1
LT1432-3.3
3.3V High Efficiency Step-Down
Switching Regulator Controller
U
A
O
PPLICATI
TYPICAL
Figure 1. High Efficiency 5V Buck Converter
S
FEATURE
The LT
1432-3.3 is a control chip designed to operate
with the LT1171/LT1271 family of switching regulators to
make a very high efficiency 3.3V step-down (buck) switch-
ing regulator. A minimum of external components is
needed.
Included is an accurate current limit which uses only
60mV sense voltage and uses "free" PC board trace
material for the sense resistor. Logic controlled electronic
shutdown mode draws only 15
A battery current. The
switching regulator operates down to 4.5V input.
The LT1432-3.3 has a logic controlled Burst Mode operation
to achieve high efficiency at very light load currents (0mA to
100mA) such as memory keep-alive. In normal switching
mode, the standby power loss is about 30mW, limiting
efficiency at light loads. In Burst Mode operation, standby
loss is reduced to approximately 11mW. Output current in
this mode is typically in the 5mA to 100mA range.
The LT1432-3.3 is available in 8-pin SO and PDIP pack-
ages. The LT1171/LT1271 is also available in surface
mount DD packages.
D
U
ESCRIPTIO
s
Accurate Preset 3.3V Output
s
Up to 87% Efficiency
s
Optional Burst Mode
TM
Operation for Light Loads
s
Can Be Used with Many LTC Switching ICs
s
Accurate Ultra-Low-Loss Current Limit
s
Operates with Inputs from 4.5V to 30V
s
Shutdown Mode Draws Only 15
A
s
Uses Small 30
H Inductor
U
S
A
O
PPLICATI
s
Laptop and Palmtop Computers
s
Portable Data-Gathering Instruments
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
+
V
SW
FB
GND
V
IN
V
C
LT1271
+
C1
330
F
35V
C6
0.02
F
C5
0.03
F
D1
MBR330p
C3
10
F
TANT
D2**
L1
30
H
R2*
0.013
C2
390
F
16V
V
OUT
***
3.3V
3A
+
MODE LOGIC
220pF
<0.3V = NORMAL MODE
>2.5V = SHUTDOWN
OPEN = Burst Mode
OPERATION
*R2 IS MADE FROM PC BOARD
COPPER TRACES.
**OPTIONAL CONNECTION FOR D2.
EFFICIENCY IS HIGHER, BUT MINIMUM
V
IN
INCREASES. SEE APPLICATION
INFORMATION SECTION.
***MAXIMUM CURRENT IS DETERMINED
BY THE CHOICE OF LT1071 FAMILY MAIN SWITCHER IC.
SEE APPLICATION INFORMATION SECTION.
V
IN
LT1432-3.3 TA01
OPTIONAL
OUTPUT
FILTER
100
F
16V
10
H
3A
+
V
C
GND
MODE
DIODE
V
OUT
V
LIM
V
+
LT1432-3.3
V
IN
D2
1N5818
FOR CIRCUITS WHICH DO NOT USE
Burst Mode OPERATION, C5 MAY
BE PARALLEL WITH A 680
, 0.1
F
IN SERIES TO GIVE WIDE PHASE MARGIN
WITH DIFFERENT SWITCHING ICs AND
OUTPUT CAPACITORS
0
60
EFFICIENCY (%)
70
80
90
100
1A
2A
3A
20mA
LT1432-3.3 TA02
40mA
60mA
0
Burst Mode OPERATION
(USE mA SCALE)
LT1271, L = 30
H
V
IN
= 7V
D2 CONNECTED
TO INPUT
D2 CONNECTED
TO OUTPUT
Efficiency
2
LT1432-3.3
V
IN
Pin .................................................................... 30V
V
+
Pin ..................................................................... 40V
V
C
........................................................................... 35V
V
LIM
and V
OUT
Pins ................................................... 7V
Diode Pin Voltage ................................................... 30V
Mode Pin Current (Note 2) ..................................... 1mA
Operating Temperature Range .................... 0
C to 70
C
Storage Temperature Range ................ 65
C to 150
C
Lead Temperature (Soldering, 10 sec)................. 300
C
A
U
G
W
A
W
U
W
A
R
BSOLUTE
XI
TI
S
W
U
U
PACKAGE/ORDER I FOR ATIO
T
JMAX
= 100
C,
JA
= 150
C/W (N8)
T
JMAX
= 100
C,
JA
= 170
C/W (S8)
LT1432CN8-3.3
LT1432CS8-3.3
ORDER PART
NUMBER
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Regulated Output Voltage
V
C
Current = 220
A
q
3.24
3.30
3.36
V
Output Voltage Line Regulation
V
IN
= 4V to 30V
q
5
20
mV
Input Supply Current (Note 1)
V
IN
= 4V to 30V, V
+
= V
IN
+ 5V, V
C
= V
IN
+ 1V
q
0.3
0.5
mA
Quiescent Output Load Current
0.9
1.2
mA
Mode Pin Current
V
MODE
= 0V (Current Is Out of Pin)
q
30
50
A
V
MODE
= 3.3V (Shutdown)
q
15
30
A
Mode Pin Threshold Voltage
I
MODE
= 10
A (Out of Pin)
q
0.6
0.9
1.5
V
(Normal to Burst)
V
C
Pin Saturation Voltage
V
OUT
= 3.6V (Forced)
q
0.25
0.45
V
V
C
Pin Maximum Sink Current
V
OUT
= 3.6V (Forced)
q
0.45
0.8
1.5
mA
V
C
Pin Source Current
V
OUT
= 3.0V (Forced)
q
35
60
100
A
Current Limit Sense Voltage (Note 3)
Device in Current Limit Loop
56
60
64
mV
V
LIM
Pin Current
Device in Current Limit Loop
q
30
45
70
A
(Current Is Out of Pin)
Supply Current in Shutdown
V
MODE
> 3V, V
IN
< 30V, V
C
and V
+
= 0V
15
60
A
Burst Mode Operation Output Ripple
Device in Burst Test Circuit
100
mV
p-p
Burst Mode Operation Average Output Voltage
Device in Burst Test Circuit
q
3.15
3.30
3.45
V
Clamp Diode Forward Voltage
I
F
= 1mA, All Other Pins Open
q
0.5
0.65
V
Start-up Drive Current
V
OUT
= 1.5V (Forced), V
IN
= 4V to 26V,
q
30
45
mA
V
+
= V
IN
1V, V
C
= V
IN
1.5V
Restart Time Delay
(Note 4)
0.7
1.2
10
ms
Transconductance, Output to V
C
Pin
I
C
= 150
A to 250
A
q
2700
3600
5000
mho
V
C
= 4V, V
IN
= 4V, V
+
= 8V, V
DIODE
= Open, V
LIM
= V
OUT
, V
MODE
= 0V, T
J
= 25
C
Device is in standard test loop unless otherwise noted.
Consult factory for Military and Industrial grade parts.
ELECTRICAL C
C
HARA TERISTICS
1
2
3
4
TOP VIEW
V
LIM
V
OUT
V
IN
V
+
MODE
GND
V
C
DIODE
N8 PACKAGE
8-LEAD PDIP
8
7
6
5
S8 PACKAGE
8-LEAD PLASTIC SO
3
LT1432-3.3
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Burst Mode Operation Quiescent Input Supply Current
1.6
2.2
mA
Burst Mode Operation Output Ripple Voltage
I
OUT
= 0
80
mV
p-p
I
OUT
= 50mA
120
mV
p-p
Normal Mode Equivalent Input Supply Current
Extrapolated from I
OUT
= 20mA
3.0
mA
Normal Mode Minimum Operating Input Voltage
100mA < I
OUT
< 1.5A
4.5
V
Burst Mode Operation Minimum Operating Input Voltage
5mA < I
OUT
< 50mA
4.1
V
Efficiency
Normal Mode
I
OUT
= 0.5A
86
%
Burst Mode Operation I
OUT
= 25mA
70
%
Load Regulation
Normal Mode
50mA < I
OUT
< 2A
5
15
mV
Burst Mode Operation 0 < I
OUT
< 50mA
30
mV
ELECTRICAL C
C
HARA TERISTICS
Operating parameters in standard circuit configuration.
V
IN
= 7V, I
OUT
= 0, unless otherwise noted. These parameters guaranteed where indicated, but not tested.
Figure 2
V
IN
+
+
3
4
6
5
2
7
8
MODE
CONTROL
V
LIM
3.3V
GND
MODE
S3*
S1**
S2**
* S3 IS CLOSED ONLY DURING START-UP.
** S1 AND S2 ARE SHOWN IN NORMAL
MODE. REVERSE FOR Burst Mode
OPERATION.
1
V
+
V
C
V
IN
DIODE
LT1432-3.3 F02
60mV
V
OUT
V
SW
V
C
GND
FB
V
IN
LT1271
D2
OPTIONAL
CONNECTION
OF D2
S
W
A
CHE
TIC
EQUIVALE
U
T
The
q
denotes specifications which apply over the full operating
temperature range.
Note 1: Does not include current drawn by the power IC. See operating
parameters in standard circuit.
Note 2: Breakdown voltage on the Mode pin is 7V. External current must
be limited to value shown.
Note 3: Current limit sense voltage temperature coefficient is +0.33%/
C
to match TC of copper trace material.
Note 4: V
OUT
pin switched from 3.6V to 3.0V.
4
LT1432-3.3
Shutdown Current vs Input
Voltage
Battery Current in Shutdown*
C
C
HARA TERISTICS
U
W
A
TYPICAL PERFOR
CE
Minimum Input Voltage to Start
Normal Mode (Diode to Input)
Efficiency vs Input Voltage
Efficiency vs Load Current
Minimum Running Voltage
Normal Mode*
Burst Mode Operation Minimum
Input Voltage
Minimum Input Voltage
Normal Mode (Diode to Output)
Current Limit Sense Voltage*
INPUT VOLTAGE (V)
0
60
EFFICIENCY (%)
70
80
90
100
5
10
15
20
LT1432-3.3 G01
25
30
T
J
= 25
C
LT1271, L = 50
H
DIODE TO INPUT
I
OUT
= 1A
DIODE TO OUTPUT
0
75
EFFICIENCY (%)
80
85
90
95
0.5
1.0
1.5
2.0
LT1432-3.3 G02
2.5
3.0
T
J
= 25
C
V
IN
= 7V
LOAD CURRENT (A)
LT1270
LT1271
INPUT VOLTAGE (V)
0
0
CURRENT (
A)
10
20
30
40
50
5
10
15
20
LT1432-3.3 G07
25
30
T
J
= 25
C
TEMPERATURE (
C)
0
CURRENT (
A)
20
30
100
LT1432-3.3 G08
10
0
25
50
75
40
V
IN
= 6V
V
IN
= 30V
*DOES NOT INCLUDE LT1271 SWITCH LEAKAGE.
JUNCTION TEMPERATURE (
C)
0
SENSE VOLTAGE (mV)
60
70
100
LT1432-3.3 G9
50
40
25
50
75
80
* TEMPERATURE COEFFICIENT OF SENSE VOLTAGE IS
DESIGNED TO TRACK COPPER RESISTANCE.
OUTPUT CURRENT (A)
0
INPUT VOLTAGE (V)
4.5
5.0
5.5
4
LT1432-3.3 G05
4.0
3.5
3.0
1
2
3
5
T
J
= 25
C
LT1271
LT1270
LT1171
*SEE MINIMUM INPUT VOLTAGE TO START
LOAD CURRENT (mA)
0
INPUT VOLTAGE (V)
4.5
5.0
40
LT1432-3.3 G06
4.0
3.5
10
20
30
50
5.5
T
J
= 25
C
OUTPUT CURRENT (A)
0
INPUT VOLTAGE (V)
5.5
6.0
6.5
4
LT1432-3.3 G03
5.0
4.5
4.0
1
2
3
5
T
J
= 25
C
LT1171
LT1271
LT1270
DIODE
TO INPUT
OUTPUT CURRENT (A)
0
INPUT VOLTAGE (V)
7.0
8.0
9.0
4
LT1432-3.3 G04
6.0
5.0
4.0
1
2
3
5
T
J
= 25
C
LT1171
LT1271
DIODE
TO INPUT
5
LT1432-3.3
C
C
HARA TERISTICS
U
W
A
TYPICAL PERFOR
CE
No Load Battery Current in Burst
Mode Operation
Transconductance V
OUT
to V
C
Current
Burst Mode Operation Load
Regulation
Incremental Battery Current * in
Burst Mode Operation
BATTERY VOLTAGE (V)
0
INCREMENTAL FACTOR (mA/mA)
1.0
1.5
20
LT1432-3.3 G10
0.5
0
5
10
15
25
2.0
* TO CALCULATE TOTAL BATTERY CURRENT IN Burst
Mode OPERATION, MULTIPLY LOAD CURRENT BY
INCREMENTAL FACTOR AND ADD NO-LOAD CURRENT.
T
J
= 25
C
BATTERY VOLTAGE (V)
0
BATTERY CURRENT (mA)
3
4
5
20
LT1432-3.3 G11
2
1
0
5
10
15
25
T
J
= 25
C
JUNCTION TEMPERATURE (
C)
0
TRANSCONDUCTANCE (
mho)
3000
4000
100
LT1432-3.3 G12
2000
1000
25
50
75
5000
Gm =
I(V
C
PIN)
V
OUT
Restart Load Current
Line Regulation
Mode Pin Current
Restart Time Delay
Start-up Switch Characteristics
MODE PIN VOLTAGE (V)
0
CURRENT (
A)
20
40
60
8
LT1432-3.3 G15
0
20
40
2
4
6
10
MODE DRIVE MUST
SINK
30
A AT 0V
T
J
= 25
C
LOAD CURRENT (mA)
0
OUTPUT CHANGE (mV)
25
0
80
50
75
20
40
60
100
25
LT1432-3.3 G14
T
J
= 25
C
V
IN
= 7V
JUNCTION TEMPERATURE (
C)
0
CURRENT (mA)
20
30
100
LT1432-3.3 G16
10
0
25
50
75
40
V
OUT
= 4.5V
JUNCTION TEMPERATURE (
C)
0
TIME DELAY (ms)
2
3
100
LT1432-3.3 G16
1
0
25
50
75
4
V
+
TO V
IN
VOLTAGE
2
V+ PIN CURRENT (mA)
20
0
5
20
LT1432-3.3 G18
40
60
80
1
0
10
30
T
J
= 25
C
NOTE VERTICAL AND
HORIZONTAL SCALE
CHANGES AT 0,0
0
0
20
20
LT1432-3.3 G13
20
40
5
10
15
40
Burst Mode OPERATION
NORMAL MODE
OUTPUT CHANGE (mV)
INPUT VOLTAGE (V)
T
J
= 25
C
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