MIC2196 Micrel Semiconductor, MIC2196 Datasheet - Page 9

• Manufacturer Part Number: MIC2196
• Description: 400kHz SO-8 Boost Control IC
• Manufacturer: Micrel Semiconductor

low enough to operate at the minimum V
tee the boost converter will start up.
The maximum amout of MOSFET gate charge that can be
driven is limited by the power dissipation in the MIC2196. The
power dissipated by the gate drive circuitry is calculated
below:
where:
The graph in Figure 4 shows the total gate charge which can
be driven by the MIC2196 over the input voltage range.
Higher gate charge will slow down the turn-on and turn-off
times of the MOSFET, which increases switching losses.
External Schottky Diode
In a boost converter topology, the boost diode, D1 must be
rated to handle the peak and average current. The average
current through the diode is equal to the average output
current of the boost converter. The peak current is calculated
in the current limit section of this specification.
For the MIC2196, Schottky diodes are recommended when
they can be used. They have a lower forward voltage drop
than ultra-fast rectifier diodes, which lowers power dissipa-
tion and improves efficiency. They also do not have a recov-
ery time mechanism, which results in less ringing and noise
when the diode turns off. If the output voltage of the circuit
prevents the use of a Schottky diode, then only ultra-fast
recovery diodes should be used. Slower diodes will dissipate
more power in both the MOSFET and the diode. The will also
cause excessive ringing and noise when the diode turns off.
Reference, Enable and UVLO Circuits
The output drivers are enabled when the following conditions
are satisfied:
The internal bias circuitry generates a 1.245V bandgap
reference for the voltage error amplifier and a 3V V
for the internal supply bus. The VDD pin must be decoupled
to ground with a 1 F ceramic capacitor.
August 2004
MIC2196
• The V
• The voltage on the enable pin is greater than the
Figure 4. MIC2196 Frequency vs. Gate Charge
P_gate_drive = Q_gate
Q_gate is the total gate charge of the external
MOSFET
undervoltage threshold.
enable UVLO threshold.
DD
voltage (pin 5) is greater than its
250
200
150
100
50
0
0
2
Max. Gate Charge
INPUT VOLTAGE (V)
4
6
V
8
IN
10 12 14
f
S
IN
voltage to guaran-
DD
voltage
9
The enable pin (pin 3) has two threshold levels, allowing the
MIC2196 to shut down in a micro-current mode, or turn-off
output switching in standby mode. Below 0.9V, the device is
forced into a micro power shutdown. If the enable pin is
between 0.9V and 1.5V the output gate drive is disabled but
the internal circuitry is powered on and the soft start pin
voltage is forced low. There is typically 135mV of hysteresis
below the 1.5V threshold to insure the part does not oscillate
on and off due to ripple voltage on the input. Raising the
enable voltage above the UVLO threshold of 1.5V enables
the output drivers and allows the soft start capacitor to
charge. The enable pin may be pulled up to VINA.
Oscillator and Sync
The internal oscillator is self-contained and requires no
external components. The maximum duty cycle of the MIC2196
is 85%.
Minimum duty cycle becomes important in a boost converter
as the input voltage approaches the output voltage. At lower
duty cycles, the input voltage can be closer to the output
voltage without the output rising out of regulation. Minimum
duty cycle is typically 7%.
A frequency foldback mode is enabled if the voltage on the
feedback pin (pin 2) is less than 0.3V. In frequency foldback
the oscillator frequency is reduced by approximately a factor
of 4.
Voltage Setting Components
The MIC2196 requires two resistors to set the output voltage
as shown in Figure 5.
The output voltage is determined by the equation below.
Where: V
Lower values of resistance are preferred to prevent noise
from apprearing on the VFB pin. A typically recommended
value for R1 is 10K.
Decoupling Capacitor Selection
A 1 F decoupling capacitor is used to stabilize the internal
regulator and minimize noise on the VDD pin. Placement of
this capacitor is critical to the proper operation of the MIC2196.
It must be next to the VDD and signal ground pins and routed
with wide etch. The capacitor should be a good quality
ceramic. Incorrect placement of the VDD decoupling capaci-
tor will cause jitter and/or oscillations in the switching wave-
form as well as variations in the overcurrent limit.
V
O
Figure 5. Voltage Setting Components
REF
V
REF
for the MIC2196 is nominally 1.245V.
1
R2
R1
1.245V
Amplifier
Voltage
MIC2196
V
REF
Pin
6
R1
R2
MIC2196
Micrel