EL7464CRE-T13 ELANTEC [Elantec Semiconductor], EL7464CRE-T13 Datasheet - Page 10

• Manufacturer Part Number: EL7464CRE-T13
• Description: Monolithic 4 Amp DC:DC Step-down Regulator
• Manufacturer: ELANTEC [Elantec Semiconductor]

EL7564C
Monolithic 4 Amp DC:DC Step-down Regulator
high-side switch remains on until all four comparator
inputs have summed to zero, at which time the high-side
switch is turned off and the low-side switch is turned on.
However, the maximum on-duty ratio of the high-side
switch is limited to 95%. In order to eliminate cross-con-
duction of the high-side and low-side switches a 15ns
break-before-make delay is incorporated in the switch
drive circuitry. The output enable (EN) input allows the
regulator output to be disabled by an external logic con-
trol signal.
Output Voltage Setting
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 is changed. This is shown in the performance
curves. A 100nA pull-up current from FB to VDD forces
VOUT to GND in the event that FB is floating.
NMOS Power FETs and Drive Circuitry
The EL7564C integrates low on-resistance (30m )
NMOS FETs to achieve high efficiency at 4A. In order
to use an NMOS switch for the high-side drive it is nec-
essary to drive the gate voltage above the source voltage
(LX). This is accomplished by bootstrapping the VHI
pin above the LX voltage with an external capacitor
CVHI and internal switch and diode. When the low-side
switch is turned on and the LX voltage is close to GND
potential, capacitor CVHI is charged through internal
switch to VDRV, typically 5V. At the beginning of the
next cycle the high-side switch turns on and the LX pins
begin to rise from GND to VIN potential. As the LX pin
rises the positive plate of capacitor CVHI follows and
eventually reaches a value of VDRV+VIN, typically
10V, for VDRV=VIN=5V. This voltage is then level
shifted and used to drive the gate of the high-side FET,
via the VHI pin. A value of 0.22µF for CVHI is
recommended.
Reference
A 1.5% temperature compensated bandgap reference is
integrated in the EL7564C. The external VREF capaci-
V
OUT
=
0.975V
1
+
R
------
R
2
1
10
tor acts as the dominant pole of the amplifier and can be
increased in size to maximize transient noise rejection.
A value of 0.1µF is recommended.
Oscillator
The system clock is generated by an internal relaxation
oscillator with a maximum duty-cycle of approximately
95%. Operating frequency can be adjusted through the
COSC pin or can be driven by an external source. If the
oscillator is driven by an external source care must be
taken in selecting the ramp amplitude. Since CSLOPE
value is derived from the COSC ramp, changes to COSC
ramp will change the CSLOPE compensation ramp
which determine the open-loop gain of the system.
When external synchronization is required, always
choose C
least 20% lower than that of sync source to accommo-
date component and temperature variations. Figure 1
shows a typical connection.
Junction Temperature Sensor
An internal temperature sensor continuously monitors
die temperature. In the event that die temperature
exceeds the thermal trip-point, the system is in fault state
and will be shut down. The upper and low trip-points are
set to 135°C and 115°C respectively.
The VTJ pin is an accurate indication of the internal sili-
con junction temperature (see performance curve.) The
Oscillator
External
Figure 1. Oscillator Synchronization
OSC
such that the free-running frequency is at
100pF
BAT54S
390pF
10
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EL7564C
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