LTC1874 Linear Technology, LTC1874 Datasheet - Page 6

• Manufacturer Part Number: LTC1874
• Description: Dual Constant Frequency Current Mode Step-Down DC/DC Controller
• Manufacturer: Linear Technology

OPERATIO
LTC1874
The LTC1874 is a dual, constant frequency current mode
switching regulator. The two switching regulators func-
tion identically but independent of each other. The follow-
ing description of operation is written for a single
switching regulator.
Main Control Loop
During normal operation, the external P-channel power
MOSFET is turned on by the oscillator and turned off when
the current comparator (I
peak inductor current at which I
controlled by the voltage on the I
output of the error amplifier EAMP. An external resistive
divider connected between V
EAMP to receive an output feedback voltage V
load current increases, it causes a slight decrease in V
relative to the 0.8V reference, which in turn causes the
I
current matches the new load current.
The main control loop is shut down by pulling the I
pin low. Releasing I
current source to charge up the external compensation
network. When the I
control loop is enabled with the I
pulled up to its zero current level of approximately 0.7V.
As the external compensation network continues to charge
up, the corresponding output current trip level follows,
allowing normal operation.
Comparator OVP guards against transient overshoots
greater than 7.5% by turning off the external P-channel
power MOSFET and keeping it off until the fault is
removed.
Burst Mode Operation
The controller enters Burst Mode operation at low load
currents. In this mode, the peak current of the inductor is
set as if V
the voltage at the I
inductor’s average current is greater than the load require-
ment, the voltage at the I
I
6
TH
TH
/RUN voltage to increase until the average inductor
/RUN voltage goes below 0.85V, the sleep signal goes
ITH
/RUN = 1V (at low duty cycles) even though
U
TH
TH
(Refer to Functional Diagram)
/RUN pin is at a lower value. If the
TH
/RUN pin reaches 0.35V, the main
/RUN allows an internal 0.5 A
TH
CMP
/RUN pin will drop. When the
OUT
) resets the RS latch. The
CMP
TH
and ground allows the
/RUN pin, which is the
TH
resets the RS latch is
/RUN voltage then
FB
. When the
TH
/RUN
FB
high, turning off the external MOSFET. The sleep signal
goes low when the I
and the controller resumes normal operation. The next
oscillator cycle will turn the external MOSFET on and the
switching cycle repeats.
Dropout Operation
When the input supply voltage decreases towards the
output voltage, the rate of change of inductor current
during the ON cycle decreases. This reduction means that
the external P-channel MOSFET will remain on for more
than one oscillator cycle since the inductor current has not
ramped up to the threshold set by EAMP. Further reduc-
tion in input supply voltage will eventually cause the
P-channel MOSFET to be turned on 100%, i.e., DC. The
output voltage will then be determined by the input voltage
minus the voltage drop across the MOSFET, the sense
resistor and the inductor.
Undervoltage Lockout
To prevent operation of the P-channel MOSFET below safe
input voltage levels, an undervoltage lockout is incorpo-
rated into the controller. When the input supply voltage
drops below approximately 2.0V, the P-channel MOSFET
and all circuitry is turned off except the undervoltage
block, which draws only several microamperes.
Short-Circuit Protection
When the output is shorted to ground, the frequency of the
oscillator will be reduced to about 120kHz. This lower
frequency allows the inductor current to safely discharge,
thereby preventing current runaway. The oscillator’s fre-
quency will gradually increase to its designed rate when
the feedback voltage again approaches 0.8V.
Overvoltage Protection
As a further protection, the overvoltage comparator in the
controller will turn the external MOSFET off when the
feedback voltage has risen 7.5% above the reference
voltage of 0.8V. This comparator has a typical hysteresis
of 20mV.
TH
/RUN voltage goes above 0.925V