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LT1950EGN#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LT1950
10
1950fa
APPLICATIO S I FOR ATIO
WUU
U
LT1950 Input Supplies, V
REF
Output and GATE Enable
V
IN
is the main input supply for the LT1950. V
IN2
is the
input supply for the LT1950 output driver. V
IN2
can be
provided by shorting the V
IN2
pin to the V
IN
pin or by
generating V
IN2
using the BOOST pin. Waveforms of V
IN
,
V
IN2
, V
REF
and GATE switching are shown in Figures 2 and
3. Figure 2 represents low V
IN
operation with V
IN2
gener-
ated using the B00ST pin. Figure 3 represents V
IN
= V
IN2
operation with the BOOST pin open circuit or shorted to
ground.
Low V
IN
Operation
The LT1950 can be configured to provide a minimum of
10V GATE drive for an external N-channel MOSFET from
V
IN
voltages as low as 3V, if the BOOST pin is used to
generate a second supply at the V
IN2
pin (see Figure 2 and
Applications Information “ Generating V
IN2
Supply Using
BOOST
Pin”). The advantage of this configuration is that a
lower R
DS(ON)
is achieved for the external N-channel
MOSFET, improving efficiency, versus a controller run-
ning at 3V input without boosted gate drive. In addition,
typical controllers running at low input voltages have the
limitation of only being able to use logic level MOSFETs.
The LT1950 allows a greater range of usable MOSFETs.
This versatility allows optimization of the overall power
supply performance and allows applications which would
otherwise not be possible without a more complex topol-
ogy. Figure 2 shows that for V
IN
above 2V, the internal
switcher at the BOOST pin is enabled. This switch gener-
ates the V
IN2
supply. As V
IN2
ramps up above the
undervoltage lockout threshold of 6.8V the 2.5V reference
V
REF
becomes active and powers up internal control
circuitry. When V
IN2
exceeds approximately 8V, the gate
driver is enabled. V
IN2
is regulated between 10V and 11V,
providing a supply to the LT1950 output driver to ensure
a minimum of 10V drive at the GATE pin.
Figure 2. Low V
IN
Operation
Figure 3. V
IN
= V
IN2
Operation
12
8
4
0
V
IN
50µs/DIV
BOOST
V
IN2
V
IN
LT1950
L1
D1
C1
MIN
3V
4
3
2
1
0
V
REF
1950 F02
GATE
V
IN2
BOOST
V
IN2
10.2
8.5
6.8
5.1
3.4
5.0
2.5
0
10
5
0
V
IN
= V
IN2
V
REF
GATE
10µs/DIV
BOOST
V
IN2
V
IN
LT1950
TYPICAL START-UP INPUT
>8.2V
C1
*
*BOOST PIN CAN BE
LEFT OPEN OR
SHORTED TO GROUND
1950 F03
BOOST
V
IN2
V
IN
= V
IN2
Operation
If low V
IN
operation is not required below approximately
8V on V
IN
the LT1950 can be configured to run without the
use of the BOOST pin by shorting the V
IN2
pin to the V
IN
pin. Figure 3 shows that both V
IN
and V
IN2
must now
exceed 6.8V to activate the 2.5V V
REF
output and must
exceed approximately 8V to enable the output driver
(GATE pin).
LT1950
11
1950fa
APPLICATIO S I FOR ATIO
WUU
U
Shutdown and Undervoltage Lockout
Figure 4 shows how to program undervoltage lockout
(UVLO) for the V
IN
supply. Typically, UVLO is used in
situations where the input supply is current limited, or has
a relatively high source resistance. A switching regulator
draws constant power from the source, hence source
current increases as source voltage drops. This looks like
a negative load resistance to the source and can cause the
source to current limit or latch low under low source
voltage conditions. An internally set undervoltage lockout
(UVLO) threshold prevents the regulator from operating at
source voltages where these problems might occur. An
internal comparator will force the part into shutdown
below the minimum V
IN
of 2.6V. This feature can be used
to prevent excessive discharge of battery-operated sys-
tems. Alternatively, UVLO threshold is adjustable. The
shutdown threshold voltage of the SHDN pin is 1.32V.
Forcing the SHDN pin below this 1.32V threshold causes
V
REF
to be disabled and stops switching at the GATE pin.
If the SHDN pin is left open circuit, a permanent 3µA flows
out of the pin to ensure that the pin defaults high to allow
normal operation. Voltages above the 1.32V threshold
cause an extra 7µA to be sourced out of the pin, providing
current hysteresis. This can be used to set voltage hyster-
esis of the UVLO threshold using the following equations:
R
VV
A
R
V
VV
R
A
HL
H
1
7
2
132
132
1
3
=
µ
=
+µ
–
.
(–. )
V
H
= Turn on threshold
V
L
= Turn off threshold
Example: switching should not start until the input is
above 11V and is to stop if the input falls below 9V.
V
H
= 11V
V
L
= 9V
R
VV
A
k
R
V
VV
k
A
k
1
11 9
7
286
2
132
11 1 32
286
3
36
=
µ
=
=
+µ
=
–
.
(–.)
Keep the connections from the resistors to the SHDN pin
short and make sure that the interplane or surface capaci-
tance to the switching nodes are minimized. If high resis-
tance values are used, the SHDN pin should be bypassed
with a 1nF capacitor to prevent coupling problems from
the switch node.
Figure 4. Undervoltage Lockout
3µA
7µA
1.32V
GND
V
REF
V
IN
R1
R2
C1
+
1950 F04
LT1950
Generating V
IN2
Supply Using BOOST Pin
The LT1950’s BOOST pin is used to provide a “boosted”
11V supply at the V
IN2
pin for V
IN
voltages as low as 3V.
Since V
IN2
supplies the output driver for the GATE pin of
the IC, it is advantageous to generate a boosted V
IN2
. This
architecture achieves high GATE drive for an external
LT1950
12
1950fa
N-channel power MOSFET even though V
IN
voltage is very
low. High GATE drive voltage reduces MOSFET R
DS(ON)
,
improves efficiency and increases the range of MOSFETs
that can be selected. A small switching regulator at the
BOOST pin, with fixed current limit and fixed off time,
generates the V
IN2
supply. With an external inductor
connected between the BOOST pin and V
IN
(see Figure 5),
the BOOST pin will draw current until approximately
125mA is reached, turn off for 0.5µs and then turn back on.
The cycle is repeated for as long as the switcher is enabled.
By using a diode connected from BOOST to V
IN2
and a
capacitor from V
IN2
to ground, energy from the external
inductor is transferred to the V
IN2
capacitor during the off-
time of the internal switcher. An auxiliary boost converter
is realized providing a supply to the V
IN2
pin. The typical
inductor current, V
IN2
voltage and BOOST pin voltage
waveforms are shown in Figure 5. When V
IN2
reaches 11V,
the internal switcher is disabled. Since V
IN2
supplies the
output driver of the LT1950, switching at the GATE pin will
eventually discharge the V
IN2
capacitor until V
IN2
reaches
a lower level of approximately 10V. At this level the internal
switcher is re-enabled and switches until V
IN2
returns to
APPLICATIO S I FOR ATIO
WUU
U
Figure 6. Oscillator Frequency (f
OSC
) vs R
OSC
R
OSC
(kΩ)
50
FREQUENCY (kHz)
400 450 500
1950 F06
100 150 250200 300 350
500
450
400
350
300
250
200
150
100
11V. This hysteretic (burst mode) operation for the inter-
nal switcher minimizes power dissipation from V
IN
.
The V
REF
output is a 2.5V reference supplying most of the
LT1950 control circuitry. It is available for external use
with maximum current capability of 2.5mA. The pin should
be bypassed to ground using a 0.1µF capacitor. Internal
undervoltage lockout thresholds for V
IN
and V
IN2
of ap-
proximately 2.6V and 6.8V respectively must be exceeded
before V
REF
becomes active.
Programming Oscillator Frequency
The oscillator frequency of the LT1950 is programmed
using an external resistor connected between the R
OSC
pin
and ground. Figure 6 shows typical f
OSC
vs R
OSC
resistor
values. The LT1950 is programmable for a free-running
oscillator frequency in the range of 100kHz to 500kHz.
Stray capacitance and potential noise pickup on the R
OSC
pin should be minimized by placing the R
OSC
resistor as
close as possible to the R
OSC
pin and keeping the area of
the R
OSC
node as small as possible. The ground side of the
R
OSC
resistor should be returned directly to the GND
(analog ground) pin.
Figure 5. V
IN2
Generation Using the BOOST Pin
BOOST
V
IN2
V
IN
LT1950
L1
D1
C1
MIN
3V
5µs/DIV
12
10
0.25
0
0.25
0
15
0
(V)
(V)
(A)
(A)
1950 F05
BOOST
V
IN2
I
D1
I
L1
BOOST
V
IN2
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LT1950EGN#TRPBF

Mfr. #:
Buy LT1950EGN#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 1x Switch PWM Cntr w/ Auxiliary Boost Co
Lifecycle:
New from this manufacturer.
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