MIC2194_05 Datasheet, PDF (8/10 Pages) Micrel Semiconductor – 400kHz SO-8 Buck Control IC

MIC2194
1.5V
V_IN
Typical
MIC2194
R1
Bias
Circuitry
EN/UVLO
R2
(3)
140mV
Hysteresis
(typical)
Figure 3. UVLO Circuitry
The line voltage turn on trip point is:
V_INPUT
_ ENABLE⁼
V_THRESHOLD
×
R2
R1+ R2
where:
V_THRESHOLD is the voltage level of the internal
comparator reference, typically 1.5V.
The input voltage hysteresis is equal to:
V_{INPUT _ HYST}=
V_HYST
×
R1+ R2
R2
where:
V_HYST is the internal comparator hysteresis level,
typically 140mV.
V_{INPUT_HYST} is the hysteresis at the input voltage
The MIC2194 will be disabled when the input voltage drops
back down to:
V_{INPUT_OFF} =
V_{INPUT_ENABLE} – V_{INPUT_HYST} =
(V_THRESHOLD
–
V_HYST)
×
R2
R1+ R2
Either of 2 UVLO conditions will pull the soft start capacitor
low:
• When the V_DD voltage drops below its
undervoltage lockout level.
• When the enable pin drops below the its enable
threshold
The internal bias circuit generates an internal 1.245V band-
gap reference voltage for the voltage error amplifier and a 3V
VDD voltage for the internal control circuitry. The VDD pin
must be decoupled with a 1µF ceramic capacitor. The capaci-
tor must be placed close to the VDD pin. The other end of the
capacitor must be connected directly to the ground plane.
MOSFET Gate Drive
The MIC2194 is designed to drive a high-side P-channel
MOSFET. The source pin of the P-channel MOSFET is
connected to the input of the power supply. It is turned on
when OUTP pulls the gate of the MOSFET low. The advan-
tage of using a P-channel MOSFET is that it does not require
a bootstrap circuit to boost the gate voltage higher than the
input, as would be required for an N-channel MOSFET. The
VIN pin (pin 8) supplies the drive voltage to the gate drive pin,
OUTP.
Micrel
MOSFET Selection
The P-channel MOSFET must have a V_GS threshold voltage
equal to or lower than the input voltage when used in a buck
converter topology. There is a limit to the maximum gate
charge the MIC2194 will drive. MOSFETs with high gate
charge will have slower turn-on and turn-off times. Slower
transition times will cause higher power dissipation in the
MOSFET due to higher switching transition losses.
The MOSFET gate charge is also limited by power dissipation
in the MIC2194. The power dissipated by the gate drive
circuitry is calculated below:
P_{GATE_DRIVE} = Q_GATE × V_IN × f_S
where: Q_GATE is the total gate charge of both the N- and P-
channel MOSFETs.
f_S is the switching frequency
V_IN is the gate drive voltage
The graph in Figure 4 shows the total gate charge that can be
driven by the MIC2194 over the input voltage range, for
different values of switching frequency.
Max Gate Charge
250
200
150
100
50
00
5
10
15
INPUT VOLTAGE (V)
Figure 4. MIC2194 V_IN vs Max. Gate Charge
Oscillator
The internal oscillator is free running and requires no external
components. The maximum duty cycle for both frequencies
is 100%. This is another advantage of using a P-channel
MOSFET for the high-side drive; it can be continuously turned
on.
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. Frequency foldback is used to limit the energy delivered
to the output during a short circuit fault condition.
Voltage Setting Components
The MIC2194 requires two resistors to set the output voltage
as shown in Figure 5.
MIC2194
8
April 2005