RT2657BQ
Application Information
Low Supply Operation
The basic RT2657BQ application circuit is shown in Typical
Application Circuit. External component selection is
determined by the maximum load current and begins with
the selection of the inductor value and operating frequency
The RT2657BQ is designed to operate down to an input
supply voltage of 2.7V. One important consideration at
low input supply voltages is that the RDS(ON) of the P-
Channel andN-Channel power switches increases. Users
should calculate the power dissipation when the
RT2657BQ is used at 100% duty cycle with low input
voltages to ensure that thermal limits are not exceeded.
followed by CIN and COUT
.
Output Voltage Setting
The output voltage is set by an external resistive divider
according to the following equation :
R1
Under-Voltage Protection (UVP)
VOUT VREF x (1
)
R2
The output voltage is continuously monitored for under-
voltage protection. When the output voltage is less than
33% of its set voltage threshold after OCP occurs, the
under-voltage protection circuit will be triggered to auto
re-softstart.
where VREF equals to 0.6V typically. The resistive divider
allows the FB pin to sense a fraction of the output voltage
as shown in Figure 1.
V
OUT
R1
Input Over-Voltage protection (VIN OVP)
FB
RT2657BQ
When the input voltage (VIN) is higher than 6V, VIN OVP
will be triggered and the IC stops switching. Once the
input voltage drops below 6V, the IC will return to normal
operation.
R2
GND
Figure 1. Setting the Output Voltage
Output Over-Voltage Protection (VOUT OVP)
Soft-Start
When the output voltage exceeds more than 5% of the
nominal reference voltage, the feedback loop forces the
internal switches off within 50μs. Therefore, the output
over-voltage protection is automatically triggered by the
loop.
The RT2657BQ contains an internal soft-start clamp that
gradually raises the clamp on the FB pin. Time from active
ENto reach 95% of VOUT nominal is within typical 300μs.
100% Duty Cycle Operation
When the input supply voltage decreases toward the output
voltage, the duty cycle increases toward the maximum
on-time. Further reduction of the supply voltage forces
the main switch to remain on for more than one cycle,
eventually reaching 100% duty cycle.
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.
The output voltage will then be determined by the input
voltage minus the voltage drop across the internal
P-MOSFET and the inductor.
Table 1. Inductors
Component
Supplier
Series
Inductance (H)
2.2H
DCR (m)
Current Rating (mA)
Dimensions (mm)
NR5018
T2R2M
TAIYO YUDEN
40
3000
4 X 4 X 1.8
Copyright 2014 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
10
DS2657BQ-00 July 2014
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
