LTC1553L_15 Datasheet, PDF (12/20 Pages) Linear Technology – 5-Bit Programmable Synchronous Switching Regulator Controller

LTC1553L
APPLICATIONS INFORMATION
Table 4. Recommended Minimum R_IMAX Resistor (kΩ) vs Maximum Operating Load Current and External MOSFET Q1
MAXIMUM OPERATING
LOAD CURRENT (A)
SUD50N03-10
SUD50N03-10
(TWO IN PARALLEL)
MTD20N03HDL
MTD20N03HDL
(TWO IN PARALLEL)
12
2.4
1.2
4.3
2.2
14
2.7
1.3
5.1
2.7
16
3.0
1.5
6.2
3.0
18
3.6
1.8
6.8
3.3
20
3.9
2.0
7.5
3.6
OUTEN and Thermistor Input
The LTC1553L includes a low power shutdown mode,
controlled 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
LTC1553L quiescent current will drop to about 130µA.
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. Elec-
trically, 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 LTC1553L disables both FET drivers. If
OUTEN is less than 1.2V, the LTC1553L will enter shut-
down mode. To activate any of these three modes, the
OUTEN voltage must drop below the respective threshold
for longer than 30µs.
V_CC
PENTIUM II
SYSTEM
R2
NTC THERMISTOR
MOUNT IN CLOSE
THERMAL PROXIMITY
TO Q1
5.6k
OT
G1
V_CC
R1
LTC1553L
OUTEN G2
V_IN
Q1
L_O
V_OUT
+
Q2
C_OUT
1553L F06
Figure 6. OUTEN Pin as a Thermistor Input
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 operating 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 LTC1553L will decrease. Since the loop gain is in-
versely proportional to the amplitude of the sawtooth, the
compensation network may need to be adjusted slightly.
Note that the temperature sensing 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
operation. This higher voltage can be supplied with a
separate supply, or it can be generated using a simple
charge pump as shown in Figure 7. The 85% typical
maximum duty cycle ensures sufficient off-time to refresh
the charge pump during each cycle.
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