MP1495 – SYNCHRONOUS STEP-DOWN CONVERTER
OPERATION
The MP1495 is a high-frequency, synchronous,
rectified, step-down, switch-mode converter
with built-in power MOSFETs. It offers a very
compact solution to achieve 3A continuous
output current with excellent load and line
regulation over a wide input supply range.
The MP1495 operates in a fixed-frequency,
peak-current–control mode to regulate the
output voltage. An internal clock initiates a
PWM cycle. The integrated high-side power
MOSFET turns on and remains on until its
current reaches the value set by the COMP
voltage. When the power switch is off, it
remains off until the next clock cycle starts. If
the current in the power MOSFET does not
reach the current value set by COMP within
95% of one PWM period, the power MOSFET
will be forced to turn off.
Internal Regulator
The 5V internal regulator power most of the
internal circuitries. This regulator takes the V
IN
input and operates in the full V
IN
range: When
V
IN
exceeds 5.0V, the output of the regulator is
in full regulation; when V
IN
falls below 5.0V, the
output decreases and requires a 0.1µF
decoupling ceramic capacitor.
Error Amplifier
The error amplifier compares the FB pin voltage
against the internal 0.8V reference (REF) and
outputs a COMP voltage—this COMP voltage
controls the power MOSFET current. The
optimized internal compensation network
minimizes the external component count and
simplifies the control loop design.
Enable/SYNC control
EN/Sync is a digital control pin that turns the
regulator on and off: Drive EN high to turn on
the regulator, drive it low to turn it off. An
internal 1MΩ resistor from EN/Sync to GND
allows EN/Sync to be floated to shut down the
chip.
The EN pin is clamped internally using a 6.7V
series Zener diode, as shown in Figure 2.
Connect the EN input pin through a pullup
resistor to any voltage connected to the V
IN
pin—the pullup resistor limits the EN input
current to less than 100µA.
MP1495 Rev. 1.04
12/26/2012
For example, with 12V connected to V
IN
,
R
PULLUP
≥
(12V – 6.5V) ÷ 100µA = 55kΩ.
Connecting the EN pin is directly to a voltage
source without any pullup resistor requires
limiting voltage amplitude to
≤
6V to prevent
damage to the Zener diode.
Figure 2: 6.5V-type Zener Diode
Connect an external clock with a range of
200kHz to 2MHz 2ms after output voltage is set
to synchronize the internal clock rising edge to
the external clock rising edge. The pulse width
of external clock signal should be less than
1.7μs.
Under-Voltage Lockout
Under-voltage lockout (UVLO) protects the chip
from operating at an insufficient supply voltage.
The MP1495 UVLO comparator monitors the
output voltage of the internal regulator, VCC.
The UVLO rising threshold is about 3.9V while
its falling threshold is 3.25V.
Internal Soft-Start
The soft-start prevents the converter output
voltage from overshooting during startup. When
the chip starts, the internal circuitry generates a
soft-start voltage (SS) that ramps up from 0V to
1.2V. When SS is lower than REF, SS
overrides REF so the error amplifier uses SS as
the reference. When SS exceeds REF, the
error amplifier uses REF as the reference. The
SS time is internally set to 1.5ms.
Over-Current Protection and Hiccup
The MP1495 has cycle-by-cycle over current
limit for when the inductor current peak value
exceeds the set current limit threshold. If the
output voltage starts to drop until FB is below
the Under-Voltage (UV) threshold—typically
50% below the reference—the MP1495 enters
hiccup mode to periodically restart the part.
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