RT8238A
11
RT8238A-07 January 2014
www.richtek.com
©
Copyright 2014 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Figure 1. Boundary Condition of CCM/DEM
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 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 degrade load transient response
(especially at low input voltage levels).
Audio-Skipping Mode
When the MODE pin is pulled to
2.5V, the controller
operates in audio skipping mode with a minimum switching
frequency of 25kHz. This mode eliminates audio-frequency
modulation that would otherwise be present when a lightly
loaded controller automatically skips pulses. In audio
skipping mode, the low side switch gate driver signal is
ORed with an internal oscillator (>25kHz). O
nce the
internal oscillator is triggered, the audio skipping controller
pulls LGATE logic high, turning on the low side MOSFET
to induce a negative inductor current. After the output
voltage rises above V
REF
, the controller turns off the low
side MOSFET (LGATE pulled logic low) and triggers a
constant on-time operation (UGATE driven logic high).
When the on-time operation expires, the controller re-
enables the low side MOSFET until the inductor current
drops below the zero-crossing threshold.
Forced-CCM Mode
The low noise, forced-CCM mode (MODE = GND) 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
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 to 40mA, depending on the external
MOSFETs.
Current Limit Setting (OCP)
The RT8238A has cycle-by-cycle current limiting control.
The current limit circuit employs a unique
“
valley
”
current
sensing algorithm. If PHASE voltage plus the current-limit
threshold is below zero, the PWM is not allowed to initiate
a new cycle (Figure 2). In order to provide both good
accuracy and a cost effective solution, the RT8238A
supports temperature compensated MOSFET R
DS(ON)
sensing. The CS pin should be connected to GND through
the trip voltage setting resistor, R
CS
. With the 10
μ
A CS
terminal source current, I
CS
, and the setting resistor, R
CS
the CS trip voltage, V
CS
, can be calculated as shown in
the following equation.
V
CS
(mV) = R
CS
(k
Ω
) x 10 (
μ
A) x (1 / 10)
I
L
t
0
t
ON
Slope = (V
IN
-V
OUT
) / L
I
PEAK
I
LOAD
= I
PEAK
/ 2
Inductor current is monitored by the voltage between the
PGND pin and the PHASE pin, so the PHASE pin should
be connected to the drain terminal of the low side
MOSFET. I
CS
has positive temperature coefficient to
compensate the temperature dependency of the R
DS(ON)
.
PGND is used as the positive current sensing node so
PGND should be connected to the source terminal of the
bottom MOSFET.
As the comparison is done during the OFF state, V
CS
sets the valley level of the inductor current. Thus, the
load current at over current threshold, I
LOAD_OC
, can be
calculated as follows.
(
)
CS
Ripple
LOAD_OC
DS(ON)
IN
OUT
OUT
CS
DS(ON)
IN
V
I
I
=
+
R
2
V V
V
V
1
=
+
R
2 L f
V
−
×
×
× ×
