PM6685 STMicroelectronics, PM6685 Datasheet - Page 19

• Manufacturer Part Number: PM6685
• Description: IC CTLR DUAL SYNC STDN 5X5VFQFPN
• Manufacturer: STMicroelectronics

Specifications of PM6685

• Applications: Controller, Notebook Power System
• Voltage - Input: 6 ~ 28 V
• Number Of Outputs: 2
• Voltage - Output: -3.3V, 5V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-VFQFN, 32-VFQFPN
• Output Voltage: 5 V, 3.3 V
• Output Current: 0.1 A
• Input Voltage: 5.5 V to 28 V
• Mounting Style: SMD/SMT
• Maximum Operating Temperature: + 125 C
• Minimum Operating Temperature: - 10 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

PM6685
7
7.1
Device description
The PM6685 is a dual step-down controller dedicated to provide logic voltages for notebook
computers.
It is based on a Constant On Time control architecture. This type of control offers a very fast
load transient response with a minimum external component count. A typical application
circuit is shown in
V and 3.3 V. The switching frequency of the two sections can be adjusted to approximately
200/300 kHz, 300/400 kHz or 400/500 kHz respectively. In order to maximize the efficiency
at light load condition, a pulse skipping mode can be selected. The PM6685 includes also
two linear regulators (LDO5 and LDO3) that allow the shutdown of the respective switching
sections in low consumption status. On the other hand, to maximize the efficiency in higher
consumption status, the linear regulators can be turned off and their outputs can be
supplied directly from the switching outputs. The PM6685 provides protection versus
overvoltage, undervoltage and overtemperature as well as power good signals for
monitoring purposes. An external 1.230 V reference is available.
Constant on time PWM control
If the SKIP pin is tied to 5 V, the device works in PWM mode. Each power section has an
independent on time control.The PM6685 implements a pseudo-fixed switching frequency,
constant on time (COT) controller as core of the switched mode section. Each power section
has an independent COT control.
The COT controller is based on a relatively simple algorithm and uses the ripple voltage due
to the output capacitor’s ESR to trigger the fixed on-time one-shot generator. In this way, the
output capacitor’s ESR acts as a current sense resistor providing the appropriate ramp
signal to the PWM comparator. On-time one-shot duration is directly proportional to the
output voltage VOUT, sensed at the OUT5/OUT3 pins, and inversely proportional to the
input voltage VIN, sensed at the VIN pin, as follows:
Equation 1
This leads to a nearly constant switching frequency, regardless of input and output voltages.
When the output voltage goes lower than the regulated voltage Vreg, the on-time one shot
generator directly drives the high side MOSFET for a fixed on time allowing the inductor
current to increase; after the on time, an off time phase, in which the low side MOSFET is
turned on, follows.
waveforms in PWM mode.
Figure 27 on page
Figure 28 on page 20
Doc ID 11674 Rev 8
18. The PM6685 regulates two fixed output voltages: 5
T
ON
shows the inductor current and the output voltage
=
K
×
V
V
OUT
IN
Device description
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