LM3311SQ National Semiconductor Corporation, LM3311SQ Datasheet - Page 17

• Manufacturer Part Number: LM3311SQ
• Description: Step-up Pwm Dc/dc Converter With Integrated Ldo, Op-amp, And Gate Pulse Modulation Switch
• Manufacturer: National Semiconductor Corporation

VGH = 20V, VDPM = 3.3V, C
R2 = 1.2kΩ, VFLK at 50% duty cycle and 30kHz
1) VFLK, 2V/div, DC
3) VGHM, 5V/div, DC
T = 4µs/div
Operation
CONTINUOUS CONDUCTION MODE
The LM3311 contains a current-mode, PWM boost regulator.
A boost regulator steps the input voltage up to a higher output
voltage. In continuous conduction mode (when the inductor
current never reaches zero at steady state), the boost regu-
lator operates in two cycles.
In the first cycle of operation, shown in Figure 1 (a), the tran-
sistor is closed and the diode is reverse biased. Energy is
collected in the inductor and the load current is supplied by
C
The second cycle is shown in Figure 1 (b). During this cycle,
the transistor is open and the diode is forward biased. The
energy stored in the inductor is transferred to the load and
output capacitor.
OUT
.
GPM Transient Waveforms
VGHM
= 4.7nF, R
(a) First Cycle of Operation (b) Second Cycle Of Operation
E
= 2.4kΩ, C
FIGURE 1. Simplified Boost Converter Diagram
E
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= open, R1 = 13kΩ,
17
VGH = 20V, VDPM = 3.3V, C
R2 = 1.2kΩ, VFLK at 50% duty cycle and 64kHz
1) VFLK, 2V/div, DC
3) VGHM, 5V/div, DC
T = 2µs/div
The ratio of these two cycles determines the output voltage.
The output voltage is defined approximately as:
where D is the duty cycle of the switch, D and D
quired for design calculations.
SETTING THE OUTPUT VOLTAGE (BOOST CONVERTER
AND LDO)
The output voltage is set using the feedback pin and a resistor
divider connected to the output as shown in the typical oper-
ating circuit. The feedback pin voltage is 1.263V for both the
GPM Transient Waveforms
VGHM
= 4.7nF, R
E
= 750Ω, C
E
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= open, R1 = 13kΩ,
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will be re-