EL7556BCM ELANTEC [Elantec Semiconductor], EL7556BCM Datasheet - Page 11

• Manufacturer Part Number: EL7556BCM
• Description: Integrated Adjustable 6 Amp Synchronous Switcher
• Manufacturer: ELANTEC [Elantec Semiconductor]

parator will terminate the high-side switch. If I LMAX
has not been reached, the regulator output voltage is then
compared to the reference voltage VREF. The resultant
error voltage is summed with the current feedback and
slope compensation ramp. The high-side switch remains
on until all three comparator inputs have summed to
zero, at which time the high-side switch is turned off and
the low-side switch is turned on. In order to eliminate
cross-conduction of the high-side and low-side switches
a 10ns break-before-make delay is incorporated in the
switch driver circuitry. In the continuous mode of opera-
tion the low-side switch will remain on until the end of
the oscillator period. In order to improve the low current
efficiency of the EL7556BC, a zero-crossing comparator
senses when the inductor transitions through zero. Turn-
ing off the low-side switch at zero inductor current
prevents forward conduction through the internal clamp-
ing diodes (LX to VSSP) when the low-side switch turns
off, reducing power dissipation. The output enable
(OUTEN) input allows the regulator output to be dis-
abled by an external logic control signal.
Output Voltage Mode Select
The VCC2DET multiplexes the FB1 and FB2 pins to the
PWM controller. A logic 1 on VCC2DET selects the
FB2 input and forces the output voltage to the internally
programmed value of 3.50V. A logic zero on VCC2DET
selects FB1 and allows the output to be programmed
from 1.0 to 3.8V. In general:
However, due to the relatively low open loop gain of the
system, gain errors will occur as the output voltage and
loop-gain are changed. This is shown in the performance
curves. (The output voltage is factory trimmed to mini-
mize error at a 2.50V output). A 2uA pull-up current
from FB1 to VIN forces VOUT to GND in the event that
FB1 is not used and the VCC2DET is inadvertently tog-
gled between the internal and external feedback mode of
operation.
NMOS Power FETS and Drive Circuitry
The EL7556BC integrates low resistance (25m )
NMOS FETS to achieve high efficiency at 6A. Gate
V
OUT
=
1V
1
+
R
----- -
R
3
4
Volt
Integrated Adjustable 6 Amp Synchronous Switcher
11
drive for both the high-side and low-side switches is
derived through a charge pump consisting of the CP pin
and external components D1-D3 and C5-C6. The CP
output is a low resistance inverter driven at one-half the
oscillator frequency. This is used in conjunction with
D2-D3 to generate a 7.5V (typical) voltage on the C2V
pin which provides gate drive to the low-side NMOS
switch and associated level shifter. In order to use an
NMOS switch for the high-side drive it is necessary to
drive the gate voltage above the source voltage (LX).
This is accomplished by boot-strapping the VHI pin
above the C2V voltage with capacitor C6 and diode D1.
When the low-side switch is turned on the LX voltage is
close to GND potential and capacitor C6 is charged
through diodes D1-D3 to approximately 6.9V. At the
beginning of the next cycle the high side switch turns on
and the LX pin begins to rise from GND to VDD poten-
tial. As the LX pin rises the positive plate of capacitor
C6 follows and eventually reaches a value of approxi-
mately 11.2V
shifted and used to drive the gate of the high-side FET,
via the VHI pin.
Reference
A 1% temperature compensated band gap reference is
integrated in the EL7556BC. The external CREF capac-
itor acts as the dominant pole of the amplifier and can be
increased in size to maximize transient noise rejection.
A value of 0.1uF is recommended.
Oscillator
The system clock is generated by an internal relaxation
oscillator with a maximum duty-cycle of approximately
96%. Operating frequency can be adjusted through the
COSC pin or can be driven by an external clock source.
If the oscillator is driven by an external source, care
must be taken in the selection of CSLOPE. Since the
COSC and CSLOPE values determine the open loop
gain of the system, changes to COSC require corre-
sponding changes to CSLOPE in order to maintain a
constant gain ratio. The recommended ratio of COSC to
CSLOPE is 1.5:1
Temperature Sensor
An internal temperature sensor continuously monitors
die temperature. In the event that die temperature
,
for VDD=5V. This voltage is then level
EL7556BC