FAN5069_08 Datasheet, PDF (10/22 Pages) Fairchild Semiconductor – PWM and LDO Controller Combo

Block Diagram
Vcc
R_{ILIM ILIM}
Internal Vcc 5.6V Max.
Shunt Reg _10μA
Current Limit
Comparator
Internal
Boot Diode
COMP
FB
Vref
Error
Amplifier
Vcc
PWM
Comparator
10μA
OSC
SS
V_{IN RRAMP}
R(RAMP)
EN
Ramp
Generator
Enable
Summing
Σ
Amplifier
PW M
RQ
S
Adaptive
Gate Drive
Circuit
Current
Sense
Amplifier
C_BOOT
BOOT
V_IN
HDRV
SW
LDRV
PGND
L_O
Vout
C_O
Figure 20. Block Diagram
Detailed Operation Description
FAN5069 combines a high-efficiency, fixed-frequency
PWM controller designed for single-phase synchronous
buck Point-Of-Load converters with an integrated LDO
controller to support GTL-type loads. This controller is
ideally suited to deliver low-voltage, high-current power
supplies needed in desktop computers, notebooks,
workstations, and servers. The controller comes with an
integrated boot diode which helps reduce component
cost and increase space savings. With this controller, the
input to the power supply can be varied from 3V to 24V
and the output voltage can be set to regulate at 0.8V to
15V on the switcher output. The LDO output can be con-
figured to regulate between 0.8V to 3V and the input to
the LDO can be from 1.5V to 5V, respectively. An internal
shunt regulator at the VCC pin facilitates the controller
operation from either a 5V or 12V power source.
V_CC Bias Supply
FAN5069 can be configured to operate from 5V or 12V
for V_CC. When 5V supply is used for V_CC, no resistor is
required to be connected between the supply and the
V_CC. When the 12V supply is used, a resistor R_VCC is
connected between the 12V supply and the V_CC, as
shown in Figure 1. The internal shunt regulator at the
VCC pin is capable of sinking 150mA of current to
ensure that the controller’s internal V_CC is maintained at
5.6V maximum.
Choose a resistor such that:
■ It is rated to handle the power dissipation.
■ Current sunk within the controller is minimized to
prevent IC temperature rise.
R_VCC Selection (IC)
The selection of R_VCC is dependent on:
■ Variation of the 12V supply
■ Gate charge of the top and bottom FETs (Q_FET)
■ Switching frequency (F_SW)
■ Shunt regulator minimum current (1mA)
■ Quiescent current of the IC (I_Q)
Calculate R_VCC based on the minimum input voltage for
the V_CC:
R_VCC = ₍---_I--_Q-----+-----₁-----_•----₁---₀--^V--_–--₃ⁱ--ⁿ--+--^M---_Q-^I--^N--_F--–--_E---⁵_T---^.-_•-⁶----_F---_S----_W------_•----₁---_.--₂----₎
(EQ. 1)
For a typical example, where: Vin_MIN = 11.5V, I_Q = 3mA,
Q_FET = 30nC, F_SW = 300KHz, R_VCC is calculated to be
398.65Ω.
PWM Section
The FAN5069’s PWM controller combines the conven-
tional voltage mode control and current sensing through
lower MOSFET R_{DS_ON} to generate the PWM signals.
This method of current sensing is loss-less and cost
effective. For more accurate current sense requirements,
an optional external resistor can be connected with the
bottom MOSFET in series.
© 2005 Fairchild Semiconductor Corporation
FAN5069 Rev. 1.1.5
10
www.fairchildsemi.com