UM3500 Datasheet, PDF (5/11 Pages) List of Unclassifed Manufacturers – 40 Watts DC-DC Converters

UM3500
Function description
UM3500 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 600mA 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. During normal operation, the internal main
switch is turned on for a certain time to ramp the inductor current at each rising edge of the
internal oscillator, and turned off when the peak inductor current reaches the controlled value.
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,
IRCMP, or the beginning of the next clock cycle.
Pulse Skipping Mode Operation
At very light loads, the UM3500 will automatically enter Pulse Skipping Mode to increase
efficiency, further extending battery life. In this mode, the control loop skips PWM pulses while
maintaining output in regulation, and the switching frequency depends on the load condition. This
is a kind of PFM mode operation.
Dropout Operation
When the input voltage decreases toward the value of the output voltage, the UM3500 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 UM3500 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 Jun.2012
5/11