Main Control Loop
The JTMA7176 has dual independent constant frequency,current-mode PWM step-down converters. During nor-
mal operation, the internal P-channel power MOSFET is turned on each cycle when the oscillator sets an internal
RS latch and is turned off when an internal comparator (ICMP) resets the latch. The peak inductor current at which
ICMP resets the RS latch is controlled by the voltage on the COMP, which is the output of the error amplifier
(EAMP). An external resistive divider connected between VOUT and ground allows the EAMP to receive an output
feedback voltage VFB at FB pin. When the load current increases, it causes a slightly decrease in VFB associated
with the 0.6V reference, which in turn causes the COMP voltage to increase until the average inductor current
matches the new load current.
 
Soft-Start
Each channel in the JTMA7176 has a built-in soft-start to control the output voltage rise during start-up. During soft-
start, an internal ramp, connected to the one of the posi- tive inputs of the error amplifier, raise up to replace the
reference voltage (0.6V typical) until the ramp voltage reaches the reference voltage.
 
Short Circuit Protection
For each channel, when the output is shortened to the ground, the frequency of the oscillator will be reduced to
210kHz. This lower frequency allows the inductor current to safely discharge, thereby preventing current runaway.
The oscillator’s frequency will gradually increase to its designed rate when the feedback voltage on the FB again
approaches 0.6V.
 
Over-Temperature Protection (OTP)
 
For each channel, the over-temperature circuit limits the junction temperature of the JTMA7176. When the junction
temperature exceeds 150oC, a thermal sensor turns off the power MOSFETs, allowing the channels to cool down.
The thermal sensor allows the converter to start a soft- start process and to regulate the output voltage again
after the junction temperature cools by 40ïC. The OTP is designed with a 40ïC hysteresis to lower the average
Junction Temperature (TJ) during continuous thermal overload conditions, increasing the lifetime of the device.
 
Enable/Shutdown
For each channel, driving EN to the ground places the channel in shutdown mode. When in shutdown, the inter-
nal power MOSFETs are turned off, all internal circuitry shuts down, and the quiescent supply current reduces to
1µA maximally.
 
Automoatic PFM/PWM mode Switch
The JTMA7176 is a fixed frequency PWM peak current modulation control step-down converter. At light loads,
the JTMA7176 will automatically enter in pulse frequency modulation operation to reduce the dominant switching
losses. In PFM operation, the inductor current may reach zero or reverse on each pulse. A zero current comparator
turns off the N-channel synchronous MOSFET, forcing DCM (Discontinuous Current Mode) operation at light
load. These controls get very low quiescent, help to main- tain high efficiency over the complete load range.