LTC1625
No R
SENSE
TM
Current Mode
Synchronous Step-Down
Switching Regulator
FEATURES
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DESCRIPTION
The LTC
®
1625 is a synchronous step-down switching
regulator controller that drives external N-Channel power
MOSFETs using few external components. Current mode
control with MOSFET V
DS
sensing eliminates the need for
a sense resistor and improves efficiency. The frequency of
a nominal 150kHz internal oscillator can be synchronized
to an external clock over a 1.5:1 frequency range.
Burst Mode
TM
operation at low load currents reduces
switching losses and low dropout operation extends oper-
ating time in battery-powered systems. A forced continu-
ous mode control pin can assist secondary winding
regulation by disabling Burst Mode operation when the
main output is lightly loaded.
Fault protection is provided by foldback current limiting
and an output overvoltage comparator. An external ca-
pacitor attached to the RUN/SS pin provides soft start
capability for supply sequencing. A wide supply range
allows operation from 3.7V (3.9V for LTC1625I) to 36V at
the input and 1.19V to V
IN
at the output.
, LTC and LT are registered trademarks of Linear Technology Corporation.
No R
SENSE
and Burst Mode are trademarks of Linear Technology Corporation.
Highest Efficiency Current Mode Controller
No Sense Resistor Required
Stable High Current Operation
Dual N-Channel MOSFET Synchronous Drive
Wide V
IN
Range: 3.7V to 36V
Wide V
OUT
Range: 1.19V to V
IN
±1%
1.19V Reference
Programmable Fixed Frequency with Injection Lock
Very Low Drop Out Operation: 99% Duty Cycle
Forced Continuous Mode Control Pin
Optional Programmable Soft Start
Pin Selectable Output Voltage
Foldback Current Limit
Output Overvoltage Protection
Logic Controlled Micropower Shutdown: I
Q
< 30µA
Available in 16-Lead Narrow SSOP and SO Packages
APPLICATIONS
s
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Notebook and Palmtop Computers, PDAs
Cellular Telephones and Wireless Modems
Battery Chargers
Distributed Power
TYPICAL APPLICATION
Efficiency vs Load Current
SYNC
C
SS
0.1µF
RUN/SS
LTC1625
I
TH
R
C
10k
C
C
2.2nF
SW
BOOST
V
PROG
INTV
CC
SGND
BG
D
B
CMDSH-3
C
B
0.22µF
C
VCC
4.7µF
D1
MBRS140T3
M2
Si4410DY
V
IN
TK
TG
M1
Si4410DY
L1
10µH
C
IN
10µF
30V
×
2
V
IN
5V TO
28V
100
V
IN
= 10V
V
OUT
= 5V
+
+
+
V
OUT
3.3V
C
OUT
4.5A
100µF
10V
×
3
EFFICIENCY (%)
V
OSENSE
PGND
1625 F01
Figure 1. High Efficiency Step-Down Converter
U
U
U
90
V
OUT
= 3.3V
80
70
60
0.01
0.1
1
LOAD CURRENT (A)
10
1625 TA01
1
LTC1625
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER I FOR ATIO
TOP VIEW
EXTV
CC
1
SYNC 2
RUN/SS 3
FCB 4
I
TH
5
SGND 6
V
OSENSE
7
V
PROG
8
16 V
IN
15 TK
14 SW
13 TG
12 BOOST
11 INTV
CC
10 BG
9
PGND
Input Supply Voltage (V
IN
, TK) ................. 36V to – 0.3V
Boosted Supply Voltage (BOOST) ............. 42V to – 0.3V
Boosted Driver Voltage (BOOST – SW) ...... 7V to – 0.3V
Switch Voltage (SW).....................................36V to – 5V
EXTV
CC
Voltage ...........................................7V to – 0.3V
I
TH
Voltage ................................................2.7V to – 0.3V
FCB, RUN/SS, SYNC Voltages .....................7V to – 0.3V
V
OSENSE
, V
PROG
Voltages ........(INTV
CC
+ 0.3V) to – 0.3V
Peak Driver Output Current < 10µs (TG, BG) ............ 2A
INTV
CC
Output Current ........................................ 50mA
Operating Ambient Temperature Range
LTC1625C............................................... 0°C to 70°C
LTC1625I (Note 5) .............................. – 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
LTC1625CGN
LTC1625CS
LTC1625IGN
LTC1625IS
GN PACKAGE
S PACKAGE
16-LEAD PLASTIC SSOP 16-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 130°C/W (GN)
T
JMAX
= 125°C,
θ
JA
= 110°C/W (S)
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL
I
IN
V
OSENSE
V
OUT
PARAMETER
Feedback Current
Regulated Output Voltage
1.19V (Adjustable) Selected
3.3V Selected
5V Selected
Reference Voltage Line Regulation
Output Voltage Load Regulation
Forced Continuous Threshold
Forced Continuous Current
Output Overvoltage Lockout
V
PROG
Input Current
3.3V V
OUT
5V V
OUT
Input DC Supply Current
Normal Mode
Shutdown
RUN/SS Pin Threshold
Soft Start Current Source
TG Transition Time
Rise Time
Fall Time
Main Control Loop
T
A
= 25°C, V
IN
= 15V unless otherwise noted.
MIN
TYP
10
q
q
q
CONDITIONS
V
PROG
Pin Open, I
TH
= 1.19V (Note 3)
I
TH
= 1.19V (Note 3)
V
PROG
Pin Open
V
PROG
= 0V
V
PROG
= INTV
CC
V
IN
= 3.6V to 20V, I
TH
= 1.19V (Note 3),
V
PROG
Pin Open
I
TH
= 2V (Note 3)
I
TH
= 0.5V (Note 3)
V
FCB
Ramping Negative
V
FCB
= 1.19V
V
PROG
Pin Open
V
PROG
= 0V
V
PROG
= 5V
EXTV
CC
= 5V (Note 4)
V
RUN/SS
= 0V, 3.7V < V
IN
< 15V
q
q
q
q
MAX
50
1.202
3.380
5.100
0.01
– 0.2
0.2
1.22
–2
1.32
–7
7
UNITS
nA
V
V
V
%/V
%
%
V
µA
V
µA
µA
µA
µA
V
µA
mV
ns
ns
1.178
3.220
4.900
1.190
3.300
5.000
0.001
– 0.020
0.035
V
LINEREG
V
LOADREG
V
FCB
I
FCB
V
OVL
I
PROG
1.16
1.24
1.19
–1
1.28
– 3.5
3.5
500
15
I
Q
30
2
4
170
150
150
V
RUN/SS
I
RUN/SS
0.8
1.2
120
1.4
2.5
150
50
50
V
RUN/SS
= 0V
V
OSENSE
= 1V, V
PROG
Pin Open
C
LOAD
= 3300pF
C
LOAD
= 3300pF
∆V
SENSE(MAX)
Maximum Current Sense Threshold
TG t
R
TG t
F
2
U
W
U
U
W W
W
LTC1625
ELECTRICAL CHARACTERISTICS
SYMBOL
BG t
R
BG t
F
V
INTVCC
V
LDOINT
V
LDOEXT
V
EXTVCC
Oscillator
f
OSC
f
H
/f
OSC
V
SYNC
R
SYNC
Oscillator Freqency
Maximum Synchronized Frequency Ratio
SYNC Pin Threshold (Figure 4)
SYNC Pin Input Resistance
PARAMETER
BG Transition Time
Rise Time
Fall Time
Internal V
CC
Voltage
INTV
CC
Load Regulation
EXTV
CC
Voltage Drop
EXTV
CC
Switchover Voltage
T
A
= 25°C, V
IN
= 15V unless otherwise noted.
MIN
TYP
50
50
q
CONDITIONS
C
LOAD
= 3300pF
C
LOAD
= 3300pF
6V < V
IN
< 30V, V
EXTVCC
= 4V
I
CC
= 20mA, V
EXTVCC
= 4V
I
CC
= 20mA, V
EXTVCC
= 5V
I
CC
= 20mA, V
EXTVCC
Ramping Positive
q
MAX
150
150
5.4
–2
300
UNITS
ns
ns
V
%
mV
V
Internal V
CC
Regulator
5.0
5.2
–1
180
4.5
135
Ramping Positive
4.7
150
1.5
0.9
50
1.2
V
kΩ
165
kHz
The
q
denotes specifications which apply over the full operating
temperature range.
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 formula:
LTC1625CGN/LTC1625IGN: T
J
= T
A
+ (P
D
• 130°C/W)
LTC1625CS/LTC1625IS: T
J
= T
A
+ (P
D
• 110°C/W)
Note 3:
The LTC1625 is tested in a feedback loop that adjusts V
OSENSE
to
achieve a specified error amplifier output voltage (I
TH
).
Note 4:
Typical in application circuit with EXTV
CC
tied to V
OUT
= 5V,
I
OUT
= 0A and FCB = INTV
CC
. Dynamic supply current is higher due
to the gate charge being delivered at the switching frequency. See
Applications Information.
Note 5:
Minimum input supply voltage is 3.9V at – 40°C for industrial
grade parts.
3
LTC1625
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Load Current
100
BURST
MODE
OPERATION
100
95
CONTINUOUS
MODE
90
I
LOAD
= 200mA
85
80
75
70
0.1
0.01
1
LOAD CURRENT (A)
10
1625 G01
90
EFFICIENCY (%)
EFFICIENCY (%)
80
EFFICIENCY (%)
70
60
V
IN
= 10V
V
OUT
= 5V
EXTV
CC
= V
OUT
50
0.001
Load Regulation
0
FIGURE 1 CIRCUIT
– 0.05
∆V
OUT
(%)
V
ITH
(V)
– 0.10
CONTINUOUS
MODE
1.5
1.0
V
IN
– V
OUT
(mV)
– 0.15
– 0.20
– 0.25
0
1
3
2
LOAD CURRENT (A)
Input and Shutdown Current
vs Input Voltage
1000
EXTV
CC
OPEN
800
INPUT CURRENT (µA)
600
SHUTDOWN
400
30
–1.0
EXTV
CC
– INTV
CC
(mV)
∆INTV
CC
(%)
200
EXTV
CC
= 5V
0
0
5
20
15
10
25
INPUT VOLTAGE (V)
30
35
4
U W
4
Efficiency vs Input Voltage,
V
OUT
= 5V
FIGURE 1 CIRCUIT
I
LOAD
= 2A
100
95
Efficiency vs Input Voltage,
V
OUT
= 3.3V
FIGURE 1 CIRCUIT
I
LOAD
= 2A
90
85
I
LOAD
= 200mA
80
75
70
0
5
10
15
20
INPUT VOLTAGE (V)
25
30
1625 G02
0
5
10
15
20
INPUT VOLTAGE (V)
25
30
1625 G02
I
TH
Pin Voltage vs Load Current
3.0
FIGURE 1 CIRCUIT
V
IN
= 20V
2.5 V
OUT
= 5V
2.0
400
V
IN
– V
OUT
Dropout Voltage
vs Load Current
FIGURE 1 CIRCUIT
V
OUT
= 5V – 5% DROP
300
200
0.5
0
5
1625 G04
Burst Mode
OPERATION
100
0
1
4
3
5
2
LOAD CURRENT (A)
6
7
0
0
1
3
2
LOAD CURRENT (A)
4
5
1625 G06
1625 G05
INTV
CC
Load Regulation
50
0
500
EXTV
CC
Switch Drop
vs INTV
CC
Load Current
40
– 0.5
400
SHUTDOWN CURRENT (µA)
300
20
–1.5
200
10
– 2.0
100
0
– 2.5
0
10
30
40
20
INTV
CC
LOAD CURRENT (mA)
50
1625 G08
0
0
10
30
40
20
INTV
CC
LOAD CURRENT (mA)
50
1625 G09
1625 G07
LTC1625
TYPICAL PERFOR A CE CHARACTERISTICS
Maximum Current Sense Voltage
vs Duty Cycle
MAXIMUM CURRENT SENSE VOLTAGE (mV)
MAXIMUM CURRENT SENSE VOLTAGE (mV)
200
150
FREQUENCY (kHz)
100
50
0
0
0.2
0.5
0.4
DUTY CYCLE
FCB Pin Current vs Temperature
0
– 0.25
– 0.50
– 0.75
–1.00
–1.25
–1.50
– 40 –15
0
RUN/SS CURRENT (µA)
FCB CURRENT (µA)
60
35
85
10
TEMPERATURE (°C)
Transient Response
V
OUT
50mV/DIV
V
IN
= 20V
V
OUT
= 5V
I
LOAD
= 1A TO 4A
FIGURE 1 CIRCUIT
200µs/DIV
U W
0.8
110
Maximum Current Sense Voltage
vs Temperature
160
300
250
155
200
Oscillator Frequency
vs Temperature
SYNC = 1.5V
150
SYNC = 0V
150
100
50
145
1.0
1625 G10
140
– 40 –15
85
10
35
60
TEMPERATURE (°C)
110
135
0
– 40 –15
60
35
85
10
TEMPERATURE (°C)
110
135
1625 G11
1625 G12
RUN/SS Pin Current
vs Temperature
INDUCTOR
CURRENT
2A/DIV
Soft Start:
Load Current vs Time
–1
–2
RUN/SS
2V/DIV
–3
20ms/DIV
–4
1625 F06
V
IN
= 20V
V
OUT
= 5V
R
LOAD
= 1Ω
FIGURE 1 CIRCUIT
–15
60
10
85
35
TEMPERATURE (°C)
110
135
135
–5
–40
1625 G13
1625 G14
Transient Response
(Burst Mode Operation)
V
OUT
50mV/DIV
V
OUT
50mV/DIV
I
TH
100mV/DIV
Burst Mode Operation
1625 F07
V
IN
= 20V
V
OUT
= 5V
I
LOAD
= 50mA TO 1A
FIGURE 1 CIRCUIT
500µs/DIV
1625 F08
V
IN
= 20V
V
OUT
= 5V
I
LOAD
= 50mA
FIGURE 1 CIRCUIT
50µs/DIV
1625 F09
5
Test/Measurement
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