LTC3772B Datasheet, PDF (8/20 Pages) Linear Technology – Micropower No RSENSE Constant Frequency Step-Down DC/DC Controller

LTC3772B
U
OPERATIO (Refer to the Functional Diagram)
Main Control Loop (Normal Operation)
The LTC3772B is a constant frequency current mode step-
down switching regulator controller. During normal op-
eration, the external P-channel MOSFET is turned on each
cycle when the oscillator sets the RS latch and turned off
when the current comparator resets the latch. The peak
inductor current at which the current comparator trips is
controlled by the voltage on the I_TH/RUN pin, which is the
output of the error amplifier. The negative input to the error
amplifier is the output feedback voltage V_FB, which is
generated by an external resistor divider connected be-
tween V_OUT and ground. When the load current increases,
it causes a slight decrease in V_FB relative to the 0.8V
reference, which in turn causes the I_TH/RUN voltage to
increase until the average inductor current matches the
new load current.
The main control loop is shut down by pulling the I_TH/RUN
pin to ground. Releasing the I_TH/RUN pin allows an
internal 1µA current source to charge up the external
compensation network. When the I_TH/RUN pin voltage
reaches approximately 0.6V, the main control loop is
enabled and the I_TH/RUN voltage is pulled up by a clamp
to its zero current level of approximately one diode
voltage drop (0.7V). As the external compensation net-
work continues to charge up, the corresponding peak
inductor current level follows, allowing normal operation.
The maximum peak inductor current attainable is set by a
clamp on the I_TH/RUN pin at 1.2V above the zero current
level (approximately 1.9V).
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
at some input-output differential, the external P-channel
MOSFET will remain on for more than one oscillator cycle
(start dropping off-cycles) since the inductor current has
not ramped up to the threshold set by the error amplifier.
Further reduction in input supply voltage will eventually
cause the external 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
sense resistor, the MOSFET and the inductor.
Undervoltage Lockout Protection
To prevent operation of the external P-channel MOSFET
with insufficient gate drive, an undervoltage lockout cir-
cuit is incorporated into the LTC3772B. When the input
supply voltage drops below approximately 2V, the
P-channel MOSFET and all internal circuitry other than the
undervoltage block itself are turned off. Input supply
current in undervoltage is approximately 1µA.
Short-Circuit Protection
If the output is shorted to ground, the frequency of the
oscillator is folded back from 550kHz to approximately
200kHz while maintaining the same minimum on time.
This lower frequency allows the inductor current to safely
discharge, thereby preventing current runaway. After the
short is removed, the oscillator frequency will gradually
increase back to 550kHz as V_FB rises through 0.3V on its
way back to 0.8V.
Overvoltage Protection
If V_FB exceeds its regulation point of 0.8V by more than
10% for any reason, such as an output short-circuit to a
higher voltage, the overvoltage comparator will hold the
external P-channel MOSFET off. This comparator has a
typical hysteresis of 40mV.
Peak Current Sense Voltage Selection and Slope
Compensation (I_PRG Pins)
When a controller is operating below 20% duty cycle, the
maximum sense voltage allowed across the external
P-channel MOSFET is 138mV, 70mV or 208mV for the
three respective states of the I_PRG pin.
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