MP1495S – SYNCHRONOUS STEP-DOWN CONVERTER
OPERATION
The
MP1495S
is
a
high-frequency,
pin—the pullup resistor limits the EN input
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.
current to less than 100µA.
For example, with 12V connected to VIN,
RPULLUP ≥ (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.
The MP1495S 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.
Figure 2: 6.5V Zener Diode
Connect an external clock with a range of
300kHz 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.
Internal Regulator
The 5V internal regulator power most of the
internal circuitries. This regulator takes the VIN
input and operates in the full VIN range: When
VIN exceeds 5.0V, the output of the regulator is
in full regulation; when VIN falls below 5.0V, the
Under-Voltage Lockout
Under-voltage lockout (UVLO) protects the chip
from operating at an insufficient supply voltage.
The MP1495S UVLO comparator monitors the
output voltage of the internal regulator, VCC.
The UVLO rising threshold is about 3.8V while
its falling threshold is 3.15V.
output decreases and requires
decoupling ceramic capacitor.
a
0.1µF
Error Amplifier
Internal Soft-Start
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.
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.
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.
Over-Current Protection and Hiccup
The MP1495S 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 MP1495S enters
hiccup mode to periodically restart the part.
The EN pin is clamped internally using a 6.5V
series Zener diode, as shown in Figure 2.
Connect the EN input pin through a pullup
resistor to any voltage connected to the VIN
MP1495S Rev. 1.0
12/13/2013
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
10
MPS [ MONOLITHIC POWER SYSTEMS ]
