EL7531 Просмотр технического описания (PDF) - Renesas Electronics
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EL7531 Datasheet PDF : 12 Pages
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EL7531
Applications Information
Product Description
The EL7531 is a synchronous, integrated FET 1A step-down
regulator which operates from an input of 2.5V to 5.5V. The
output voltage is user-adjustable with a pair of external
resistors.
When the load is very light, the regulator automatically
operates in the PFM mode, thus achieving high efficiency at
light load (>70% for 1mA load). When the load increases to
typically 250mA, the regulator automatically switches over to a
voltage-mode PWM operating at nominal 1.4MHz switching
frequency. The efficiency is up to 94%.
It can also operate in a fixed PWM mode or be synchronized to
an external clock up to 12MHz for improved EMI performance.
PFM Operation
The heart of the EL7531 regulator is the automatic PFM/PWM
controller.
If the SYNC pin is connected to ground, the regulator operates
automatically in either the PFM or PWM mode, depending on
load. When the SYNC pin is connected to VIN, the regulator
operates in the fixed PWM mode. When the pin is connected to
an external clock ranging from 1.6MHz to 12MHz, the regulator
is in the fixed PWM mode and synchronized to the external
clock frequency.
In the automatic PFM/PWM operation, when the load is light,
the regulator operates in the PFM mode to achieve high
efficiency. The top P channel MOSFET is turned on first. The
inductor current increases linearly to a preset value before it is
turned off. Then the bottom N channel MOSFET turns on, and
the inductor current linearly decreases to zero current. The N
channel MOSFET is then turned off, and an anti-ringing
MOSFET is turned on to clamp the VLX pin to VO. The inductor
current looks like triangular pulses. The frequency of the
pulses is mainly a function of output current. The higher the
load, the higher the frequency of the pulses until the inductor
current becomes continuous. At this point, the controller
automatically changes to PWM operation.
When the controller transitions to PWM mode, there can be a
perturbation to the output voltage. This perturbation is due to
the inherent behavior of switching converters when
transitioning between two control loops. To reduce this effect, it
is recommended to use the phase-lead capacitor (C4) shown
in the Typical Application Diagram on page 1. This capacitor
allows the PWM loop to respond more quickly to this type of
perturbation. To properly size C4, refer to the Component
Selection section.
PWM Operation
The regulator operates the same way in the forced PWM or
synchronized PWM mode. In this mode, the inductor current is
always continuous and does not stay at zero.
In this mode, the P channel MOSFET and N channel MOSFET
always operate complementary. When the PMOSFET is on
and the NMOSFET off, the inductor current increases linearly.
The input energy is transferred to the output and also stored in
the inductor. When the P channel MOSFET is off and the N
channel MOSFET on, the inductor current decreases linearly,
and energy is transferred from the inductor to the output.
Hence, the average current through the inductor is the output
current. Since the inductor and the output capacitor act as a
low pass filter, the duty cycle ratio is approximately equal to VO
divided by VIN.
The output LC filter has a second order effect. To maintain the
stability of the converter, the overall controller must be
compensated. This is done with the fixed internally
compensated error amplifier and the PWM compensator.
Because the compensations are fixed, the values of input and
output capacitors are 10µF to 22µF ceramic and inductor is
1.5µH to 2.2µH.
Forced PWM Mode/SYNC Input
Pulling the SYNC pin HI (>2.5V) forces the converter into PWM
mode in the next switching cycle regardless of output current.
The duration of the transition varies depending on the output
current. Figures 22 and 23 (under two different loading
conditions) show the device goes from PFM to PWM mode.
Note: In forced PWM mode, the IC will continue to start-up in
PFM mode to support pre-biased load applications.
Start-Up and Shut-Down
When the EN pin is tied to VIN, and VIN reaches approximately
2.4V, the regulator begins to switch. The inductor current limit
is gradually increased to ensure proper soft-start operation.
When the EN pin is connected to a logic low, the EL7531 is in
the shut-down mode. All the control circuitry and both
MOSFETs are off, and VOUT falls to zero. In this mode, the
total input current is less than 1µA.
When the EN reaches logic HI, the regulator repeats the start-
up procedure, including the soft-start function.
Current Limit and Short-Circuit Protection
The current limit is set at about 2A for the PMOS. When a
short-circuit occurs in the load, the preset current limit restricts
the amount of current available to the output, which causes the
output voltage to drop below the preset voltage. In the
meantime, the excessive current heats up the regulator until it
reaches the thermal shut-down point.
Thermal Shut-Down
Once the junction reaches about 145°C, the regulator shuts
down. Both the P channel and the N channel MOSFETs turn
FN7428 Rev 10.00
December 10, 2015
Page 9 of 12
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