V to 2.4V
PGATE Peak Output Current (< 10µs) ....................... 1A
Storage Ambient Temperature Range ... – 65°C to 150°C
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Junction Temperature (Note 3) ............................. 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
1
2
3
4
5
6
7
8
16 PV
IN1
15 PGATE1
14 PGND1
13 I
TH
/RUN1
12 V
FB2
11 GND2
10 SENSE2
–
9
V
IN2
ORDER PART
NUMBER
LTC1874EGN
GND1
V
FB1
I
TH
/RUN2
PGND2
PGATE2
PV
IN2
GN PART MARKING
1874
GN PACKAGE
16-LEAD NARROW PLASTIC SSOP
T
JMAX
= 150°C,
θ
JA
= 135°C/ W
Consult factory for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER
Input DC Supply Current (Per Controller)
Normal Operation
Sleep Mode
Shutdown
UVLO
Undervoltage Lockout Threshold
Shutdown Threshold (at I
TH
/RUN)
Start-Up Current Source
Regulated Feedback Voltage
Output Voltage Line Regulation
Output Voltage Load Regulation
V
FB
Input Current
Overvoltage Protect Threshold
Overvoltage Protect Hysteresis
Oscillator Frequency
Gate Drive Rise Time
Gate Drive Fall Time
Peak Current Sense Voltage
All specifications apply to each controller. 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 2)
CONDITIONS
Typicals at V
IN
= 4.2V (Note 4)
2.4V
≤
V
IN
≤
9.8V
2.4V
≤
V
IN
≤
9.8V
2.4V
≤
V
IN
≤
9.8V, V
ITH
/RUN = 0V
V
IN
< UVLO Threshold
V
IN
Falling
V
IN
Rising
V
ITH
/RUN = 0V
T
A
= 0°C to 70°C (Note 5)
T
A
= – 40°C to 85°C (Note 5)
2.4V
≤
V
IN
≤
9.8V (Note 5)
I
TH
/RUN Sinking 5µA (Note 5)
I
TH
/RUN Sourcing 5µA (Note 5)
(Note 5)
Measured at V
FB
V
FB
= 0.8V
V
FB
= 0V
C
LOAD
= 3000pF
C
LOAD
= 3000pF
(Note 6)
0.820
500
q
q
q
q
MIN
TYP
270
230
8
6
MAX
420
370
22
10
2.35
2.40
0.55
0.85
0.820
0.830
UNITS
µA
µA
µA
µA
V
V
V
µA
V
V
mV/V
mV/µA
mV/µA
1.55
1.85
0.15
0.25
0.780
0.770
2.0
2.3
0.35
0.5
0.800
0.800
0.05
2.5
2.5
10
0.860
20
550
120
40
40
120
50
0.895
650
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC1874E 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 formula:
T
J
= T
A
+ (P
D
•
θ
JA
°C/W)
Note 4:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 5:
Each controller in the LTC1874 is individually tested in a feedback
loop that servos V
FB
to the output of the error amplifier.
Note 6:
Peak current sense voltage is reduced dependent upon duty cycle
to a percentage of value as given in Figure 2.
2
U
W
U
U
W W
W
nA
V
mV
kHz
kHz
ns
ns
mV
LTC1874
TYPICAL PERFORMANCE CHARACTERISTICS
Reference Voltage
vs Temperature
825
820
815
V
IN
= 4.2V
10
8
NORMALIZED FREQUENCY (%)
V
FB
VOLTAGE (mV)
805
800
795
790
785
780
775
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1874 G01
2
0
–2
–4
–6
–8
–10
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1874 G02
TRIP VOLTAGE (V)
810
Maximum (V
IN
– SENSE
–
) Voltage
vs Duty Cycle
130
120
TRIP VOLTAGE (mV)
I
TH
/RUN VOLTAGE (mV)
110
100
90
80
70
60
50
20
30
40
50 60 70 80
DUTY CYCLE (%)
90
100
U W
Normalized Oscillator Frequency
vs Temperature
V
IN
= 4.2V
2.24
2.20
2.16
2.12
2.08
2.04
2.00
1.96
1.92
1.88
Undervoltage Lockout Trip
Voltage vs Temperature
V
IN
FALLING
6
4
1.84
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1874 G03
Shutdown Threshold
vs Temperature
600
560
520
480
440
400
360
320
280
240
200
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1874 G05
V
IN
= 4.2V
T
A
= 25°C
V
IN
= 4.2V
1874 G04
3
LTC1874
PIN FUNCTIONS
V
IN1
(Pin 1):
Main Supply Pin for Controller #1. This pin
delivers the Input DC Supply Current (listed in the Electri-
cal Characteristics table) plus a small amount of logic
switching current. Must be connected to PV
IN1
(Pin 16)
and closely decoupled to GND1 (Pin 3).
SENSE1
–
(Pin 2):
The Negative Input to the Current
Comparator of Controller #1.
GND1 (Pin 3):
Signal Ground for Controller #1. Must be
connected to PGND1 (Pin 14).
V
FB1
(Pin 4):
Receives the feedback voltage from an
external resistive divider across the output of Controller
#1.
I
TH
/RUN2 (Pin 5):
This pin performs two functions. It
serves as the error amplifier compensation point as well as
the run control input of Controller #2. The current com-
parator threshold increases with this control voltage.
Nominal voltage range for this pin is 0.7V to 1.9V. Forcing
this pin below 0.35V causes Controller #2 to be shut down.
In shutdown, all functions of Controller #2 are disabled
and PGATE2 (Pin 7) is held high.
PGND2 (Pin 6):
Power Ground for Controller #2. Must be
connected to GND2 (Pin 11).
PGATE2 (Pin 7):
Gate Drive for the External P-Channel
MOSFET of Controller #2. This pin swings from 0V to the
voltage of PV
IN2
.
PV
IN2
(Pin 8):
Power Supply Pin for Controller #2. This pin
delivers the dynamic switching current that drives the gate
of the external P-channel MOSFET of Controller #2. Must
be connected to V
IN2
(Pin 9) and closely decoupled to
PGND2 (Pin 6).
V
IN2
(Pin 9):
Main Supply Pin for Controller #2. This pin
delivers the Input DC Supply Current (listed in the Electri-
cal Characteristics table) plus a small amount of logic
switching current. Must be connected to PV
IN2
(Pin 8) and
closely decoupled to GND2 (Pin 11).
SENSE2
–
(Pin 10):
The Negative Input to the Current
Comparator of Controller #2.
GND2 (Pin 11):
Signal Ground for Controller #2. Must be
connected to PGND2 (Pin 6).
V
FB2
(Pin 12):
Receives the feedback voltage from an
external resistive divider across the output of Controller
#2.
I
TH
/RUN1 (Pin 13):
This pin performs two functions. It
serves as the error amplifier compensation point as well as
the run control input of Controller #1. The current com-
parator threshold increases with this control voltage.
Nominal voltage range for this pin is 0.7V to 1.9V. Forcing
this pin below 0.35V causes Controller #1 to be shut down.
In shutdown, all functions of Controller #1 are disabled
and PGATE1 (Pin 15) is held high.
PGND1 (Pin 14):
Power Ground for Controller #1. Must be
connected to GND1 (Pin 3).
PGATE1 (Pin 15):
Gate Drive for the External P-Channel
MOSFET of Controller #1. This pin swings from 0V to the
voltage of PV
IN1
.
PV
IN1
(Pin 16):
Power Supply Pin for Controller #1. This
pin delivers the dynamic switching current that drives the
gate of the external P-channel MOSFET of Controller #1.
Must be connected to V
IN1
(Pin 1) and closely decoupled
to PGND1 (Pin 14).
4
U
U
U
LTC1874
FUNCTIONAL DIAGRA
V
IN1
1
SENSE1
–
2
OSC
FREQ
FOLDBACK
SHORT-CIRCUIT
DETECT
0.15V
V
IN
0.5µA
+
V
IN
0.3V
–
VOLTAGE
REFERENCE
V
REF
0.8V
GND1
3
UNDERVOLTAGE
LOCKOUT
V
IN2
9
SENSE2
–
10
GND2
11
CONTROLLER #2 IS THE SAME AS CONTROLLER #1
–
+
W
Controller #1
U
U
+
I
CMP
PV
IN1
16
RS1
R
Q
SLOPE
COMP
S
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
PGATE1
15
PGND1
14
–
0.3V
+
–
BURST
CMP
SLEEP
OVP
+
–
V
REF
+
60mV
EAMP
+
–
V
REF
0.8V
V
FB1
4
V
IN
0.35V
+
SHDN
CMP
SHDN
UV
–
1.2V
13
I
TH
/RUN1
Controller #2
PV
IN2
8
PGATE2
7
PGND2
6
V
FB2
12
1874FD
5
I
TH
/RUN2
5
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