DS6550A Datasheet, PDF (10/16 Pages) Richtek Technology Corporation – Synchronous DC-DC Step-Down Controller with 5V LDO

RT6550A
Application Information
The RT6550A is a low quiescent, Mach Response^TM DRV^TM
mode synchronous Buck controller targeted for Ultrabook
system power supply solutions. Richtek's Mach
Response^TM technology provides fast response to load
steps. The topology solves the poor load transient
response timing problems of fixed frequency current mode
PWMs, and avoids the problems caused by widely varying
switching frequencies in CCR (constant current ripple)
constant on-time PWM schemes. A special adaptive on-
time control trades off the performance and efficiency over
wide input voltage range. The RT6550A includes 5V (LDO5)
linear regulators. The LDO5 linear regulator steps down
the battery voltage to supply internal circuitry and gate
drivers. The synchronous switch gate drivers are directly
powered by LDO5.
PWM Operation
The Mach Response^TM DRV^TM mode controller relies on
the output filter capacitor's Effective Series Resistance
(ESR) to act as a current sense resistor, so that the output
ripple voltage provides the PWM ramp signal. Referring to
the RT6550A's Function Block Diagram, the synchronous
high-side MOSFET is turned on at the beginning of each
cycle. After the internal one-shot timer expires, the
MOSFET will be turned off. The pulse width of this one-
shot is determined by the converter's input output voltages
to keep the frequency fairly constant over the entire input
voltage range. Another one-shot sets a minimum off-time
(200ns typ.). The on-time one-shot will be triggered if the
error comparator is high, the low-side switch current is
below the current limit threshold, and the minimum off-
time one-shot has timed out.
PWM Frequency and On-time Control
For each specific input voltage range, the Mach
Response^TM control architecture runs with pseudo constant
frequency by feed forwarding the input and output voltage
into the on-time one-shot timer. The high-side switch
on-time is inversely proportional to the input voltage as
measured by V_IN and proportional to the output voltage.
The inductor ripple current operating point remains
relatively constant, resulting in easy design methodology
and predictable output voltage ripple.
The RT6550A adaptively changes the operation frequency
according to the input voltage. Higher input voltage usually
comes from an external adapter, so the RT6550A operates
with higher frequency to have better performance. Lower
input voltage usually comes from a battery, so the
RT6550A operates with lower switching frequency for
lower switching losses. For a specific input voltage range,
the switching cycle period is given by :
For 5V VOUT,
Period (usec.) =
V_IN  2.025
V_IN  3.79
where the VIN is in volt.
The on-time guaranteed in the Electrical Characteristics
table is influenced by switching delays in the external
high-side power MOSFET.
Forced-CCM Mode
The low noise, forced-CCM mode disables the zero-
crossing comparator, which controls the low side switch
on-time. This causes the low side gate-drive waveform to
become the complement of the high side gate-drive
waveform. This in turn causes the inductor current to
reverse at ligt loads as the PWM loop to maintain a duty
ratio V_OUT/V_IN. The benefit of forced-CCM mode is to keep
the switching frequency fairly constant, but it comes at a
cost : The no-load battery current can be up to 10mA,
depending on the external MOSFETs.
Linear Regulators (LDO5)
The RT6550A includes 5V (LDO5) linear regulators. The
regulator can supply up to 100mA for external loads.
Bypass LDO with 1μF (min) to 4.7μF (max), and the
recommended value is 1μF ceramic capacitor.
Current Limit Setting
The RT6550A has cycle-by-cycle current limit control and
the OCP function only operation at CCM , it is disabled at
FCCM in order to reduce quiescent current. The current
limit circuit employs an unique “valley” current sensing
algorithm. If the magnitude of the current sense signal at
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DS6550A-00 September 2016