9
FN7434.6
July 12, 2006
Applications Information
Product Description
The EL7530 is a synchronous, integrated FET 600mA 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,
the regulator automatically switches over to a voltage-mode
PWM operating at nominal 1.4MHz switching frequency. The
efficiency is up to 95%.
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 EL7530 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 V
IN
,
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
(C
4
) 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 C
4
, 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 V
O
divided by V
IN
.
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 V
IN
, and V
IN
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 EL7530 is
in the shut-down mode. All the control circuitry and both
MOSFETs are off, and V
OUT
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 1.2A 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
off. The output voltage will drop to zero. With the output
MOSFETs turned off, the regulator will soon cool down.
Once the junction temperature drops to about 130°C, the
regulator will restart again in the same manner as EN pin
connects to logic HI.
