ISL8201M(2009) Ver la hoja de datos (PDF) - Intersil
Número de pieza
componentes Descripción
Fabricante
ISL8201M Datasheet PDF : 16 Pages
| |||
ISL8201M
the output enabled during the transition through this region, it
may be acceptable. The user should monitor the output for
their application to see if there is any problem. If PVCC powers
up first and the VIN is not present by the time the initialization
is done, then the soft-start will not be able to ramp the output,
and the output will later follow part of the VIN ramp when it is
applied. If this is not desired, then change the sequencing of
the supplies, or use the COMP/EN pin to disable VOUT until
both supplies are ready.
Figure 20 shows a simple sequencer for this situation. If
PVCC powers up first, Q1 will be off, and R3 pulling to PVCC
will turn Q2 on, keeping the ISL8201M in shut-down. When
VIN turns on, the resistor divider R1 and R2 determines when
Q1 turns on, which will turn off Q2 and release the
shut-down. If VIN powers up first, Q1 will be on, turning Q2
off; so the ISL8201M will start-up as soon as PVCC comes
up. The VENDIS trip point is 0.4V nominal, so a wide variety
of N-MOSFET or NPN BJT or even some logic IC's can be
used as Q1 or Q2. However, Q2 must be low leakage when
off (open-drain or open-collector) so as not to interfere with
the COMP output. Q2 should also be placed near the
COMP/EN pin.
FIGURE 20. SEQUENCE CIRCUIT
The VIN range can be as low as ~1V (for VOUT as low as the
0.6V reference) and as high as 20V. There are some
restrictions for running high VIN voltage. The maximum
PHASE voltage is 30V. The VIN + PVCC + any ringing or
other transients on the PHASE pin must be less than 30V. If
VIN is 20V, it is recommended to limit PVCC to 5V.
Switching Frequency
The switching frequency is a fixed 600kHz clock, which is
determined by the internal oscillator. However, all of the other
timing mentioned (POR delay, OCP sample, soft-start, etc.) is
independent of the clock frequency (unless otherwise noted).
Selection of the Input Capacitor
The input filter capacitor should be based on how much
ripple the supply can tolerate on the DC input line. The larger
the capacitor, the less ripple expected but consideration
should be taken for the higher surge current during
power-up. The ISL8201M provides the soft-start function that
controls and limits the current surge. The value of the input
capacitor can be calculated by Equation 5:
CIN = -I-I--N--Δ----×V----Δ-----t
(EQ. 5)
Where:
CIN is the input capacitance (µF)
IIN is the input current (A)
Δt is the turn on time of the high-side switch (µs)
ΔV is the allowable peak-to-peak voltage (V)
In addition to the bulk capacitance, some low Equivalent
Series Inductance (ESL) ceramic capacitance is
recommended to decouple between the drain terminal of the
high side MOSFET and the source terminal of the low side
MOSFET. This is used to reduce the voltage ringing created
by the switching current across parasitic circuit elements.
Output Capacitors
The ISL8201M is designed for low output voltage ripple. The
output voltage ripple and transient requirements can be met
with bulk output capacitors (COUT) with low enough
Equivalent Series Resistance (ESR). COUT can be a low
ESR tantalum capacitor, a low ESR polymer capacitor or a
ceramic capacitor. The typical capacitance is 330µF and
decoupled ceramic output capacitors are used. The
internally optimized loop compensation provides sufficient
stability margins for all ceramic capacitor applications with a
recommended total value of 400µF. Additional output filtering
may be needed if further reduction of output ripple or
dynamic transient spike is required.
Layout Guide
To achieve stable operation, low losses, and good thermal
performance some layout considerations are necessary.
FIGURE 21. RECOMMENDED LAYOUT
12
FN6657.1
July 16, 2009
Share Link: 