RC5051M Fairchild Semiconductor, RC5051M Datasheet - Page 11

• Manufacturer Part Number: RC5051M
• Description: DC/DC Switching Controllers DC-DC Converter Prog Sync
• Manufacturer: Fairchild Semiconductor

Specifications of RC5051M

• Number Of Outputs: 1
• Output Voltage: 1.3 V to 3.5 V
• Output Current: 15 A
• Mounting Style: SMD/SMT
• Package / Case: SOIC-20 Wide
• Maximum Operating Temperature: + 70 C
• Minimum Operating Temperature: 0 C
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

PRODUCT SPECIFICATION
developed across the sense resistor exceeds the 120mV com-
parator threshold voltage, the RC5051 reduces the output
duty cycle to help protect the power devices. The DC-DC
converter returns to normal operation after the fault has been
removed.
Oscillator
The RC5051 oscillator section uses a fixed current capacitor
charging configuration. An external capacitor (CEXT) is
used to set the oscillator frequency between 80KHz and
1MHz. This scheme allows maximum flexibility in choosing
external components.
In general, a higher operating frequency decreases the peak
ripple current flowing in the output inductor, thus allowing
the use of a smaller inductor value. In addition, operation at
higher frequencies decreases the amount of energy storage
that must be provided by the bulk output capacitors during
load transients due to faster loop response of the controller.
Unfortunately, the efficiency losses due to switching of the
MOSFETs increase as the operating frequency is increased.
Thus, efficiency is optimized at lower frequencies. An oper-
ating frequency of 300KHz is a typical choice which opti-
mizes efficiency and minimizes component size while
maintaining excellent regulation and transient performance
under all operating conditions.
Design Considerations and Component
Selection
Additional information on design and component selection
may be found in Fairchild Semiconductor’s Application
Note 53.
MOSFET Selection
This application requires N-channel Logic Level Enhance-
ment Mode Field Effect Transistors. Desired characteristics
are as follows:
• Low Static Drain-Source On-Resistance,
• Low gate drive voltage, V
• Power package with low Thermal Resistance
• Drain-Source voltage rating > 15V.
The on-resistance (R
MOSFET selection. The on-resistance determines the power
dissipation within the MOSFET and therefore significantly
affects the efficiency of the DC-DC Converter. For details
and a spreadsheet on MOSFET selection, refer to Applica-
tions Bulletin AB-8
REV. 1.0.4 4/2/01
R
DS,ON
< 20m (lower is better)
DS,ON
) is the primary parameter for
GS
= 4.5V rated
MOSFET Gate Bias
The high side MOSFET gate driver can be biased by one of
two methods–Charge Pump or 12V Gate Bias. The charge
pump method has the advantage of requiring only +5V as an
input voltage to the converter, but the 12V method will real-
ize increased efficiency by providing an increased V
high side MOSFETs.
Method 1. Charge Pump (Bootstrap)
Figure 3 shows the use of a charge pump to provide gate bias
to the high side MOSFET when +12V is unavailable. Capac-
itor CP is the charge pump used to boost the voltage of the
RC5051 output driver. When the MOSFET Q1 switches off,
the source of the MOSFET is at approximately 0V because
of the MOSFET Q2. (The Schottky D2 conducts for only a
very short time, and is not relevent to this discussion.) CP is
charged through the Schottky diode D1 to approximately
4.5V. When the MOSFET Q1 turns on, the voltage at the
source of the MOSFET is equal to 5V. The capacitor voltage
follows, and hence provides a voltage at VCCQP equal to
almost 10V. The Schottky diode D1 is required to provide
the charge path when the MOSFET is off, and reverses
biases when VCCQP goes to 10V. The charge pump capaci-
tor (CP) needs to be a high Q, high frequency capacitor. A
1 F ceramic capacitor is recommended here.
Method 2. 12V Gate Bias
Figure 4 illustrates how a 12V source can be used to bias
VCCQP. A 47 resistor is used to limit the transient current
into the VCCQP pin and a 1µF capacitor is used to filter the
VCCQP supply. This method provides a higher gate bias
voltage (V
pump method, and therefore reduces the R
resulting power loss within the MOSFET. In designs where
efficiency is a primary concern, the 12V gate bias method is
recommended. A 6.2V Zener diode, D1, is used to clamp the
voltage at VCCQP to a maximum of 12V and ensure that the
absolute maximum voltage of the IC will not be exceeded.
PWM/PFM
Control
Figure 3. Charge Pump Configuration
GS
) to the high side MOSFET than the charge
VCCQP
GNDP
LODRV
HIDRV
CP
D1
Q1
Q2
L2
DS,ON
D2
RS
+5V
and the
GS
65-5051-06
C
RC5051
OUT
VO
to the
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