HT7718S Datasheet, PDF (5/29 Pages) Holtek Semiconductor Inc – 100mA PFM Synchronous Step-up DC/DC Converter

HT77xxS
Functional Description
The HT77xxS is a constant on time synchronous step-up
converter, which uses a pulse frequency modulation
(PFM) controller scheme. The PFM control scheme is
inherently stable. The required input/output capacitor
and inductor selections will not create situations of
instability.
The device includes a fully integrated synchronous
rectifier which reduces costs (includes reduce L and C
sizes, eliminates Schottky diode cost etc.) and board
area. A true load disconnect function ensures that the
device is completely shutdown.
Low Voltage Start-up
The devices have a very low start up voltage down to
0.7V. When power is first applied, the synchronous
switch will be initially off but energy will be trans-
ferred to the load through its intrinsic body diode.
Shutdown
During normal device operation, the EN pin should be
either high or connected to the VOUT pin or the VIN
power source. When the device is in the shutdown
mode, that is when the EN pin is pulled low, the inter-
nal circuitry will be switched off. During shutdown,
the PMOS power transistor will be switched off thus
placing the output into a floating condition.
Synchronous Rectification
A dead time exists between the N channel and P chan-
nel MOSFET switching operations. In synchronous
rectification, the P channel is replaced by a Schottky
diode. Here the P channel switch must be completely
off before the N channel switch is switched on. After
each cycle, a 30ns delay time is inserted to ensure the
N channel switch is completely off before the P channel
switch is switched on to maintain a high efficiency
over a wide input voltage and output power range.
Application Information
Inductor Selection
Selecting a suitable inductor is an important consider-
ation as it is usually a compromise situation between
the output current requirements, the inductor saturation
limit and the acceptable output voltage ripple. Lower
values of inductor values can provide higher output
currents but will suffer from higher ripple voltages
and reduced efficiencies. Higher inductor values can
provide reduced output ripple voltages and better
efficiencies, but will be limited in their output current
capabilities. For all inductors it must be noted however
that lower core losses and lower DC resistance values
will always provide higher efficiencies.
The peak inductor current can be calculated using the
following equation:
I_{L(PEAK )}
= ^{VOUT × IO}
V_IN ×η
+ ^{VIN × (VOUT}
2 ×V_OUT
−V_IN )
×L
Where
V_IN = Input Voltage
V_OUT = Output Voltage
I_O = Output Current
η = Efficiency
L = Inductor
Capacitor Selection
As the output capacitor selected affects both efficien-
cy and output ripple voltage, it must be chosen with
care to achieve best results from the converter. Output
voltage ripple is the product of the peak inductor current
and the output capacitor equivalent series resistance
or ESR for short. It is important that low ESR value
capacitors are used to achieve optimum performance.
One method to achieve low ESR values is to connect
two or more filter capacitors in parallel. The capacitors
values and rated voltages are only suggested values.
Rev. 1.00
5
February 22, 2012