LT1950 Linear Technology, LT1950 Datasheet - Page 12

• Manufacturer Part Number: LT1950
• Description: Single Switch PWM Controller
• Manufacturer: Linear Technology

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LT1950
APPLICATIO S I FOR ATIO
(V)
(A)
(A)
(V)
12
N-channel power MOSFET even though V
low. High GATE drive voltage reduces MOSFET R
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
connected between the BOOST pin and V
the BOOST pin will draw current until approximately
125mA is reached, turn off for 0.5 s 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
capacitor from V
inductor is transferred to the V
time of the internal switcher. An auxiliary boost converter
is realized providing a supply to the V
inductor current, V
waveforms are shown in Figure 5. When V
the internal switcher is disabled. Since V
output driver of the LT1950, switching at the GATE pin will
eventually discharge the V
a lower level of approximately 10V. At this level the internal
switcher is re-enabled and switches until V
0.25
0.25
12
10
15
0
0
0
Figure 5. V
IN2
IN2
5 s/DIV
IN2
U
IN2
Generation Using the BOOST Pin
to ground, energy from the external
supply. With an external inductor
voltage and BOOST pin voltage
U
IN2
capacitor until V
IN2
BOOST
V
IN2
I
I
D1
L1
capacitor during the off-
W
MIN
3V
IN2
LT1950
IN
IN
IN2
IN2
pin. The typical
V
BOOST
(see Figure 5),
IN
voltage is very
IN2
V
reaches 11V,
IN2
supplies the
IN2
returns to
U
IN2
reaches
DS(ON)
L1
and a
D1
C1
1950 F05
,
11V. This hysteretic (burst mode) operation for the inter-
nal switcher minimizes power dissipation from V
The V
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 0.1 F capacitor. Internal
undervoltage lockout thresholds for V
proximately 2.6V and 6.8V respectively must be exceeded
before V
Programming Oscillator Frequency
The oscillator frequency of the LT1950 is programmed
using an external resistor connected between the R
and ground. Figure 6 shows typical f
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
pin should be minimized by placing the R
close as possible to the R
the R
R
(analog ground) pin.
OSC
OSC
REF
resistor should be returned directly to the GND
REF
Figure 6. Oscillator Frequency (f
node as small as possible. The ground side of the
output is a 2.5V reference supplying most of the
becomes active.
500
450
400
350
300
250
200
150
100
50
100 150
200
OSC
R
250
OSC
pin and keeping the area of
(k )
300 350
OSC
OSC
400 450 500
IN
) vs R
vs R
and V
OSC
1950 F06
OSC
OSC
resistor as
IN2
sn1950 1950fs
IN
resistor
OSC
of ap-
.
OSC
pin