RT7295D Datasheet, PDF (11/14 Pages) Richtek Technology Corporation – The RT7295D is a synchronous step-down converter

RT7295D
Feed-forward Capacitor (C_ff)
The RT7295D is optimized for ceramic output capacitors
and for low duty cycle applications. However for high-output
voltages, with high feedback attenuation, the circuit's
response becomes over-damped and transient response
can be slowed. In high-output voltage circuits (V_OUT > 3.3V)
transient response is improved by adding a small “feed-
forward” capacitor (C_ff) across the upper FB divider resistor
(Figure 1), to increase the circuit's Q and reduce damping
to speed up the transient response without affecting the
steady-state stability of the circuit. Choose a suitable
capacitor value that following below step.
 Get the BW the quickest method to do transient
response form no load to full load. Confirm the damping
frequency. The damping frequency is BW.
Enable Operation (EN)
For automatic start-up the EN pin can be connected to
V_IN through a 100kΩ resistor. Its large hysteresis band
makes EN useful for simple delay and timing circuits. EN
can be externally pulled to V_IN by adding a resistor-
capacitor delay (R_EN and C_EN in Figure 2). Calculate the
delay time using EN's internal threshold where switching
operation begins (1.4V, typical).
An external MOSFET can be added to implement digital
control of EN when no system voltage above 2V is available
(Figure 3). In this case, a 100kΩ pull-up resistor, R_EN, is
connected between VIN and the EN pin. MOSFET Q1 will
be under logic control to pull down the EN pin. To prevent
enabling circuit when VIN is smaller than the VOUT target
value or some other desired voltage level, a resistive voltage
divider can be placed between the input voltage and ground
and connected to EN to create an additional input under
voltage lockout threshold (Figure 4).
BW
V_OUT
FB
RT7295D
GND
R1 C_ff
R2
EN
R_EN
V_IN
C_EN
EN
RT7295D
GND
Figure 2. External Timing Control
R_EN
V_IN ^100k
Enable
EN
Q1 ^RT7295D
GND
Figure 1. C_ff Capacitor Setting
 C_ff can be calculated base on below equation.
C_ff

1
2 3.1412R1BW  0.8
Internal Soft-Start (SS)
The RT7295D soft-start uses an internal soft-start time
800μs.
Figure 3. Digital Enable Control Circuit
V_IN
R_EN1
R_EN2
EN
RT7295D
GND
Figure 4. Resistor Divider for Lockout Threshold Setting
Copyright ©2015 Richtek Technology Corporation. All rights reserved.
DS7295D-05 July 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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