RT7277
11
DS7277-01 March 2013 www.richtek.com
©
Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
OUT OUT
L
IN
VV
I = 1
fL V
⎡⎤⎡ ⎤
Δ×−
⎢⎥⎢ ⎥
×
⎣⎦⎣ ⎦
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. High frequency with small ripple current can achieve
highest efficiency operation. However, it requires a large
OUT OUT
L(MAX) IN(MAX)
VV
L = 1
fI V
⎡⎤⎡ ⎤
×−
⎢⎥⎢ ⎥
×Δ
⎣⎦⎣ ⎦
Input and Output Capacitors Selection
The input capacitance, C
IN
, is needed to filter the
trapezoidal current at the source of the high side MOSFET.
A low ESR input capacitor with larger ripple current rating
should be used for the maximum RMS current. The RMS
current is given by :
OUT
IN
RMS OUT(MAX)
IN OUT
V
V
I = I 1
VV
−
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/ 2. This simple worst-case condition is
commonly used for design because even significant
deviations do not offer much relief.
Choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to
meet size or height requirements in the design. For the
input capacitor, two 10μF and 0.1μF low ESR ceramic
capacitors are recommended.
The selection of C
OUT
is determined by the required ESR
to minimize voltage ripple.
Moreover, the amount of bulk capacitance is also a key
for C
OUT
selection to ensure that the control loop is stable.
The output ripple, ΔV
OUT
, is determined by :
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 4.
Under Voltage Lockout Protection
The RT7277 has Under Voltage Lockout Protection (UVLO)
that monitors the voltage of PVCC pin. When the V
PVCC
voltage is lower than UVLO threshold voltage, the RT7277
will be turned off in this state. This is non-latch protection.
Over Temperature Protection
The RT7277 equips an Over Temperature Protection (OTP)
circuitry to prevent overheating due to excessive power
dissipation. The OTP will shut down switching operation
when junction temperature exceeds 150°C. Once the
junction temperature cools down by approximately 20°C
the main converter will resume operation. To keep operating
at maximum, the junction temperature should be prevented
from rising above 150°C.
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and an output
voltage. The ripple current ΔI
L
increases with higher V
IN
and decreases with higher inductance.
Figure 4. Output Voltage Setting
The output voltage is set by an external resistive divider
according to the following equation. It is recommended to
use 1% tolerance or better divider resistors.
)
OUT
R1
V = 0.765(1
R2
×+
GND
FB
R1
R2
V
OUT
RT7277
inductor to achieve this goal. For the ripple current
selection, the value of ΔI
L
= 0.2(I
MAX
) will be a reasonable
starting point. The largest ripple current occurs at the
highest V
IN
. To guarantee that the ripple current stays
below the specified maximum, the inductor value should
be chosen according to the following equation :
OUT L
OUT
1
VIESR
8fC
⎡⎤
Δ≤Δ +
⎢⎥
⎣⎦
The output ripple will be highest at the maximum input
voltage since ΔI
L
increases with input voltage. Multiple
capacitors placed in parallel may need to meet the ESR
and RMS current handling requirements.
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at input and
