7
Applications Information
Circuit Description
General
The EL7551 is a fixed frequency, current mode controlled
DC:DC converter with integrated N-channel power
MOSFETs and a high precision reference. The device
incorporates all the active circuitry required to implement a
cost effective, user-programmable 1A synchronous step-
down regulator suitable for use in DSP core power supplies.
Theory of Operation
The EL7551 is composed of 5 major blocks:
1. PWM Controller
2. NMOS Power FETs and Drive Circuitry
3. Bandgap Reference
4. Oscillator
5. Thermal Shut-down
PWM Controller
The EL7551 regulates output voltage through the use of
current-mode controlled pulse width modulation. The three
main elements in a PWM controller are the feedback loop
and reference, a pulse width modulator whose duty cycle is
controlled by the feedback error signal, and a filter which
averages the logic level modulator output. In a step-down
(buck) converter, the feedback loop forces the time-averaged
output of the modulator to equal the desired output voltage.
Unlike pure voltage-mode control systems, current-mode
control utilizes dual feedback loops to provide both output
voltage and inductor current information to the controller.
The voltage loop minimizes DC and transient errors in the
output voltage by adjusting the PWM duty-cycle in response
to changes in line or load conditions. Since the output
voltage is equal to the time-averaged of the modulator
output, the relatively large LC time constant found in power
supply applications generally results in low bandwidth and
poor transient response. By directly monitoring changes in
inductor current via a series sense resistor the controller's
response time is not entirely limited by the output LC filter
and can react more quickly to changes in line and load
conditions. This feed-forward characteristic also simplifies
AC loop compensation since it adds a zero to the overall
loop response. Through proper selection of the current-
feedback to voltage-feedback ratio the overall loop response
will approach a one-pole system. The resulting system offers
several advantages over traditional voltage control systems,
including simpler loop compensation, pulse by pulse current
limiting, rapid response to line variation and good load step
response.
The heart of the controller is an input direct summing
comparator which sum voltage feedback, current feedback,
slope compensation ramp and power tracking signals
together. Slope compensation is required to prevent system
instability that occurs in current-mode topologies operating
at duty-cycles greater than 50% and is also used to define
the open-loop gain of the overall system. The slope
compensation is fixed internally and optimized for 500mA
inductor ripple current. The power tracking will not contribute
any input to the comparator steady-state operation. Current
feedback is measured by the patented sensing scheme that
senses the inductor current flowing through the high-side
switch whenever it is conducting. At the beginning of each
oscillator period the high-side NMOS switch is turned on.
The comparator inputs are gated off for a minimum period of
time of about 150ns (LEB) after the high-side switch is
turned on to allow the system to settle. The Leading Edge
Blanking (LEB) period prevents the detection of erroneous
voltages at the comparator inputs due to switching noise. If
the inductor current exceeds the maximum current limit
(ILMAX) a secondary over-current comparator will terminate
the high-side switch on time. If ILMAX has not been reached,
the feedback voltage FB derived from the regulator output
voltage VOUT is then compared to the internal feedback
reference voltage. The resultant error voltage is summed
with the current feedback and slope compensation ramp.
The 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-conduction 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 control 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 EL7551 integrates low on-resistance (60mΩ) NMOS
FETs to achieve high efficiency at 1A. 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 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
V
OUT
0.975V 1
R
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R
1
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EL7551
