10
FN7333.6
October 9, 2007
Schottky Diode
In selecting the Schottky diode, the reverse break down
voltage, forward current and forward voltage drop must be
considered for optimum converter performance. The diode
must be rated to handle 1.5A, the current limit of the
EL7516. The breakdown voltage must exceed the maximum
output voltage. Low forward voltage drop, low leakage
current, and fast reverse recovery will help the converter to
achieve the maximum efficiency.
Input Capacitor
The value of the input capacitor depends on the input and
output voltages, the maximum output current, the inductor
value and the noise allowed to put back on the input line. For
most applications, a minimum 10µF is required. For
applications that run close to the maximum output current
limit, input capacitor in the range of 22µF to 47µF is
recommended.
The EL7516 is powered from the V
IN
. High frequency 0.1µF
by-pass cap is recommended to be close to the V
IN
pin to
reduce supply line noise and ensure stable operation.
Loop Compensation
The EL7516 incorporates an transconductance amplifier in
its feedback path to allow the user some adjustment on the
transient response and better regulation. The EL7516 uses
current mode control architecture, which has a fast current
sense loop and a slow voltage feedback loop. The fast
current feedback loop does not require any compensation.
The slow voltage loop must be compensated for stable
operation. The compensation network is a series RC
network from COMP pin to ground. The resistor sets the high
frequency integrator gain for fast transient response and the
capacitor sets the integrator zero to ensure loop stability. For
most applications, the compensation resistor in the range of
2k to 7.5k and the compensation capacitor in the range of
3nF to 10nF.
Soft-Start
The soft-start is provided by an internal 6µA current source,
which charges the external C
SS
. The peak MOSFET current
is limited by the voltage on the capacitor. This in turn controls
the rising rate of the output voltage. The regulator goes
through the start-up sequence as well after the SHDN
pin is
pulled to HI.
Frequency Selection
The EL7516 switching frequency can be user selected to
operate at either at constant 620kHz or 1.25MHz.
Connecting F
SEL
pin to ground sets the PWM switching
frequency to 620kHz. When connect F
SEL
high or V
DD
,
switching frequency is set to 1.25MHz.
Shut-Down Control
When the Shut-down pin is pulled down, the EL7516 is shut-
down, reducing the supply current to <3µA.
EL7516 does not use a level translator or ground-referenced
threshold for the SHDN input. For different supply voltages,
please refer to Figure 32 to choose the right input threshold
voltages for SHDN, where VTP is about 1V. It is
recommended that V
IH
= (V
IN
- VTP/2) and V
IL
= (V
IN
/4).
If the consistent SHDN threshold is desired in the
application, an external active level shifter must be used.
The simplest circuit requires 1 NMOS and 1 resistor, as
shown in Figure 33 where the gate of the NMOS is
connected to supply of PWRON logic circuit, and the source
of the NMOS goes to PWRON pin of the converter.
Maximum Output Current
The MOSFET current limit is nominally 1.5A and guaranteed
1.3A. This restricts the maximum output current I
OMAX
based on the following formula:
FIGURE 32. SHDN INPUT THRESHOLD vs INPUT SUPPLY
VOLTAGE
V
IH
, UPPER
0V
V
IN
(V
IN
- VTP)
(V
IN
/2)
KEEP OUT
V
IL
, LOWER
V
IN
V
IN
= 3.3V V
IN
= 5.5V
SHDN INPUT THRESHOLDS
LOGIC THRESHOLD
LOGIC THRESHOLD
FIGURE 33. LEVEL SHIFTER CIRCUIT
SUPPLY INPUT VOLTAGE
TO EL7516
PIN3 SHDN
20k
3VD
20k
PWRON
PIN OF THE
CONVERTER
I
L
I
L-AVG
12⁄ΔI
L
×()+=
(EQ. 7)
EL7516
