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RC5053
PRODUCT SPECIFICATION
10
P
Figure 5. Current Limit Setting
OUTEN and Thermistor Input
The RC5053 includes a low power shutdown mode, con-
trolled by the logic at the OUTEN pin. A high at OUTEN
allows the part to operate normally. A low level at OUTEN
stops all internal switching, pulls COMP and SS to ground
internally and turns Q1 and Q2 off. OT and PWRGD are
pulled low, and FAULT is left floating. In shutdown, the
RC5053 quiescent current will drop to about 760
μ
A.
The remaining current is used to keep the thermistor sensing
circuit at OUTEN alive. Note that the leakage current of the
external MOSFETs may add to the total shutdown current
consumed by the circuit, especially at elevated temperature.
OUTEN is designed with multiple thresholds to allow it to
also be utilized for over-temperature protection. The power
MOSFET operating temperature can be monitored with an
external negative temperature coefficient (NTC) thermistor
mounted next to the external MOSFET which is expected to
run the hottest ––often the high-side device, Q1. Electrically,
the thermistor should form a voltage divider with another
resistor, R1, connected to V
CC
. Their midpoint should be
connected to OUTEN (see Figure 6). As the temperature
increases, the OUTEN pin voltage is reduced. Under normal
operating conditions, the OUTEN pin should stay above 2V.
All circuits will function normally, and the OT pin will
remain in a high state. If the temperature gets abnormally
high, the OUTEN pin voltage will eventually drop below 2V.
OT will switch to a logic low, providing an over-temperature
warning to the system. As OUTEN drops below 1.7V, the
RC5053 disables both FET drivers. If OUTEN is less than
1.2V, the RC5053 will enter shutdown mode. To activate any
of these three modes, the OUTEN voltage must drop below
the respective threshold for longer than 30
μ
s.
Figure 6. OUTEN Pin as a Thermistor Input
Q1
180
μ
A
G1
Q2
C
IN
L
O
V
OUT
C
OUT
R
IMAX
V
IN
–
+
CC
G2
56
RC5053
I
MAX
7
I
FB
8
+
+
Q1
Q2
L
O
V
OUT
C
OUT
5.6k
V
IN
V
CC
V
CC
R1
R2
NTC THERMISTOR
MOUNT IN CLOSE
THERMAL PROXIMITY
TO Q1
RC5053
PENTIUM II
SYSTEM
G1
G2
OT
OUTEN
+
4.7
4.7
Clock Synchronization
The internal oscillator can be synchronized to an external
clock by applying the external clocking signal to the OUTEN
pin. The synchronizing range extends from the initial operat-
ing frequency up to 500kHz. If the external frequency is
much higher than the natural free-running frequency, the
peak-to-peak sawtooth amplitude within the RC5053 will
decrease. Since the loop gain is inversely proportional to the
amplitude of the sawtooth, the compensation network may
need to be adjusted slightly. Note that the temperature sens-
ing circuitry does not operate when external synchronization
is used.
MOSFET Gate Drive
Power for the internal MOSFET drivers is supplied by
PV
CC
. This supply must be above the input supply voltage
by at least one power MOSFET V
GS(ON)
for efficient opera-
tion. This higher voltage can be supplied with a separate sup-
ply, or it can be generated using a simple charge pump as
shown in Figure 7. The 82% typical maximum duty cycle
ensures sufficient off-time to refresh the charge pump during
each cycle.
Figure 7. Doubling Charge Pump
Upon power-down, G1 and G2 will both be held low to
prevent output voltage under shoot. On power-up or wake-up
from thermal shutdown, the driver is designed such G2 will
be held low until after G1 first goes high.
Power MOSFETs
Two N-channel power MOSFETs are required for most
RC5053 circuits. They should be selected based primarily on
gate threshold and on-resistance considerations. The required
MOSFET threshold should be determined based on the avail-
able power supply voltages and/or the complexity of the gate
driver charge pump scheme. In 5V input designs where a
12V supply is used to power PV
CC
, standard MOSFETs with
R
DS(ON)
specified at V
GS
= 5V or 6V can be used with good
results. However, logic level devices will improve efficiency.
The current drawn from the 12V supply varies with the
MOSFETs used and the RC5053 operating frequency, but is
generally less than 50mA.
Q1
G1
20
Q2
0.1
μ
F
L
O
V
OUT
C
OUT
C
IN
V
= 5V
IN
PV
CC
1N5817
G2
1
RC5053
2
+
+
4.7
4.7