RT2652
Application Information
The RT2652 is a single-phase buck PWM converter. It
provides single feedback loop, current mode control with
fast transient response. An internal 0.6V reference allows
the output voltage to be precisely regulated for low output
voltage applications.Afixed switching frequency (1.2MHz)
oscillator and internal compensation are integrated to
minimize external component count.
⎡
⎤ ⎡
× 1−
⎤
V
f × ΔI
V
OUT
V
IN(MAX)
OUT
L =
⎢
⎥ ⎢
⎥
L(MAX)
⎣
⎦ ⎣
⎦
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant
frequency architectures by preventing sub-harmonic
oscillations at duty cycles greater than 50%. It is
accomplished internally by adding a compensating ramp
to the inductor current signal. Normally, the maximum
inductor peak current is reduced when slope compensation
is added. In this IC, however, separated inductor current
signal is used to monitor over current condition and this
keeps the maximum output current relatively constant
regardless of duty cycle.
Output Voltage Setting
The output voltage is set by an external resistive voltage
divider according to the following equation :
R1
R2
⎛
⎝
⎞
⎟
⎠
VOUT = VREF 1+
⎜
Where VREF is equals 0.6V (typ.).
The resistive voltage divider allows the FB pin to sense a
fraction of the output voltage as shown in Figure 1.
Low Dropout Operation
The RT2652 is designed to operate down to an input supply
voltage of 2.7V. One important consideration at low input
supply voltage is that the RDS(ON) of the P-Channel andN-
Channel power switches increases. The user should
calculate the power dissipation when the RT2652 is used
at 100% duty cycle with low input voltages to ensure that
thermal limits are not exceeded. Slope compensation and
inductor peak current slope compensation provides
stability in constant frequency architectures by preventing
sub-harmonic oscillations at duty cycles greater than
50%.It is accomplished internally by adding a
compensating ramp to the inductor current signal.
Normally, the maximum inductor peak current is reduced
when slope compensation is added. In the RT2652,
however, separated inductor current signals are used to
monitor over current condition. This keeps the maximum
output current relatively constant regardless of duty cycle.
V
OUT
R1
FB
RT2652
GND
R2
Figure 1. Setting the Output Voltage
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ΔIL increases with higher VIN and decreases
with higher inductance.
V
f ×L
VOUT
V
IN
⎡
OUT ⎤ ⎡
× 1−
⎥ ⎢
⎤
ΔIL =
⎢
⎣
⎥
⎦
⎦ ⎣
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. Highest efficiency operation is achieved by reducing
ripple current at low frequency, but a large inductor is
required to attain this goal. For ripple current selection,
the value of ΔIL = 0.4(IMAX) is a reasonable starting point.
The largest ripple current occurs at the highest VIN. To
guarantee that the ripple current stays below a specified
maximum value, the inductor should be chosen according
to the following equation :
Short Circuit Protection
When the output is shorted to ground, the inductor current
decays very slowly during a single switching cycle. A
current runaway detector is used to monitor inductor
current.As current increases beyond the control of current
loop, switching cycles will be skipped to prevent current
runaway from occurring.
Copyright 2012 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
10
DS2652-00 November 2012
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
