FAN1577A Datasheet, PDF (11/15 Pages) Fairchild Semiconductor – Dual Synchronous DC/DC Controller

Functional Description
The FAN1577A is a dual-channel voltage-mode PWM
controller with two sets of synchronous MOSFET driving
circuits. The two channels are running 180-degrees out
of phase. FAN1577A has the following advantages.
Soft Start
An internal startup current (35/15µA) flows out of SS/EN
pin to charge an external capacitor. During the startup
sequence, FAN1577A isn’t enabled until the SS/ENB
pin is higher than 1.2V. From 1.2V to (1.2 + 1.6 x DON /
DON_MAX) V, the PWM duty cycle gradually increases
following the SS/ENB pin voltage to bring output rising.
After (1.2 + 1.6 x DON / DON_MAX) V, the soft-start period
ends and the SS/ENB pin continually rises to 4.8V.
When input power is abnormal, the external capacitor
on the SS pin is shorted to ground to disable the chip.
5
C _SS1
× (1.4V
- 1.2V)
= 35uA
× t₁;
C _SS1
× (1.2V
+ 1.6 ×
12
0.9
- 1.4V)
= 15μA × t ₂
; t₁ + t ₂ = t _SS1
3.3
C _SS2
× (1.4V
- 1.2V)
= 35μA × t₁;
C _SS2
× (1.2V
+ 1.6 ×
12
0.9
- 1.4V)
= 15μA × t ₂
(1)
; t ₁ + t ₂ = t _SS2
C _SS1 × 1.2V
= 35 μA × t ₃ ;
C _SS2 × 0.3V
15 μA
+ ^{C SS2 × (1.2V - 0.3V)}
35 μA
= t₄
; t ₄ - t ₃ = t _{time -shift}
Over-Current Protection (OCP)
Over-current protection is implemented by sensing the
voltage drop across the drain and source of the external
high-side MOSFET. Over-current protection is triggered
when the voltage drop on external high-side MOSFET
RDS(ON) is greater than the programmable current-limit
voltage threshold. 120µA flowing through an external
resistor between input voltage and the CLP pin sets the
threshold of current limit voltage. When over-current
condition is TRUE, the system is protected against the
cycle-by-cycle current limit. A counter counts a series of
over-current peak values for eight cycles; the soft-start
capacitor is discharged by a 50µA current until the
voltage on SS pin reaches 1.2V. During the discharge
period, the high-side driver is turned off and the low-
side driver is turned on. Once the voltage on the
SS/ENB pin is under 1.2V, the normal soft-start
sequence is initiated and the 35/15µA current charges
the soft-start capacitor again.
Error Amplifier
The IN1 and IN2 pins are connected to the
corresponding internal error amplifier’s inverting input
and the outputs of the error amplifiers are connected to
the corresponding COMP1 and COMP2 pins. The
COMP1 and COMP2 pins are available for control-loop
compensation externally. Non-inverting inputs are
internally tied to a fixed 0.7V ±1.5% reference voltage.
Oscillator Operation
The FAN1577A has a programmable-frequency
oscillator. The oscillator is running at 61kHz when the
RT pin is floating. The oscillator frequency can be
adjusted from 61kHz up to 340kHz by an external
resistor RRT between the RT pin and ground. The
oscillator generates a sawtooth wave that has a 90%
rising duty. The sawtooth wave voltage threshold is
from 1.2V to 2.8V. The frequency of oscillator can be
programmed according to the following equation:
fOSC, RT(kHz) = 61kHz + 8522 / RRT(kΩ)
(3)
Setting the Output Voltage
The FAN1577A can be set from 0.7V to 90% of VIN. The
output voltages are independently adjusted by voltage
dividers (R1 and Rf in Figure 31) connected to INx. The
external resister dividers can be calculated by:
VOUT = 0.7 ^× (1+R1/Rf)
(4)
Output Driver
The high-side gate drivers need an external
bootstrapping circuit to provide the required boost
voltage. The highest gate driver’s output (15V is the
allowed) on the high-side and low-side MOSFETs
forces external MOSFETs to have the lowest RDS(ON),
which results in higher efficiency.
Over-Temperature Protection (OTP)
The device is over-temperature protected. When chip
temperature is over 150°C, the chip enters 3-state
(high-side driver is turned off). The hysteresis is 20^°C.
IL(OCP)= [(RSENSE x IOCSET + VOFFSET) / RDS(ON) -
(VIN - VOUT) x VOUT / (fOSC x LOUT x VIN x 2) ]
(2)
where VOFFSET ( ≒ 10mV) is the offset voltage
contributed by the internal OCP comparator.
© 2010 Fairchild Semiconductor Corporation
FAN1577A • Rev. 1.0.0
11
www.fairchildsemi.com