B3800M Bay Linear, Inc., B3800M Datasheet - Page 8

• Manufacturer Part Number: B3800M
• Description: 5.5mA 1MHz switching regulator controller step up low voltage (BIPOLAR)
• Manufacturer: Bay Linear, Inc.

Application Descriptions
Power Supply Control Function
Stand- by mode (supply current 1 mA or less) can be set by
connecting the BR/ CTL pin (pin 4) to VCC or by making the
pin open circuit.
Switching Regulator Function
The reference voltage circuit generates a temperature-
compensated reference voltage (@1.25V) from voltage
supplied from the power supply pin (pin 3). In addition to
providing the reference voltage for the switching regulator, the
circuit also sets the idle period.
Sawtooth wave oscillator
The sawtooth oscillator generates a sawtooth wave (up to 1
MHz) that is stable with respect to the supply voltage and
temperature. The capacitor and resistor that set the oscillation
frequency are connected to the OSC pin (pin 7).
Error amplifier (Error Amp.)
The error amplifier detects the output voltage of the switching
regulator and outputs the PWM control signal. The voltage
gain is fixed, and connecting a phase compensation capacitor
to the FB pin (pin 8) provides stable phase compensation for
the system.
PWM comparator (PWM Comp.)
The voltage comparator has one inverting and three non-
inverting inputs. The comparator is a voltage/ pulse width
converter that controls the ON time of the output pulse
depending on the input voltage. The output level is high (H)
when the sawtooth wave is lower than the error amplifier
output voltage, soft start setting voltage, and idle period setting
voltage.
Reference voltage circuit
Bay Linear, Inc
2418 Armstrong Street, Livermore, CA 94550 Tel: [phone], Fax: [phone]
Output circuit
The output circuit has a totem pole type configuration and
can drive an external NPN transistor directly. The value
of the ON/ OFF current can be set by a resistor connected
to the BR/ CTL pin (pin 4).
Other Functions
Soft start and short circuit detection
Soft start operation is set by connecting capacitor CPE to
the SCP pin (pin 2). Soft start prevents a current spike on
start- up.
On completion of soft start operation, the SCP pin (pin 2)
stays low and enters the short circuit detection wait state.
When an output short circuit occurs, the error amplifier
output is fixed at VOM+ and capacitor CPE starts
charging. After charging to approximately 0.8 V, the
output pin (pin 5) is set low and the SCP pin (pin 2) stays
low.
Once the protection circuit operates, the circuit can be
restored by resetting the power supply. (See “n HOW TO
SET THE TIME CONSTANT FOR SOFT START AND
SHORT CIRCUIT DETECTION”.)
Circuit to prevent malfunction at low input voltage
Transients when powering on or instantaneous glitches in
the supply voltage can lead to malfunction of the control
IC and cause system damage or failure. The circuit to
prevent malfunction at low input voltage detects a low
input voltage by comparing the supply voltage to the
internal reference voltage. On detection, the circuit fixes
the output pin to low.
The system recovers when the supply voltage rises back
above the threshold voltage of the malfunction prevention
circuit.
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B3800