MIC3808BMM Micrel Inc, MIC3808BMM Datasheet - Page 8

• Manufacturer Part Number: MIC3808BMM
• Description: IC CTRLR DC/DC PUSH-PULL 8-MSOP
• Manufacturer: Micrel Inc

Specifications of MIC3808BMM

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 1MHz
• Duty Cycle: 50%
• Voltage - Supply: 2 V ~ 14 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Frequency-max: 1MHz
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

MIC3808/3809
Functional Diagram
Functional Description
The MIC3808/9 is a high-speed power supply controller with
push-pull output drive capability. MIC3808 has a higher V
turn-on threshold and more hysteresis between V
and turn-off than the MIC3809. The outputs of the controller
operate in a push-pull fashion with a guaranteed dead time
between them. A block diagram of the MIC3808/9 controller
is shown in Figure 1.
V
The oscillator and output gate drive signals are disabled
when V
output drivers eliminates glitching or random pulsing during
the start-up sequence. The oscillator is enabled when V
applied and reaches the turn-on threshold. The V
parator also turns off the internal soft-start discharge FET,
slowly bringing up the COMP pin voltage.
The V
clamp voltage, the V
current draw of the IC. Exceeding the V
voltage may cause excessive power dissipation in the
MIC3808/9.
Soft-Start
The soft start feature helps reduce surge currents at the
power supply input source. An internal current source and
capacitor ramp up from 0V to near Vdd at a typical rate of
1V/ms. The softstart feature limits the output voltage of the
MIC3808/3809
DD
and Turn-on Sequence
Reference
DD
Voltage
DD
pin is internally clamped. As V
is lower than the turn on threshold. Circuitry in the
V
DD
V
DD
OK
Overcurrent
Comparator
—1V
0.5V
DD
current will increase over the normal
Soft Start
0.75V
2.2V
FB
2
2.0V
R
S1
S2
R
S
Amplifier
Error
DD
3.65V
Q
Q
Slope = 1V/ms
DD
approaches this
Figure 1. MIC3808 Block Diagram
zener shunt
V
DD
DD
DD
COMP
turn-on
1
com-
DD
1.2R
R
DD
is
CS
3
0.8V
8
Comparator
0.5V
PWM
Peak Current
error amplifier at the COMP pin. As the softstart voltage rises,
it allows the COMP pin voltage to rise, which in turn allows the
duty cycle of the output drivers to increase. The internal
softstart voltage is discharged and remains discharged dur-
ing the following conditions:
Once the internal softstart discharge FET is turned on, it
cannot be turned off until the internal softstart voltage drops
down below 0.5V. This insures a clean restart.
Oscillator
The oscillator operates at twice the switching frequency of
either OUTA or OUTB. The oscillator generates a sawtooth
waveform on the RC pin. The rising edge of the waveform is
controlled by the external resistor/capacitor combination.
The fall time is set by the on-resistance of the discharge FET
(see Figure 2). The fall time sets the delay (dead time)
between the turn-off of one output driver and the turn-on of the
other driver. A toggle flip-flop insures that drive signals to
OUTA and OUTB are alternated and therefore insures a
maximum duty cycle of less than 50% for each output driver.
Graphs of component values vs. oscillator frequency and
dead time are shown in the typical characteristic section of
this specification.
Comparator
Oscillator
1. The V
2. The voltage on the CS pin exceeds the
RC
4
threshold
overcurrent comparator threshold
DD
voltage drops below the turn-off
S
R
PWM
Latch
Q
14V
T
/Q
Q
April 2005
8
7
6
5
Micrel
V
OUTA
OUTB
GND
DD