EL7531 Datasheet, PDF (8/10 Pages) Intersil Corporation – Monolithic 1A Step-Down Regulator with Low Quiescent Current

EL7531
Performance Curves and Waveforms (Continued)
All waveforms are taken at V_IN=3.3V, V_O=1.8V, I_O=1A with component values shown on page 1 at room ambient temperature, unless otherwise
noted.
JEDEC JESD51-3 LOW EFFECTIVE THERMAL
CONDUCTIVITY TEST BOARD
0.6
0.5 ^486mW
0.4
0.3
θ_{JA =2M0S6°OCP/W10}
0.2
0.1
0
0
25
50
75 85 100
125
AMBIENT TEMPERATURE (°C)
FIGURE 25. PACKAGE POWER DISSIPATION vs AMBIENT
TEMPERATURE
JEDEC JESD51-7 HIGH EFFECTIVE THERMAL
CONDUCTIVITY TEST BOARD
1
0.9 ^870mW
0.8
0.7
0.6
0.5
θ_{JA =1M15S°OCP/W10}
0.4
0.3
0.2
0.1
0
0
25
50
75 85 100
125
AMBIENT TEMPERATURE (°C)
FIGURE 26. PACKAGE POWER DISSIPATION vs AMBIENT
TEMPERATURE
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 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 V_LX pin to V_O.
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.
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.
8
FN7428.6
August 5, 2005