UM3510 Datasheet, PDF (5/10 Pages) Union Semiconductor, Inc. – 1.0A, 1.5MHz, Synchronous Step-Down DC-DC Converter

UM3510
Function description
UM3510 is a monolithic switching mode Step-Down DC-DC converter. It utilizes internal
MOSFETs to achieve high efficiency and can generate very low output voltage by using internal
reference at 0.6V. It operates at a fixed switching frequency, and uses the slope compensated
current mode architecture. This Step-Down DC-DC Converter supplies 1.0A output current at V_IN
=3.6V with input voltage range from 2.5V to 5.5V.
Current Mode PWM Control
Slope compensated current mode PWM control provides stable switching and cycle-by-cycle
current limit for excellent load and line responses and protection of the internal main switch (P-Ch
MOSFET) and synchronous rectifier (N-CH MOSFET). During normal operation, the internal
P-Ch MOSFET is turned on for a certain time to ramp the inductor current at each rising edge of
the internal oscillator, and switched off when the peak inductor current is above the error voltage.
The current comparator, I_COMP, limits the peak inductor current. When the main switch is off, the
synchronous rectifier will be turned on immediately and stay on until either the inductor current
starts to reverse, as indicated by the current reversal comparator, I_ZERO, or the beginning of the
next clock cycle. The OVDET comparator controls output transient overshoots by turning the
main switch off and keeping it off until the fault is no longer present.
Pulse Skipping Mode Operation
At very light loads, the UM3510 automatically enters Pulse Skipping Mode. In the Pulse Skipping
Mode, the inductor current may reach zero or reverse on each pulse. The PWM control loop will
automatically skip pulses to maintain output regulation. The bottom MOSFET is turned off by the
current reversal comparator, I_ZERO, and the switch voltage will ring. This is discontinuous mode
operation, and is normal behavior for the switching regulator.
Dropout Operation
When the input voltage decreases toward the value of the output voltage, the UM3510 allows the
main switch to remain on for more than one switching cycle and increases the duty cycle (Note 1)
until it reaches 100%. The output voltage then is the input voltage minus the voltage drop across
the main switch and the inductor. At low input supply voltage, the R_DS(ON) of the P-Channel
MOSFET increases, and the efficiency of the converter decreases. Caution must be exercised to
ensure the heat dissipated not to exceed the maximum junction temperature of the IC.
Note 1: The duty cycle D of a step-down converter is defined as:
D=T_ON×f_OSC×100%≌V_OUT/V_IN×100%
Where T_ON is the main switch on time and f_OSC is the oscillator frequency (1.5Mhz).
Maximum Load Current
The UM3510 will operate with input supply voltage as low as 2.5V, however, the maximum load
current decreases at lower input due to large IR drop on the main switch and synchronous rectifier.
The slope compensation signal reduces the peak inductor current as a function of the duty cycle to
prevent sub-harmonic oscillations at duty cycles greater than 50%. Conversely the current limit
increases as the duty cycle decreases.
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http://www.union-ic.com Rev.02 Dec.2011
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