10
FN7102.7
May 8, 2006
It also allows 100% turn-on of the upper PMOS switch,
achieving V
O
close to V
IN
. The maximum achievable V
O
is:
Where R
L
is the DC resistance on the inductor and R
DSON1
is the PMOS on-resistance, nominally 30mΩ at room
temperature with a temperature coefficient of 0.2mΩ/°C.
OUTPUT VOLTAGE SELECTION
The output voltage can be as high as the input voltage minus
the PMOS and inductor voltage drops (as seen previously in
Equation 2). Referring to the Typical Application Circuit on
page 2, use R
1
and R
2
to set the output voltage according to
the following formula:
Some standard values of R
1
and R
2
are listed in Table 1.
It is important that the series combination of R1 and R2 is
large enough as to not draw excessive current from the
output.
VOLTAGE MARGINING
The EL7566 has built-in 5% load stress test (commonly
called voltage margining) function. Combinations of TM and
SEL set the margins shown in Table 2. When this function is
not used, both pins should be connected to SGND, either
directly or through a 10kΩ resister. Figure 16 shows this
feature.
SWITCHING FREQUENCY
The regulator has a programmable switching frequency of
200kHz to 1MHz. The switching frequency is generated by a
relaxation comparator and adjusted by a capacitor from the
OSC pin to GND (C
OSC
). The triangle waveform has 95%
duty ratio and runs from 0.2V to 1.2V. Refer to the curve in
Figure 6 for the appropriate value of C
OSC
for the desired
frequency. If external synchronization is desired, the circuit
in Figure 21 can be used.
Always choose the converter self-switching frequency 20%
lower than the sync frequency to accommodate component
variations.
Protection Features
The EL7566 features a wide range of protective measures to
prevent the persistence of damaging system conditions.
These features are overvoltage, overcurrent, Power-On-
Reset (POR), and Thermal Shutdown protection.
OVERVOLTAGE PROTECTION (OVP)
The EL7566 monitors the output voltage and will shut down
if it exceeds 110% of the set regulation point. This is
accomplished by comparing the reference to the FB pin
voltage. If an overvoltage condition is met, the controller will
turn the high-side switch off, the low-side switch on, and pull
PGOOD low. The converter will not latch off and will proceed
with a soft-start as soon as the fault condition is cleared.
OVERCURRENT PROTECTION (OCP)
The current information for PWM ramp generation is also
used for overcurrent protection. The measured current is
compared against a preset Overcurrent threshold (~7-10A).
If the output current exceeds the threshold, the output will
shut down by turning off the high-side switch and turning the
low-side switch on. This event, like OVP, will not latch the
converter off. A soft-start will be initiated when the fault is
cleared.
POWER-ON RESET (POR)
To ensure proper regulator operation, a power-on reset
feature monitors the input voltage. When adequate input
voltage is achieved (V
DD
> 2.8V), the converter is allowed to
soft-start. However, if V
DD
falls below 2.5V, the regulator will
shut down in the same manner as OVP or OCP.
THERMAL PROTECTION AND JUNCTION
TEMPERATURE INDICATOR
An internal temperature sensor continuously monitors the
junction temperature. If the junction temperature exceeds
135°C, the regulator is in a fault condition and will shut
down. When the temperature falls back below 110°C, the
regulator goes through the soft-start procedure again.
TABLE 1.
V
O
(V) R
1
(kΩ)R
2
(kΩ)
0.8 2 Open
12.4910
1.2 4.99 10
1.5 10 11.5
1.8 12.7 10.2
2.5 21.5 10
3.3 36 11.5
TABLE 2.
CONDITION TM SEL V
O
Normal 0 X Nominal
High Margin 1 1 Nominal + 5%
Low Margin 1 0 Nominal - 5%
V
O
V
IN
R
L
R
DSON1
+()I
O
×–=
V
O
0.8 1
R
1
R
2
-------
+
⎝⎠
⎜⎟
⎛⎞
×=
EL7566
C
OSC
100pF
EXTERNAL SYNC
SOURCE
FIGURE 20. EXTERNAL SYNC CIRCUIT
EL7566
