RT8253A RICHTEK [Richtek Technology Corporation], RT8253A Datasheet - Page 12

• Manufacturer Part Number: RT8253A
• Description: 3A, 23V, 340kHz Synchronous Step-Down Converter
• Manufacturer: RICHTEK [Richtek Technology Corporation]

RT8253A
Thermal Shutdown
The device implements an internal thermal shutdown
function to protect itself when the junction temperature
exceeds 150°C. The thermal shutdown forces the device
to stop switching when the junction temperature exceeds
the thermal shutdown threshold. Once the die temperature
decreases below 150°C, the device reinstates the power
up sequence.
Loop Compensation
The RT8253A is a current mode converter and requires
external compensation to have an accurate output voltage
regulation with fast transient response. The main concern
of compensation deals with the position of the capacitor
ESR zero and mid frequency to high frequency gain boost.
The RT8253A uses a high gain Operational
Transconductance Amplifier (OTA) as the error amplifier.
As Figure 4 shows, the OTA works as the voltage
controlled current source. The characteristic of OTA is
shown as below :
Figure 5 shows a typical buck control loop using Type-II
compensator. The control loop consists of the power stage,
current comparator and a compensation network. The
current comparator compares V
current sense and slope comp to provide Pulse Width
Modulated (PWM) gate driving signal with the oscillator.
The PWM wave is smoothed out by the output filter, L
and C
the inverting input of the error amplifier.
www.richtek.com
12
GM
and V
Figure 4. Operational Transconductance Amplifier, OTA
=
Δ
V+
OUT
V-
Δ
Δ
COMP
I
OUT
V
. The output voltage (V
M
,where V
= Δ
+
-
I
OUT
GM
Δ
x Z
M
=
OUT
(
V
+ −
I
OUT
) (
OUT
COMP
V
) is sensed and fed to
−
Z
)
OUT
with the sum of
V
COMP
The modulator transfer function is the small signal transfer
function of V
amplifier output). According to the derivation of the Ridley’s
thesis, the transfer function is dominated by a DC gain, a
double pole, a low frequency pole, and an ESR zero as
shown in Figure 6.
where R
transconductance; m
duty cycle; T
where
And
F (s)
ω
F (s)
V
Current Sense
V
p
h
p
COMP
Slope Comp
Figure 5. Typical Current Mode Buck Converter with
OUT
=
C
x m x (1 D) 0.5
=
=
OUT
1
[
1 sC
LOAD
≅
+
+
R
c
ω
x
x F (s) x F (s)
Oscillator
LOAD
x R
n p
1
S
1
S
is the load resistor; g
Q
1
OUT
p
OUT
1
+
is the switching period. And :
+
Comparator
LOAD
GM
+
+
-
R
−
ω
Current
+
/V
s
LOAD
Type-II Compensator
x g
-
+
p
ω
x ESR
S
COMP
n
2
2
−
CS
h
C
L
+
is the slope comp value; D is the
L x C
x T
V
(output voltage over the error
]
REF
Control
S
PWM
T
S
COMP
OUT
x m x (1 D) 0.5
1
[
DS8253A-02 March 2011
FB
C
CS
is the current sense
C
V
R
C
IN
C
−
R2
C
R1
ESR
L
OUT
−
C
]
P
V
OUT
V
COMP