HT7465_14 Datasheet, PDF (5/9 Pages) Holtek Semiconductor Inc – Step-Down DC to DC Converter

HT7465
Functional Description
Setting the Output Voltage
The HT7465 is a step down switching DC/DC
converter device. The device switching output, SW, is
connected to a standard converter LC filter circuit for
buck conversion. The output current is sensed using
an external voltage divider network connected to the
load output voltage and ground, and the sense voltage
feedback to the device on the FB pin. By comparing
this sense voltage with an internally generated
0.92 voltage reference, suitable regulation can be
implemented to achieve the required output voltage.
By selecting suitable values of external voltage
divider resistors, the desired output voltage can be
adjusted to the required level. The output voltage can
be calculated using the following formula:
V_OUT
=
R₁ + R₂
R₂
x 0.92
where R2 is the resistor divider lower resistor and R1
is the upper resistor.
Compensation Components
Care must be taken when selecting external
components. The COMP pin is the output of the
error amplifier and must be properly connected to an
external RC network to ensure regulator loop stability.
The values of the compensation components given
in Table 1 yield a stable control loop for the output
voltage and capacitor given.
Table 1 Compensation Values for Typical Output
Voltage/Capacitor Combinations.
V_OUT
L
R3 C4
C6
C5
2.5 10/15mH 9.1K 2.2n 22mF Ceramic —
3.3 10/15mH 10K 3.9n 22mF Ceramic —
5.0 10/15mH 18K 1.5n 22mF Ceramic —
Current Limit Protection
The device has a cycle-by-cycle current limit to
protect the internal power MOSFET. If the inductor
current reaches the current limit threshold of 3.4A,
the MOSFET oscillator frequency will be transformed
from 380kHz to a short circuit frequency of 240kHz
and the output current will remain at its current limit
value.
Over — Temperature Protection
A thermal shutdown function is implemented to
prevent damages due to excessive heat and power
dissipation. Typically the thermal shutdown threshold
temperature is 150°C.When the thermal shutdown is
triggered the device stops switching and will require
a power on input voltage signal, before resuming
switching operation.
Application Information
Inductor
The inductor is required to supply a constant current
to the output load while being driven by the switched
input voltage. The choice of inductor affects steady
state operation as well as transient behavior and
loop stability. There are three important electrical
parameters which need to be considered when
choosing an inductor:
• The inductor value
• DCR - copper wire resistance
• The saturation current
Inductor choice is especially important as it is
important to ensure the inductor does not saturate
under its peak current conditions. Also ensure that the
inductor has a low DCR to minimise power losses.
Input/Output Capacitor
Input Capacitor
Because the V_IN pin is connected to the internal power
MOSFET, when the power MOSFET is switching and
the input current is discontinuous, therefore an input
capacitor C1 is required to supply the AC current
to the step-down converter while maintaining the
DC input voltage. A low ESR ceramic capacitor is
required to keep noise to a minimum.
Output Capacitor
The output capacitor is required to maintain the DC
output voltage. Ceramic or low ESR electrolytic
capacitors is the rule to choice of external load
capacitor is also critical and low ESR value capacitors
must be used to ensure stability. This capacitor must
be connected very close to the inductor, with short
traces for good noise performance.
Schottky Diode
The output diode conducts when the internal power
MOSFET is switched off. There are three important
electrical parameters to consider when choosing the
diode:
• The diode maximum reverse voltage value must
greater than the maximum input voltage
• Short recovery time -- use Schottky diode
• Diode current rating should be greater than the
maximum load current
Rev. 1.10
5
April 18, 2014