RT8204BGQW Richtek USA Inc, RT8204BGQW Datasheet - Page 13

• Manufacturer Part Number: RT8204BGQW
• Description: IC SYNC BUCK W/LDO CTRLR 16WQFN
• Manufacturer: Richtek USA Inc

Specifications of RT8204BGQW

• Topology: Step-Down (Buck) Synchronous (1), Linear (LDO) (1)
• Function: Any Function
• Number Of Outputs: 2
• Voltage/current - Output 1: Controller
• Voltage/current - Output 2: Controller
• W/led Driver: No
• W/supervisor: No
• W/sequencer: No
• Voltage - Supply: 4.5 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-WFQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The switching waveforms may appear noisy and
asynchronous when light loading causes diode-emulation
operation, but this is a normal operating condition that
results in high light-load efficiency. Trade-off in DEM noise
vs. light-load efficiency is made by varying the inductor
value. Generally, low inductor values produce a broader
efficiency vs. load curve, while higher values result in higher
full-load efficiency (assuming that the coil resistance
remains fixed) and less output voltage ripple. The
disadvantages for using higher inductor values include
larger physical size and degrades load-transient response
(especially at low input-voltage levels).
Forced-CCM Mode (EN/DEM = floating)
The low noise, forced-CCM mode (EN/DEM = floating)
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 light loads as the PWM loop to
maintain a duty ratio V
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 to 40mA, depending on the external
MOSFETs.
Current Limit Setting (OCP)
The RT8204B has cycle-by-cycle current limiting control.
The current limit circuit employs a unique “valley” current
sensing algorithm. If the magnitude of the current sense
signal at OC is above the current limit threshold, the PWM
is not allowed to initiate a new cycle (Figure 2).
DS8204B-03 March 2011
I
0
L
Figure 2. Valley Current-Limit
OUT
/V
IN
. The benefit of forced-CCM
I
I
I
L, peak
Load
LIM
t
Current sensing of the RT8204B can be accomplished in
two ways. Users can either use a current sense resistor
or the on-state of the low side MOSFET (R
resistor sensing, a sense resistor is placed between the
source of low side MOSFET and PGND (Figure 3(a)).
R
3(b)). There is a compromise between current limit
accuracy and sense resistor power dissipation.
In both cases, the R
PHASE pin sets the over current threshold. This resistor
R
RT8204B which is turned on when the low side MOSFET
turns on. When the voltage drop across the sense resistor
or low side MOSFET equals the voltage across the R
resistor, positive current limit will be activated. The high
side MOSFET will not be turned on until the voltage drop
across the sense element (resistor or MOSFET) falls
below the voltage across the R
Choose a current limit resistor by following equation :
R
Carefully observe the PC board layout guidelines to ensure
that noise and DC errors do not corrupt the current sense
signal seen by OC and PGND. Mount the IC close to the
low side MOSFET and sense resistor with short, direct
traces, making a Kelvin sense connection to the sense
resistor.
DS(ON)
ILIM
ILIM
= I
is connected to a 20μA current source within the
sensing is more efficient and less expensive (Figure
LIMIT
Figure 3. Current Sense Methods
x R
SENSE
PHASE
LGATE
PHASE
LGATE
ILIM
OC
/ 20μA
OC
resistor between the OC pin and
(a)
(b)
R
ILIM
R
ILIM
ILIM
resistor.
RT8204B
www.richtek.com
DS(ON)
). For
ILIM
13