MC33470 Datasheet, PDF (9/16 Pages) ON Semiconductor – SYNCHRONOUS RECTIFICATION DC/DC CONVERTER PROGRAMMABLE INTEGRATED CONTROLLER

MC33470
the compensation pin to ground. The error amplifier input is
tied to the sense pin which also has an internal 20 µA current
source to ground. The current source is intended to provide a
24 mV offset when an external 1.2 k resistor is placed
between the output voltage and the sense pin. The 24 mV
offset voltage is intended to allow a greater dynamic load
regulation range within a given specified tolerance for the
output voltage. The offset may be increased by increasing
the resistor value. The offset can be eliminated by connecting
the sense pin directly to the regulated output voltage.
The voltage reference consists of an internal, low
temperature coefficient, reference circuit with an added offset
voltage. The offset voltage level is the output of the
digital–to–analog converter. Control bits VID0 through VID4
control the amount of offset voltage which sets the value of
the voltage reference, as shown in Table 1. The VID0–4 input
bits each have internal 10 k pullup resistances. Therefore,
the reference voltage, and the output voltage, may be
programmed by connecting the VID pins to ground for logic
“0” or by an open for a logic “1”. Typically, a logic “1” will be
recognized by a voltage > 0.67 x VCC. A logic “0” is a voltage
< VCC/3.
MOSFET Switch Outputs
The output MOSFETs are designed to switch a maximum
of 18 V, with a peak drain current of 2.0 A. Both G1 and G2
output drives are designed to switch N–channel MOSFETs.
Output drive controls to G1 and G2 are phased to prevent
cross conduction of the internal IC output stages. Output
dead time is typically 100 nanoseconds between G1 and G2
in order to minimize cross conduction of the external
switching MOSFETs.
Current Limit and Soft–Start Controls
The soft–start circuit is used both for initial power
application and during current limit operation. A single
external capacitor and an internal 10 µA current source
control the rate of voltage increase at the error amplifier
output, establishing the circuit turn on time. The G1 output
will increase from zero duty cycle as the voltage across the
soft–start capacitor increases beyond about 0.5 V. When the
soft–start capacitor voltage has reached about 1.5 V, normal
duty cycle operation of G1 will be allowed.
An overcurrent condition is detected by the current limit
amplifier. The current limit amplifier is activated whenever the
G1 output is high. The current limit amplifier compares the
voltage drop across the external MOSFET driven by G1, as
measured at the IFB pin, with the voltage at the Imax pin.
Because the Imax pin draws 190 µA of input current, the
overcurrent threshold is programmed by an external resistor.
Referring to Figure 13, the current limit resistor value can be
determined from the following equation:
where:
+ R1
[( IL(max))(RDS(on))]
( Imax)
+ ⁾ IL(max)
IO Iripple
2
IO = Maximum load current
Iripple = Inductor peak to peak ripple current
OUTEN Input and OT Output Pins
On and off control of the MC33470 may be implemented
with the OUTEN pin. A logic “1” applied the OUTEN pin,
where a logic “1” is above 2.0 V, will allow normal operation of
the MC33470. The OUTEN pin also has multiple thresholds
to provide over temperature protection. An negative
temperature coefficient thermistor can be connected to the
OUTEN pin, as shown in Figure 15. Together with RS, a
voltage divider is formed. The divider voltage will decrease
as the thermistor temperature increases. Therefore, the
thermistor should be mounted to the hottest part on the circuit
board. When the OUTEN voltage drops below 2.0 V typically,
the MC33470 OT pin open collector output will switch from a
logic “1” to a logic “0”, providing a warning to the system. If
the OUTEN voltage drops below 1.7 V, both G1 and G2
output driver pins are latched to a logic “0” state.
Figure 15. OUTEN/OT Overtemperature Function
VCC
10 k
OT
VCC
RS
MC33470
OUTEN
NTC
Thermistor
APPLICATIONS INFORMATION
Design Example
Given the following requirements, design a switching
dc–to–dc converter:
VCC = 5.0 V
VCCP = 12 V
VID4–0 bits ^{= 10111 – Output Voltage = 2.8 V}
Output current ^{= 0.3 A to 14 A}
Efficiency > 80% at full load
Output ripple voltage ≈ 1% of output voltage
1. Choose power MOSFETs.
In order to meet the efficiency requirement, MOSFETs
should be chosen which have a low value of RDS(on).
However, the threshold voltage rating of the MOSFET must
also be greater than 1.5 V, to prevent turn on of the
synchronous rectifier MOSFETs due to dv/dt coupling
through the Miller capacitance of the MOSFET
drain–to–source junction. Figure 16 shows the gate voltage
transient due to this effect.
MOTOROLA ANALOG IC DEVICE DATA
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