RT8202AGQW Richtek USA Inc, RT8202AGQW Datasheet - Page 11

• Manufacturer Part Number: RT8202AGQW
• Description: IC PWM CTRLR SYNC BUCK 16WQFN
• Manufacturer: Richtek USA Inc

Specifications of RT8202AGQW

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Voltage - Supply: 4.5 V ~ 5.5 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 16-WFQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Duty Cycle: -

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-offs in DEM noise
vs. light-load efficiency are 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)
RT8202/A/B 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).
DS8202/A/B-04 March 2011
Figure 1. Boundary condition of CCM/DEM
I
0
L
Slope = (V
t
ON
IN
OUT
-V
OUT
/V
) / L
IN
. The benefit of forced-CCM
i
i
L, peak
Load
t
= i
L, peak
/ 2
Current sensing of the RT8202/A/B 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
RT8202/A/B 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
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
DS(ON)
ILIM
ILIM
PHASE
LGATE
= I
OC
is connected to a 20μA current source within the
sensing is more efficient and less expensive (Figure
LIMIT
(a)
Figure 3. Current-Sense Methods
I
0
ILIM
x R
L
R
Figure 2. Valley Current-Limit
ILIM
SENSE
resistor, positive current limit will activate.
ILIM
/ 20μA
resistor between the OC pin and
RT8202/A/B
PHASE
LGATE
OC
I
I
I
L, peak
Load
LIM
R
www.richtek.com
(b)
ILIM
t
DS(ON)
ILIM
resistor.
). For
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