MP1542 MPS, MP1542 Datasheet - Page 6

• Manufacturer Part Number: MP1542
• Description: Monolithic Power Systems
• Manufacturer: MPS

At the beginning of each cycle, the N-Channel
MOSFET switch is turned on, forcing the
inductor current to rise. The current at the
source of the switch is internally measured and
converted to a voltage by the current sense
amplifier. That voltage is compared to the error
voltage at COMP. The voltage at the output of
the error amplifier is an amplified version of the
difference between the 1.25V reference voltage
and the feedback voltage. When these two
voltages are equal, the PWM comparator turns
APPLICATION INFORMATION
Components referenced below apply to Typical Application Circuit on page 1.
(Selecting the Soft-Start Capacitor
The MP1542 includes a soft-start timer that
limits the voltage at COMP during startup to
prevent excessive current at the input. This
prevents premature termination of the source
voltage
overshoot. When power is applied to the
MP1542, and enable is asserted, a 6µA internal
current source charges the external capacitor at
SS. As the SS capacitor is charged, the voltage
at SS rises. The MP1542 internally clamps the
voltage at COMP to 700mV above the voltage
at SS. The soft-start ends when the voltage at
SS reaches 0.45V. This limits the inductor
current at start-up, forcing the input current to
rise slowly to the current required to regulate
the output voltage.
The soft-start period is determined by the
equation:
Where C
from SS to GND, and t
period.
Determine the capacitor required for a given
soft-start period by the equation:
Setting the Output Voltage
Set the output voltage by selecting the resistive
voltage divider ratio. Use 10kΩ for the low-side
resistor R2 of the voltage divider. Determine the
high-side resistor R1 by the equation:
01/05, Rev. 1.0
at
SS
(in nF) is the soft-start capacitor
startup
C
SS
t
SS
=
=
. 0
7
SS
due
0133
5
×
(in µs) is the soft-start
C
×
SS
to
t
SS
MPS Proprietary Information. © 2005 All Rights Reserved.
MP1542 – 700KHz/1.3MHz BOOST CONVERTER WITH A 2A SWITCH
Unauthorized Photocopy and Duplication Prohibited.
input
current
off the switch forcing the inductor current to the
output capacitor through the external rectifier.
This causes the inductor current to decrease.
The peak inductor current is controlled by the
voltage at COMP, which in turn is controlled by
the output voltage. Thus the output voltage
controls the inductor current to satisfy the load.
The use of current mode regulation improves
transient response and control loop stability.
.
where V
For R2 = 10kΩ and V
R1 (kΩ) = 8kΩ (V
Selecting the Input Capacitor
An input capacitor is required to supply the AC
ripple current to the inductor, while limiting
noise at the input source. A low ESR capacitor
is required to keep the noise at the IC to a
minimum. Ceramic capacitors are preferred, but
tantalum or low-ESR electrolytic capacitors may
also suffice.
Use an input capacitor value greater than
4.7µF. The capacitor can be electrolytic,
tantalum or ceramic. However since it absorbs
the input switching current it requires an
adequate ripple current rating. Use a capacitor
with RMS current rating greater than the
inductor ripple current (see Selecting The
Inductor to determine the inductor ripple
current).
To insure stable operation place the input
capacitor as close to the IC as possible.
Alternately a smaller high quality ceramic 0.1µF
capacitor may be placed closer to the IC with
the larger capacitor placed further away. If
using this technique, it is recommended that the
larger capacitor be a tantalum or electrolytic
type. All ceramic capacitors should be placed
close to the MP1542.
OUT
is the output voltage.
R
1
=
OUT
R
( 2
– 1.25V).
FB
V
OUT
= 1.25V, then
V
FB
−
INITIAL RELEASE
V
FB
)
6